1.A.8 The Major Intrinsic Protein (MIP) Family
The Major Intrinsic Protein (MIP) of the human lens of the eye (Aqp0), after which the MIP family was named, represents about 60% of the protein in the lens cell. In the native form, it is an aquaporin, but during lens development, it becomes proteolytically truncated. The channel, which normally houses 6-9 water molecules, becomes constricted so only three remain, and these are trapped in a closed conformation (Gonen et al., 2004a,b). These truncated tetramers form intercellular adhesive junctions (head to head), yielding a crystalline array that mediates lens formation with cells tightly packed as required to form a clear lens (Gonen and Walz, 2006). Lipids crystallize with the protein (Gonen et al., 2005). Ion channel activity has been shown for Aquaporins 0, 1, and 6, Drosophila Big Brain and plant Nodulin-26 (Yool and Campbell, 2012). Roles of aquaporins in human cancer have been reviewed (Pareek et al. 2013) as have their folding pathways (Klein et al. 2015). AQPs may act as transmembrane osmosensors in red cells, secretory granules and microorganisms (Hill and Shachar-Hill 2015). MIP superfamly proteins and variations of their selectivity filters have been reviewed (Verma et al. 2015). Their evolution has been discussed (Ishibashi et al. 2017).
The MIP family is large and diverse, possessing thousands of members that form transmembrane channels. These channel proteins function in water, small carbohydrate (e.g., glycerol), urea, NH3, CO2, H2O2 and ion transport by energy-independent mechanisms. For example, the glycerol channel, FPS1p of Saccharomyces cerevisiae mediates uptake of arsenite and antimonite (Wysocki et al., 2001). Ion permeability appears to occur through a pathway different than that used for water/glycerol transport and may involve a channel at the 4 subunit interface rather than the channels through the subunits (Saparov et al., 2001). MIP family members are found ubiquitously in bacteria, archaea and eukaryotes. Phylogenetic clustering of the proteins is largely according to phylum of the organisms of origin, but one or more clusters are observed for each phylogenetic kingdom (plants, animals, yeast, bacteria and archaea) (Park and Saier, 1996). MIPs are classified into five subfamilies in higher plants, including plasma membrane (PIPs), tonoplast (TIPs), NOD26-like (NIPs), small basic (SIPs) and unclassified X (XIPs) intrinsic proteins. One of the plant clusters includes only tonoplast (TIP) proteins, while another includes plasma membrane (PIP) proteins (de Paula Santos Martins et al. 2015).
The known aquaporins cluster loosely together as do the known glycerol facilitators. MIP family proteins are believed to form aqueous pores that selectively allow passive transport of their solute(s) across the membrane with minimal apparent recognition. Aquaporins selectively transport water (but not glycerol) while glycerol facilitators selectively transport glycerol but not water. Some aquaporins can transport NH3 and CO2. Glycerol facilitators function as solute nonspecific channels, and may transport glycerol, dihydroxyacetone, propanediol, urea and other small neutral molecules in physiologically important processes. Some members of the family, including the yeast Fps1 protein (TC #1.A.8.5.1) and tobacco NtTIPa (TC #1.A.8.10.2) may transport both water and small solutes.
Calamita et al. 2018 review the expression, regulation and physiological roles of AQPs in adipose tissue, liver and endocrine pancreas that are involved in energy metabolism. The review also summarizes the involvement of AQPs in metabolic disorders, such as obesity, diabetes and liver diseases. Challenges and recent advances related to pharmacological modulation of AQPs expression and function to control and treat metabolic diseases are discussed (Calamita et al. 2018). Jain et al. 2018 have shown that an intra-helical salt-bridge in the Loop E half-helix can influence the transport properties of AQP1 and GlpF channels.
Zardoya and Villalba (2001) have conducted phylogenetic analyses of the MIP family, analyzing 153 homologues. They divided the proteins into six major 'paralogous' groups: (1) GLPs, or glycerol-transporting channel proteins, which include mammalian AQP3, AQP7, and AQP9, several nematode paralogues, a yeast paralogue, and Escherichia coli GLP; (2) AQPs, or aquaporins, which include metazoan AQP0, AQP1, AQP2, AQP4, AQP5, and AQP6; (3) PIPs, or plasma membrane intrinsic proteins of plants, which include PIP1 and PIP2; (4) TIPs, or tonoplast intrinsic proteins of plants, which include αTIP, γTIP, and δTIP; (5) NODs, or nodulins of plants; and (6) AQP8s, or metazoan aquaporin 8 proteins. Of these groups, AQPs, PIPs, and TIPs cluster together as noted above.
In agreement with their divergent sequences, human AQP1-9 have very different physiological functions. They are involved in (1) nephrogenic diabetes insipidus, (2) brain water balance and hearing and (3) salivary secretion (Li and Verkman, 2001). Bacterial homologues also have diverse functions. Two proteins in E. coli function as water and glycerol transporters, respectively. Lactobacillus plantarum has 6 homologues, some of which transport water, glycerol and dihydroxyacetone, and some which transporter these compounds as well as D,L-lactic acid (Bienert et al. 2013). The pH sensitivities of Aqp0 channels in lenses of tetraploid and diploid teleosts have been reported (Chauvigné et al. 2015). In te heart, AQPs are implicated in proper cardiac water homeostasis and energy balance as well as heart failure and arsenic cardiotoxicity (Verkerk et al. 2019).
Several reports of MIP family proteins transporting ions may or may not be physiologically significant. For example, the influx of arsenite and antimonite via the Fps1 protein into yeast cells is well documented (Wysocki et al., 2001). Similarly, these compounds are taken up via aquaporins in Leishmania (Gourbal et al., 2004). Moreover, AQP6 of renal epithelia have been reported to transport anions at low pH (Yasui et al., 1999). Demonstration of the involvement of the cyanobacterial channel protein (TC #1.A.8.4.1) in copper homeostasis suggests that it may transport Cu2+. Finally, Yang et al. (2005) showed that arsenite exists the Mesorhizobium meliloti cell by downhill movement through AqpS (1.A.8.15.1). The physiological functions of many MIP family proteins are unknown.
MIP family channels consist of homotetramers (e.g., GlpF of E. coli; TC #1.A.8.1.1, AqpZ of E. coli; TC #1.A.8.3.1, and MIP or Aqp0 of Bos taurus; TC #1.A.8.8.1). Each subunit spans the membrane six times as putative α-helices and arose from a 3-spanner-encoding genetic element by a tandem, intragenic duplication event. The two halves of the proteins are therefore of opposite orientation in the membrane. However, a well-conserved region between TMSs 2 and 3 and TMSs 5 and 6 dip into the membrane, each loop forming a half TMS.
Several MIPs within all domains of life have been shown to facilitate the diffusion of reduced and non-charged species of the metalloids silicon, boron, arsenic and antimony (Bienert et al., 2008). Metalloids encompass a group of biologically important elements ranging from the essential to the highly toxic. Consequently, all organisms require efficient membrane transport systems to control the exchange of metalloids with the environment. Recent genetic evidence has demonstrated a crucial role for specific MIPs in metalloid homeostasis (Bienert et al., 2008).
The crystal structure of the glycerol facilitator of E. coli was solved at 2.2 Å resolution (Fu et al., 2000). Glycerol molecules line up in single file within the amphipathic channel. In the narrow selectivity filter of the channel, the glycerol alkyl backbone is wedged against a hydrophobic corner, and successive hydroxyl groups form hydrogen bonds with a pair of acceptor and donor atoms. The two conserved D-P-A motifs in the loops between TMSs 2 and 3 and TMSs 5 and 6 form the interface between the two duplicated halves of each subunit. Thus each half of the protein forms 3.5 TMSs surrounding the channel. The structure explains why GlpF is selectively permeable to straight chain carbohydrates, and why water and ions are excluded. Aquaporin-1 (AQP1) and the bacterial glycerol facilitator, GlpF can transport O2, CO2, NH3, glycerol, urea, and water to varying degrees. For small solutes permeating through AQP1, a remarkable anticorrelation between permeability and solute hydrophobicity was observed whereas the opposite trend was observed for permeation through the membrane (Hub and Groot, 2008). AQP1 is thus a selective filter for small polar solutes, whereas GlpF is highly permeable to small solutes and less permeable to larger solutes.
Aquaporin-1 (Aqp1) from the human red blood cell has been solved by x-ray crystallography to 3.8 Å resolution (Murata et al., 2000). The aqueous pathway is lined with conserved hydrophobic residues that permit rapid water transport. Water selectivity is due to a constriction of the inner pore diameter to about 3 Å over the span of a single residue, superficially similar to that in the glycerol facilitator of E. coli.
AqpZ, a homotetramer (tAqpZ) of four water-conducting channels that facilitate rapid water movements across the plasma membrane of E. coli, has been solved to 3.2 Å resolution. All channel-lining residues in the four monomeric channels are found orientated in nearly identical positions with one marked exception at the narrowest channel constriction, where the side chain of a conserved Arg-189 adopts two distinct conformational orientations. In one of the four monomers, the guanidino group of Arg-189 points toward the periplasmic vestibule, opening up the constriction to accommodate the binding of a water molecule through a tridentate H-bond. In the other three monomers, the Arg-189 guanidino group bends over to form an H-bond with carbonyl oxygen of Thr-183 occluding the channel. Therefore, the tAqpZ structure reveals two distinct Arg-189 conformations associated with water permeation through the channel constrictions. Alternating between the two Arg-189 conformations disrupts continuous flow of water, thus regulating the open probability of the water pore. Further, the difference in Arg-189 displacements is correlated with a strong electron density found between the first transmembrane helices of two open channels, suggesting that the observed Arg-189 conformations are stabilized by asymmetrical subunit interactions in tAqpZ (Jiang et al., 2006).
The 3-D structures of the open and closed forms of plant aquaporins, PIP1 and PIP2, have been solved (Törnroth-Horsefield et al., 2006). In the closed conformation, loop D caps the channel from the cytoplasm and thereby occludes the pore. In the open conformation, loop D is displaced up to 16 Å, and this movement opens a hydrophobic gate blocking the channel entrance from the cytoplasm. These results reveal a molecular gating mechanism which appears conserved throughout all plant plasma membrane aquaporins. In plants it regulates water intake/export in response to water availability and cytoplasmic pH during anoxia (Törnroth-Horsefield et al., 2006).
The MIP superfamily includes three subfamilies: aquaporins, aquaglyceroporins and S-aquaporins. (1) The aquaporins (AQPs) are water selective. (2) The aquaglyceroporins are permeable to water, but also to other small uncharged molecules. (3) The third subfamily, with little conserved amino acid sequences around the NPA boxes, include 'superaquaporins' (S-aquaporins).The phylogeny of insect MIP family channels has been published (Finn et al. 2015). The arylsulfonamide AqB011 which selectively blocks the central ion pore of mammalian AQP1 has been shown to impair migration of HT29 colon cancer cells. Traditional herbal medicines are sources of selective AQP1 inhibitors that also slow cancer cell migration (Kourghi et al. 2018).
13 isoforms of mammalian aquaporins (AQP0 - AQP12),are known, 9 of which is localized in different parts of the renal tubular epithelium. Additional transport functions of renal AQPs (AQP3, AQP6, AQP7 and AQP8) are known. Aquaglyceroporins are most probably key elements in the renal regulation of nitrogen balance and maintenance of the correct pH of body fluids (Michalek 2016).
Otitis media (OM) refers to inflammatory diseases of the middle ear (ME), regardless of cause or pathological mechanism. The expression of aquaporins (AQPs) in the ME and Eustachian tube (ET) is relevant. Eleven types of AQPs, AQP1 to AQP11, have been found to be expressed in mammalian ME and ET (Jung et al. 2017). The distribution and levels of expression of AQPs depend on the presence or absence of inflammation. Fluid accumulation in the ME and ET is a common mechanism for all types of OM, causing edema in the tissue and inducing inflammation involving various AQPs. The expression patterns of several AQPs, especially AQP1, 4 and 5, may have immunological functions in OM.
Some classes of AQPs conduct ions, glycerol, urea, CO2 , nitric oxide, and other small solutes. Ion channel activity has been demonstrated for mammalian AQPs 0, 1, 6, Drosophila big brain (BIB), soybean nodulin 26, and rockcress AtPIP2;1 (Kourghi et al. 2017). Classification of AQPs into three categories (orthodox AQPs, aquaglyceroporins and superaquaporins) is based on their sequence similarities and substrate selectivities. In the male reproductive tract of mammals, most AQPs (except AQP6 and AQP12) are found in different organs (including testis, efferent ducts and epididymis). AQP1 and AQP9 are the most abundant AQPs in the efferent ducts and epididymis and play a crucial role for the secretion/reabsorption dynamics of luminal fluid during sperm transport and maturation. AQP3, AQP7, AQP8 and AQP11 are the most abundant AQPs in sperm and are involved in the regulation of their volumes, which is required for the differentiation of spermatids into spermatozoa during spermatogenesis, as well as in sperm transit along environments of different osmolality (male and female reproductive tracts). Mounting evidence indicates that AQP3, AQP7 and AQP11 are involved in cryotolerance as well as the sperm response to variations of osmolality and to freeze-thawing procedures (Yeste et al. 2017).
The generalized transport reaction for channel proteins of the MIP family is:
H2O (out) → H2O (in) (e.g., aquaporins)
solute (out) → solute (in) (e.g., glycerol facilitators).
Bienert, G.P., M.D. Schüssler, and T.P. Jahn. (2008). Metalloids: essential, beneficial or toxic? Major intrinsic proteins sort it out. Trends Biochem. Sci. 33: 20-26.
Glycerol facilitator, GlpF. Transports various polyols with decreasing rates as size increases (Heller et al. 1980); also transports arsenite (As(III) and antimonite (Sb(III)) (Meng et al., 2004).
GlpF of E. coli
Aqp1 of 270 aas and 6 TMSs. Induced by NH3 but not CO2, but transports both gases. Aqp1 is found in the plasma membrae as well as the ER/chloroplast. Aqp1 may be involved in photoprotection. It may facilitate the efflux of NH3, preventing the uncoupling effect of high intracellular ammonia concentrations (Matsui et al. 2018).
Aqp1 of Phaeodactylum tricornutum, a marine photoautotrophic diatoms
Aquaporin TIP2-1 (Delta-tonoplast intrinsic protein) (Delta-TIP) (Tonoplast intrinsic protein 2-1) (AtTIP2;1) (Daniels et al. 1996). Transports water and ammonia, and can be activated by mercury (Kirscht et al. 2016). The 3-d structure is known to 1.2Å resolution (Kirscht et al. 2016). It may participate in vacuolar compartmentation and detoxification of ammonium.
TIP2-1 of Arabidopsis thaliana
Uncharacterized protein of 295 aas and 6 TMSs.
UP of Volvox carteri
The Aquaporin-8 (Aqp8) transporter is permeable to water, NH3, formamide and H2O2, and it is present in the inner membrane of mitochondria and the plasma membrane (Bienert et al., 2007; Saparov et al., 2007; Soria et al., 2010). Cholesterol, via sterol regulatory element-binding protein (SREBP) transcription factors, activates or represses genes involved in its hepatic biosynthetic pathway, and also modulates the expression of hepatocyte mitochondrial aquaporin-8 (mtAQP8), a channel that can function as peroxiporin by facilitating the transmembrane diffusion of H2O2. The peroxiporin, mtAQP8, plays a role in the SREBP-controlled hepatocyte cholesterogenesis (Danielli et al. 2019).
Aqp8 of Homo sapiens (O94778)
Aqp8a.1 of 260 aas and 6 TMSs. The spaciotemporal pattern of induction of three aquaporins during embyonic development in Zebrafish has been determined, and all three, Aqp8a.1, Aqp8a.2 and Aqp8b, show distictive patterns (Koun et al. 2016).
Aqp8a.1 of Danio rerio (Zebrafish) (Brachydanio rerio)
Aquaporin of 250 aas and 6 TMSs. It is a water channel required to facilitate the transport of water across membranes; it is involved in osmotolerance (Ghosh et al. 2006).
Aquaporin of Encephalitozoon cuniculi (Microsporidian parasite)
Endoplasmic reticulum Small and Basic Intrinsic Protein; (SIP1;1) water channel (present in all plant tissues except seeds) (Ishikawa et al., 2005) May play a role in gas and water exchange between the plant and its environment via stromata (turgor-driven epidermal valves) and the hydathode pore (Pillitteri et al., 2008).
SIP1;1 of Arabidopsis thaliana (Q9M8W5)
Tip1;3 of Arabidopsis thaliana (O82598)
The pollen-specific water/urea aquaporin. Tip5;1 (Soto et al. 2008) An aquaporin specifically targeted to pollen mitochondria; probably involved in nitrogen remobilization (Soto et al., 2010).
Tip5;1 of Arabidopsis thaliana (Q9STX9)
Aquaporin-B, AqpB of 294 aas and 6 TMSs. Tyr216 in loop D is a key residue in gating, possibly involving phosphorylation. Mutation of Tyr216 to aspartate or glutamate initiated water permeability. The truncated, permanently open AqpB yielded cells with reduced capability to cope with hypotonic stress (von Bülow et al. 2015).
AqpB of Dictyostelium discoideum
Plasma membrane aquaporin 1 (Törnroth-Horsefield et al., 2006). Transports H2O, H2O2 (Dynowski et al., 2008), O2 and CO2 (Zwiazek et al. 2017). Forms active heterotetramers with PIP2;1 (1.A.8.11.4); down regulated under drought stress (Najafabadi et al., 2008); plays a role insalt tolerance (Li et al. 2018). Gated by H+, Ca2+, Mn2+ and Cd2+ (Verdoucq et al., 2008). The wheat orthologue has been described (Ayadi et al., 2011). 96% identical to PIP1;3. In Selaginella moellendorffii (Sm; spikemoss), SmPIP1;1 is retained in the ER while SmPIP2;1 is found in the plasma membrane but, upon co-expression, both isoforms are found in the plasma membrane as a heterotetramer, leading to a synergistic effect on water membrane permeability (Bienert et al. 2018). In some speices, PIP1 is inactive (e.g., in maize), but formation of a hetrotetramer with PIP2 allows transport (Vajpai et al. 2018).
PIP1.1 of Arabidopsis thaliana (P61837)
Plasma membrane intrinsic protein 2a (forms active heterotetramers with PIP1;1 (TC# 1.A.8.11.3); down regulated under drought stress (Najafabadi et al., 2008). Transports H2O2 (Dynowski et al., 2008). The Mesembryanthemum crystallinum PIP2;1 orthologue is an aquaporin impermeable to urea and glycerol. It is positively regulated by PKA- and PKC- mediated phosphorylation (Amezcua-Romero et al., 2010). PIP1;1 and PIP2;2 (Q9ATM8) co-expression modulates the membrane water permeability in the halophyte Beta vulgaris storage root through a pH regulatory response, enhancing membrane versatility to adjust its water transfer capacity (Bellati et al., 2010). The wheat orthologue has been described (Ayadi et al., 2011). Inter-TMS interactions occurring both within and between monomers play crucial roles in tetramer formation, and assembly of tetramers is critical for their trafficking from the ER to the plasma membrane as well as water permeability (Yoo et al. 2016). This protein as well as 1.A.8.11.6 is possibly orthologous to spinach PIP1;2 for which the crystal structure is available (PDP# 4JC6) (Berny et al. 2016). Plays a role in drought and salt tolerance (Wang et al. 2015). PIP-type aqauporins may also transport CO2, boric acid, glycerol, arsenic and Na+ (Byrt et al. 2017).
PIP2;1 of Arabidopsis thaliana (P43286)
Aquaporin PIP2-8 (Plasma membrane intrinsic protein 2-8) (AtPIP2;8) (Plasma membrane intrinsic protein 3b) (PIP3b). This protein as well as 1.A.8.11.4 are possibly orthologous to spinach PIP1;2 for which the crystal structure is available (PDP# 4JC6) (Berny et al. 2016).
PIP2-8 of Arabidopsis thaliana
Aquaporin PIP2;5 (PIP2-5) of 285 aas. Transports water and hydrogen peroxide (H2O2) (Bienert et al. 2014). PIP1;2 doesn't transport H2O2. TMS3 contains an LxxA motif that targets the protein to the plasma membrane from the ER. While PIP2s are in the plasma mebrane, PIP1s are retained in the ER; this motif only partly explains the difference (Chevalier and Chaumont 2015). PIP1;2 AND PIP2;5 form homo- and heterotetramers (Berny et al. 2016).
PIP2;5 of Zea mays
Aqp2 of 297 aas and 6 TMSs. Induced by both NH3 and CO2, and transports both gases. Aqp2 is found in the plasma membrane and may be involved in photoprotection. It may facilitate the efflux of NH3, preventing the uncoupling effect of high intracellular ammonia concentrations (Matsui et al. 2018).
Aqp2 of Phaeodactylum tricornutum, a marine photoautotrophic diatoms
Nodulin-26 aquaporin and glycerol facilitator, NIP (de Paula Santos Martins et al. 2015). Transports NH3 5-fold better than water in Hg2+-sensitive fashion (Hwang et al., 2010).
Nodulin-26 of Glycine max (spP08995)
Uncharacterized MIP family protein of 274 aas and 6 TMSs.
UP of Entamoeba histolytica
Uncharacterized MIP family protein of 314 aas and 8 putative TMSs. The extra 2 non-homologous TMSs appear to be N-terminal.
UP of Entamoeba histolytica
The silicon (silicic acid) (undissociated form) transporter, Lsi1 (Ma et al., 2007a, b; Mitani et al., 2008). The barley orthologue Lsi1 (also called NIP2-1) is also a silicon (silicic acid) uptake channel (Chiba et al., 2009). Rice Lsi1 also transports arsenite and pentavalent mono and dimethyl arsenite (Li et al., 2009). In addition to silicon (Si), selenite (Se) uptake is mediated by Lsi1, also called NIP2;1 (Zhao et al., 2010).
Lsi1 of Oryza sativa (Q6Z2T3)
The boric acid channel protein, NIP5;1 (expressed in the root elongation zone and root hairs in response to boron deficiency) (Takano et al., 2006). Borate is an essential nutrient in plants. The ortholog in Brassica napus ( XP_013652160.1) is 301 aas in length with 6 TMSs and is 90% identical to the A. thaliana protein. Synthesis of the B. napus protein is in induced in roots and shoots by a borate deficiency (Diehn et al. 2019). NIP2, 3, 4, 6 and 7 can also transport boric acid (Diehn et al. 2019).
NIP5;1 of Arabidopsis thaliana (NP_192776)
The silicon (silicic acid) transporter, Nip2-2 (Nip2;2) (Mitani et al., 2008). Also transports arsenite (Li et al., 2009).
Nip2-2 of Oryza sativa (Q67WJ8)
Nip7;1 arsenite and borate channel (Isayenkov and Maathuis, 2008; Li et al., 2011)
Nip7. 1 of Arabidopsis thaliana (Q8LAI1)
Aquaporin NIP1-1 (NOD26-like intrinsic protein 1-1) (AtNIP1;1) (Nodulin-26-like major intrinsic protein 1) (NodLikeMip1) (Protein NLM1). NIP-like aquaporins transport water, but also arsenic, boric acid, sliicon, glycerol, urea, lactic acid and ammonia (Mitani-Ueno et al. 2011; Hwang et al. 2010).
NIP1-1 of Arabidopsis thaliana
Hg2+-inhibitable aquaporin, AqpM (transports both water and glycerol as well as CO2) (Kozono et al., 2003; Araya-Secchi et al., 2011). Its 3-d structure has been determined to 1.7 Å. In AqpM, isoleucine replaces a key histidine residue found in the lumen of water channels, which becomes a glycine residue in aquaglyceroporins. As a result of this and other side-chain substituents in the walls of the channel, the channel is intermediate in size and exhibits differentially tuned electrostatics when compared with the other subfamilies (Lee et al. 2005).
AqpM of Methanothermobacter marburgensis
Putative aquaporin, GlpF5, of 216 aas; probably transports water, glycerol and dihydroxyacetone (Bienert et al. 2013).
GlpF5 of Lactobacillus plantarum
Aquaglyceroporin of 270 aas and 6 TMSs.
Aquaporin of Paramecium bursaria chlorella
Mixed function glycerol facilitator/aquaporin, GlpF (Froger et al. 2001).
GlpF of Lactococcus lactis
GlpF of Mycoplasma gallisepticum )
GlpF1; transports water, dihydroxyacetone and glycerol as well as D,L-lactic acid (Bienert et al. 2013).
GlpF1 of Lactobacillus plantarum
GlpF2. Transporter of water, dihydroxyacetone and glycerol (Bienert et al. 2013).
GlpF2 of Lactobacillus plantarum
GlpF3. Transports water, dihydroxyacetone and glycerol (Bienert et al. 2013).
GlpF3 of Lactobacillus plantarum
GlpF4. Transports water, dihydroxyacetone and glycerol as well as D,L-lactic acid (Bienert et al. 2013).
GlpF4 of Lactobacillus plantarum
Putative aquaporin, GlpF6. Probably transports water, glycerol and dihydroxyacetone (Bienert et al. 2013).
GlpF6 of Lactobacillus plantarum
Glycerol facilitator, GlpF, of 248 aas and 6 TMSs
GlpF of Blattabacterium sp. subsp. Blattella germanica (strain Bge) (Blattella germanica symbiotic bacterium)
Aquaporin Z water channel (aqpZ gene expression is under sigma S control; induced at the onset of stationary phase) (Mallo and Ashby, 2006). The high resolution 3-d structure is available (PDB 1RC2) revealing two re-entrant coil-helix domains from the selectivity filter (Savage et al. 2003).
AqpZ of E. coli (P60844)
Intracellular endoplasmic reticulum (ER)-localized Aquaporin 11 (Aqp11, AqpX1) water channel (important for the development of kidney proximal tubules; disruption produces neonatally fatal polycystic kidneys (Ishibashi 2006). Has a positively charged C-terminal amino acid cluster similar to the di-lysine motif (-KKXX) for ER retention (Nozaki et al., 2008)). In the horse, AQP3 and AQP11 are involved in the resilience of stallion sperm to withstand cryopreservation (Bonilla-Correal et al. 2017).
Aqp11 of Homo sapiens (Q8NBQ7)
Aquaporin-12A (AQP-12) of 295 aas and probably 7 TMSs with an extra N-terminal TMS. Bears a C-terminal KKXX-like ER retention sequence and is found intracelllularly (Ishibashi 2006). It is expressed in elevated amounts in exocrine glandular cells of the pancreas (Danielsson et al. 2014).
AQP12A of Homo sapiens
Aquaporin 10, Aqp10 of 259 aas and 6 TMSs
Aqp10 of Haemonchus contortus (Barber pole worm)
Aquaporin of 263 aas and 7 TMSs (Stavang et al. 2015).
Aquaporin of the salmon leach, Lepeophtheirus salmonis
Aquaporin of 256 aas with 6 TMSs in a 3 (N-terminus) + 3 TMS (C-terminus) arrangement (Zhou et al. 2018).
Aqp of Blomia tropicalis (mite)
FPS1 glycerol efflux facilitator (important for maintaining osmotic balance during mating-induced yeast cell fusion and for tolerating hypoosmotic shock; also transports arsenite and antimonite). FPS1 is a homotetramer (Beese-Sims et al., 2011). Fps1 is important for osmo-adaptation by regulating intracellular glycerol levels during changes in external osmolarity. Upon high osmolarity conditions, yeast accumulate glycerol by increased production of the osmolyte and by restricting glycerol efflux through Fps1. The extended cytosolic termini of Fps1 contain short domains that are important for regulating glycerol flux through the channel. The transmembrane core of the protein plays an equally important role (Geijer et al., 2012). The MAP kinase, Slt2, physically interacts with Fps1, and this interaction, dependent on phosphorylation of S537, regulates arsenite uptake (Ahmadpour et al. 2016).
FPS1 protein of Saccharomyces cerevisiae
Fps1 hyperactive orthologue of the S. cerevisiae Fps1 (1.A.8.5.1) (Geijer et al., 2012).
Fps1 of Ashbya gossypii (Q75CI7)
Aquaporin, Aqy1 (PIP2-7 7). The subangstron (0.88Å) structure is available (Kosinska Eriksson et al. 2013). the H-bond donor interactions of the NPA motif''s asparagine residues to passing water molecules are revealed. A polarized water-water H-bond configuration is observed within the channel. Four selectivity filter water positions are too closely spaced to be simultaneously occupied. Strongly correlated movements break the connectivity of selectivity filter water molecules to other water molecules within the channel, thereby preventing proton transport via a Grotthuss mechanism.
Aqy1 of Komagataella pastoris (Pichia pastoris)
Water and CO2 permeable aquaporin, AQP1, of an edible mycorhizal fungus (desert truffles) (Navarro-Ródenas et al. 2012).
AQP1 of Terfezia claveryi
Tobacco X-intrinsic protein (XIP1-1-β). Transports glycerol, urea and boric acid, but not water (Bienert et al., 2011).
XIP1-1 of Nicotiana tomentosiformis (E3UN01)
Potato X intrinsic protein, XIP1. Transports glycerol, urea and boric acid, but not water (Bienert et al., 2011).
XIP1-1 of Solanum tuberosum (E3UMZ6)
Morning glory XIP-1-1-α. Transports glycerol, urea and boric acid, but not water (Bienert et al., 2011).
XIP1 of Ipomoea nil (E3UMZ5)
Major intrinsic protein superfamily, aquaporin-like protein. MIP2, of 247 aas and 6 TMSs.
MIP2 of Chlamydomonas reinhardtii (Chlamydomonas smithii)
Aquaporin 1 (CO2-, O2- and nitrous oxide-permeable and water-selective) (Zwiazek et al. 2017). Aquaporin-1 tunes pain perception by interacting with Na(v)1.8 Na+ channels in dorsal root ganglion neurons (Zhang and Verkman, 2010). It is upregulated in skeletal muscle in muscular dystrophy (Au et al. 2008). AQP1 has been reported to first insert as a four-helical intermediate, where helices 2 and 4 are not inserted into the membrane. In a second step this intermediate is folded into a six-helical topology. During this process, the orientation of the third helix is inverted, and it can shift out the membrane core (Virkki et al. 2014). Its synthesis is regluated by Kruppel-like factor 2 (KLF2; Q9Y5W3) which also interacts directly with Aqp1 (Fontijn et al. 2015). A nanoscale ion pump has been derived artificially from Aqp1 (Decker et al. 2017). Mammalian Aquaporin-1 (AQP1) channels activated by cyclic GMP can carry non-selective monovalent cation currents, selectively blocked by arylsulfonamide compounds AqB007 (IC50 170 muM) and AqB011 (IC50 14 muM). Loop D-domain amino acids activate the channel for ion coductance (Kourghi et al. 2018). Water flux through AQP1s is inhibited by 1 - 10 mμM acetozolaminde (Gao et al. 2006).
Aquaporin 1 (AQP1) of Homo sapiens
Water and urea transporting aquaporin (cockroach) (Herraiz et al., 2011).
Aquaporin of Blatella germanica (G8YY04)
Water channel, Aqp1; inhibited by HgCl2 and tetraethylammonium. Plays a role in water homeostasis during blood feeding and humidity adaptation of A. gambiae, a major mosquito vector of human malaria in Africa (Liu et al., 2011).
Aqp1 of Anopheles gambiae (F2YNF6)
Aquaporin, Aqp1 in the gall fly. Transports water but not glycerol or urea. Promotes freeze-tolerance (Philip et al., 2011).
Aqp1 of Eurosta solidaginis (E4W5Y5)
The Drosophila melanogaster integral protein, DRIP (Ishida et al., 2012).
Aqp, DRIP of Drosophila melanogaster (Q9V5Z7)
MIP26 of Rana pipiens
Mercury-sensitive whitefly aquaporin-1 of the specialized filter chamber of the alimentary tract (Mathew et al. 2011).
Aquaporin-1 of Bemisia tabaci
Aquaporin-1 or Aquaporin1, Aqp1, of 258 aas and 6 TMSs. Three Aqp1 isoforms are differentially regluated by the function of the vasotocin (AVTR) and isotocin (ITR) receptors (Martos-Sitcha et al. 2015). Aqp1aa, one of two isoforms in teleosts, may play a role in spermatogenesis in Cynoglossus semilaevis (Guo et al. 2017).
Aqp1 of Sparus aurata (Gilthead sea bream)
Aquaporin-3, Aqp-3 of 271 aas. Transports water, glycerol, hydrogen peroxide and urea (Geadkaew et al. 2015). AQP3 induces the production of chemokines such as CCL24 and CCL22 through regulating the amount of cellular H2O2 in M2 polarized alveolar macrophages, implying a role of AQP3 in asthma (Ikezoe et al. 2016).
Aqp3 of Opisthorchis viverrini (liver fluke)
Aqp-x2 water channel in the luminal epithelium of urinary bladder cells and lungs. Responsive to Vasotocin (AVT) (Shibata et al. 2015).
Aqp-x2 of Xenopus laevis
Contractile vacuole aquaporin of 295 aas and 6 TMSs, Aqp. Shown to transport water, accounting for the high water permeability of the contractile vacuole (Nishihara et al. 2008).
Aqp of Amoeba proteus (Amoeba) (Chaos diffluens)
The lens fiber MIP aquaporin (Aqp0) of B. taurus (forms membrane junctions in vivo and double layered crystals in vitro that resemble the in vivo junctions). The water pore is closed in the in vitro structure (Gonen et al., 2004b). It interacts directly with the intracellular loop of connexin 45.6 via its C-terminal extension (Yu et al., 2005). Forms human cataract lens membranes (Buzhynskyy et al., 2007; Yang et al., 2011). A mutation that causes congenital dominant lens cataracts has been identified (Varadaraj et al. 2008). AqpO catalyzes Zn2+-modulated water permeability as a cooperative tetramer (Nemeth-Cahalan et al., 2007). It transports ascorbic acid (Nakazawa et al., 2011). The Detergent organization around solubilized aquaporin-0 using Small Angle X-ray Scattering has been reported (Berthaud et al., 2012). Aquaporin 0 (AQP0) in the eye lens is truncated by proteolytic cleavage during lens maturation. This truncated AQP0 is no longer a water channel (Berthaud et al. 2015). A mutation that causes congenital dominant lens cataracts has been identified (Varadaraj et al. 2008). Cataractogenesis in MIP mutants are probably caused by defects in MIP gene expression in mice (Takahashi et al. 2017).
Major intrinsic protein (MIP or Aqp0) of Bos taurus
Aquaporin x5 of 273 aas and 6 TMSs, Aqp-x5. The sequence reveals a mercurial-sensitive cysteine and a putative phosphorylation motif site for protein kinase A at Ser-257 (Kubota et al. 2006). A swelling assay using Xenopus oocytes revealed that AQP-x5 facilitated water permeability. Expression of AQP-x5 mRNA was restricted to the skin, brain, lungs and testes. Immunofluorescence and immunoelectron microscopical studies using an anti-peptide antibody (ST-156) against the C-terminal region of the AQP-x5 protein revealed the presence of immunopositive cells in the skin, with the label predominately localized in the apical plasma membrane of the secretory cells of the small granular glands. These glands are unique both in being close to the epidermal layer of the skin and in containing mitochondria-rich cells with vacuolar H+-ATPase dispersed among its secretory cells. Results from immunohistochemical experiments on the mucous or seromucous glands of several other anurans verified this result (Kubota et al. 2006).
Aqp-x5 of Xenopus laevis (African clawed frog)
Aqp-1A of 258 aas and 6 TMSs, DRIP1. Transports water but not glycerol or urea. Functions in water homeostasis in many tissues and stages of development (Lu et al. 2018). An aquaporin in the beet armyworm, Spodoptera exigua, (79% identical to the one in Chilo suppressalis, mediates cell shape change required for cellular immunity (Ahmed and Kim 2019).
Aqp-1A of Chilo suppressalis (Asiatic rice borer moth)
Aqp-2A of 269 aas and 6 TMSs, DRIP2. Transports water but not glycerol or urea. Functions in water homeostasis in many tissues and stages of development (Lu et al. 2018).
Aqp-2A of Chilo suppressalis (Asiatic rice borer moth)
Big brain-like protein of 309 aas and 6 probable TMSs, BibL1 (Lind et al. 2017).
BibL1 of the euryhaline bay barnacle, Balanus improvisus (Darwin, 1854) (Amphibalanus improvisus)
Aquaporin 1, AQP1, of 261 aas and 6 TMSs, which selectively transports water (Lind et al. 2017).
AQP1 of the euryhaline bay barnacle Balanus improvisus (Darwin, 1854) (Amphibalanus improvisus)
Aquaporin (Aqp) of 458 aas, 6 N-terminal TMSs and a 200 aa hydrophilic C-terminal domain.
Aqp of Blomia tropicalis (mite)
Aquaporin-4 (AQP4) is the major water channel in the central nervous system and plays an important role in the brain's water balance, including edema formation and clearance. There are 6 splice variants; the shorter ones assemble into functional, tetrameric square arrays; the longer is palmitoylated on N-terminal cysteyl residues) (Suzuki et al., 2008). The longest, Aqp4e, has a novel N-terminal domain and forms a water channel in the plasma membrane although various shorter variants don't (Moe et al., 2008). AQP4, like AQP0 (1.A.8.8.2), forms water channels but also forms adhesive junctions (Engel et al., 2008) (causes cytotoxic brain swelling in mice (Yang et al., 2008)) Mice lacking Aqp4 have impaired olfactions (Lu et al., 2008). Aqp4 is down regulated in skeletal muscle in muscular dystrophy (Au et al. 2008). The crystal structure is known to 2.8 Å resolution (Tani et al., 2009). The structure reveals 8 water molecules in each of the four channels, supporting a hydrogen-bond isolation mechanism and explains its fast and selective water conduction and proton exclusion (Tani et al., 2009; Cui and Bastien, 2011). It is an important antigen in Neuromyelitis optica (NMO) patients (Kalluri et al., 2011). A connection has been made between AQP4-mediated fluid accumulation and post traumatic syringomyelia (Hemley et al. 2013). AQP4 has increased water permeability at low pH, and His95 is the pH-dependent gate (Kaptan et al. 2015). Also transports NH3 but not NH4+ (Assentoft et al. 2016). Cerebellar damage following status epilepticus involves down regulation of AQP4 expression (Tang et al. 2017). SUR1-TRPM4 and AQP4 form a complex to increase bulk water influx during astrocyte swelling (Stokum et al. 2017). A mutation, S111T, causes intellectual disability, hearing loss, and progressive gait dysfunction (Berland et al. 2018). As in humans, the chicken ortholog, Aqp4, is found in brain > kidney > stomach (Ramírez-Lorca et al. 2006). A Molecular Dynamics Investigation on Human AQP4 has been published (Marracino et al. 2018).
AQP4 of Homo sapiens (P55087)
Aqp1 of Polypedilum vanderplanki
Aqp2 water channel of the sleeping chironomid (functions in water homeostasis during anhydrobiosis (Kikawada et al., 2008).
Aqp2 of Polypedilum vanderplanki (A5A7P0)
Vasopressin-sensitive aquaporin-2 (Aqp2) in the apical membrane of the renal collecting duct (Fenton et al., 2008). Controls cell volume and thereby influences cell proliferation (Di Giusto et al. 2012). It plays a key role in concentrating urine. Water reabsorption is regulated by AQP2 trafficking between intracellular storage vesicles and the apical membrane. This process is tightly controlled by the pituitary hormone arginine vasopressin, and defective trafficking results in nephrogenic diabetes insipidus (NDI). The crystal structure of Aqp2 has been solved to 2.75Å (Frick et al. 2014). In terrestrial vertebrates, AQP2 function is generally regulated by arginine-vasopressin to accomplish key functions in osmoregulation such as the maintenance of body water homeostasis by a cyclic AMP-independent mechanism (Olesen and Fenton 2017; Martos-Sitcha et al. 2015). AQP2 is expressed in the anterior vaginal wall and fibroblasts, and regulates the expression level of collagen I/III i, suggesting that AQP2 is associated with the pathogenesis of stress urinary incontinence through collagen metabolism during ECM remodeling (Zhang et al. 2017). As in humans, the chicken ortholog, Aqp2, is found only in the kidney (Ramírez-Lorca et al. 2006). AQP2 is critical in regulating urine concentrating ability. The expression and function of AQP2 are regulated by a series of transcriptional factors and post-transcriptional phosphorylation, ubiquitination, and glycosylation (He and Yang 2019). Mutation or functional deficiency of AQP2 leads to severe nephrogenic diabetes insipidus, and inhibition of various aquaporins leads to many water-related diseases such as, edema, cardiac arrest, and stroke. Maroli et al. 2019 reported on the molecular mechanisms of mycotoxin (citrinin, ochratoxin-A, and T-2 mycotoxin) inhibition of AQP2 and arginine vasopressin receptor 2 (AVPR2).
Aqp2 of Homo sapiens (P41181)
Aquaporin 5 (x-ray structure at 2.0 Å resolution (PDB# 3D9S) is available) (Horsefield et al., 2008). Aqp5 is a marker for proliferation and migration of human breast cancer cells (Jung et al., 2011). Plays a role in chronic obstructive pulmonary diseases (COPD) (Zhao et al. 2014). Its expression is regulated by androgens (Pust et al. 2015). As in humans, the chicken ortholog, Aqp5, is found in the intestine, the jejunum, ileum and colon (Ramírez-Lorca et al. 2006). Proteomic analyses of the ocular lens revealed palmitoylation (Wang and Schey 2018).
Aquaporin 5 of Homo sapiens (P55064)
Aquaporin 3. 95% identical to the human orthologue. Poorly permeable to water, but more permeable to glycerol and arsenic trioxide (Palmgren et al. 2017). It is expressed in the plasma membrane of basal epidermal cells in the skin; loss of function prevents skin tumorigenesis and epidermal cell proliferation (Hara-Chikuma and Verkman, 2008). The human orthologue also transports both water and glycerol and is the predominant AQP in skin (Jungersted et al. 2013). It's function is necessary for normal proliferation of colon cancer cells due to glycerol uptake (Li et al. 2016). Aqp3 is implicated in cancer progression to the metastatic state as its function promotes cell migration and cell shape plasticity. Its synthesis is regulated by the AhR (aryl hydrocarbon (pollutant) receptor or dioxin receptor), a transcription factor triggered by environmental pollutants (Bui et al. 2016). Trefoil factor (TFF) peptides increase cell water permeability and induce prodcution of Aqp3 (Marchbank and Playford 2018). Although AQP3 and other similar transmembrane proteins do not themselves transport drugs, changes in their expression levels can cause changes in cell membrane fluidity, thus affecting drug absorption rates (Ikarashi et al. 2019).
Aquaporin 3 of Rattus norvegicus (P47862)
Aqp9 or Aqp-h9 of 294 aas. Takes up glycerol as well as water, and thereby contributes to freeze tolerance (Hirota et al. 2015). An almost identical orthologue, HC-9 in Dryophytes chrysoscelis (gray treefrog), similarly facilitates glycerol permeability. Both the transcriptional and translational levels of HC-9 change in response to thermal challenges, with a unique increase in liver during freezing and thawing (Stogsdill et al. 2017).
Aqp9 of Hyla japonica
Aqp1 of 304 aas and 6 TMSs; the most abundant transmembrane protein in the tegument of Schistosoma mansoni. This protein is expressed in all developmental stages and seems to be essential in parasite survival since it plays a crucial role in osmoregulation, nutrient transport and drug uptake (Figueiredo et al. 2014).
Aqp1 of Schistosoma mansoni (Blood fluke)
Basolateral Aqp3 of 292 aas and 6 TMSs in the frog urinary bladder (Shibata et al. 2015).
Aqp3 of Xenopus laevis
Glycerolaquaporin 9, Aqp9 of 295 aas and 6 TMSs. Transports water, glycerol and arsenic trioxide, As2O3 (Palmgren et al. 2017). Primary APL cells express AQP9 significantly (2-3 logs) higher than other acute myeloid leukemia cells (AMLs), explaining their exquisite As2O3 sensitivity (Leung et al. 2007). AQP-7 and AQP-9-mediated glycerol transport in tanycyte cells may be under hormonal control to use glycerol as an energy source during the mouse estrus cycle (Yaba et al. 2017). It transports multiple neutral and ionic arsenic species including arsenic trioxide, monomethylarsenous acid (MAs(III)) and dimethylarsenic acid (DMA(V)). It also transports clinically relevant selenium species including monomethylselenic acid (MSeA), especially at acidic pH. FCCP, valinomycin and nigericin do not significantly inhibit MSeA uptake, but AQP9 also transport ionic selenite and lactate, with low efficiency compared with MSeA uptake. Selenite and lactate uptake is pH dependent and inhibited by FCCP and nigericin but not valinomycin. The selenite and lactate uptake via AQP9 can be inhibited by different lactate analogs. AQP9 transport of MSeA, selenite and lactate is inhibited by an AQP9 inhibitor, phloretin, and the AQP9 substrate, arsenite (As(III)) (Geng et al. 2017).
Aqp9 of Homo sapiens
Aquaporin 9, Aqp9, small solute channel 1 of 296 aas and 6 TMSs (Wang and Ye 2016).
Aqp9 of Echinococcus granulosus (Hydatid tapeworm)
Water/glycerol aquaglyceroporin 2, AQP2, of 294 aas and 6 TMSs (Lind et al. 2017).
AQP2 of the euryhaline bay barnacle, Balanus improvisus (Darwin, 1854) (Amphibalanus improvisus)
Glycerol-aquaporin of 332 aas and 6 TMSs (Stavang et al. 2015).
Aqp of the salmon leach, Lepeophtheirus salmonis
Aquaporin of 341 aas and 7 TMSs (Ben Amira et al. 2018).
Aqp of Hypocrea atroviridis (Trichoderma atroviride)
AQP2 (AQP9) of 312 aas and 6 TMSs; transports water, glycerol and urea as well as the drugs, melarsoprol and pentamidine (Schmidt et al. 2018).
AQP2 of Trypanosoma brucei
Aquaporin-9 (Aqp9) (permeable to glycerol, urea, polyols, carbamides, purines, pyrmidines, nucleosides, monocarboxylates, pentavalent methylated arsenicals and the arsenic chemotherapeutic drug, trisenox (McDermott et al., 2009). It is poorly permeable to water and not permeable to β-hydroxybutyrate (Carbrey et al., 2003). (Regulated by CFTR and NHERF1 in response to cyclic AMP (Pietrement et al., 2008)) The 7 Å projection structure and a homology model revealed that pore-lining residues and the hydrophobic edge of the tripathic pore of GlpF (1.A.8.1.1) provide the basis for broad substrate specificity (Viadiu et al., 2007). Important for urea transport in mouse hepatocytes (Jelen et al. 2012). Activation of the PPARα transcription factor results in reduction in the abundance of AQP9 in periportal hepatocytes, but its activation in the fed state directs glycerol into glycerolipid synthesis rather than into de novo synthesis of glucose (Lebeck et al. 2015). Azacytidine up-regulates AQP9 and enhances arsenic trioxide (As2O3)-mediated cytotoxicity in acute myeloid leukemia (AML) (Chau et al. 2015). Human Aqp9 transports hydrogen peroxide (HOOH) (Watanabe et al. 2016).
Aqp9 of Rattus norvegicus (P56627)
Aquaporin of 274 aas and 6 TMSs. See Zhou et al. 2018 for its identification.
Aqp of Blomia tropicalis (mite)
Major aquaglyceroporin, LmAQP1: transports water, glycerol, methylglyoxal, trivalent metalloids such as arsenite and antimonite, dihydroxyacetone and sugar alcohols. Also takes up the activated form or the drug, pentostam. It localizes to the flagellum of the Leishmania promastigotes and is used to regulate volume in response to hypoosmotic stress, functions in osmotaxis) (Figarella et al., 2005; Gourbal et al, 2004).
Aqp1 of Leishmania major (Q6Q1Q6)
Aquaporin 1 (permeable to water, glycerol, dihydroxyacetone and urea) (Uzcategui et al., 2004)
Aqp1 of Trypanosoma brucei (Q6ZXT4)
Aquaporin 10. Present in keratinocytes and the stratum corneum (Jungersted et al. 2013).
Aqp10 of Homo sapiens
Glycerol/water/urea/arsenic trioxide-transporting channel protein, aqaporin 7 or Aqp7, but water is a poor substrate (Palmgren et al. 2017). Present in adipose tissue where it allows glycerol efflux. Defects result in increased accumulation of triglycerides, obesity and adult onset (type 2) diabetes (Lebeck 2014). It may be a drug target for anti-type 2 diabetes (Méndez-Giménez et al. 2018). AQP-7- and AQP-9-mediated glycerol transport in tanycyte cells may be under hormonal control to use glycerol as an energy source during the mouse estrus cycle (Yaba et al. 2017). It may also influence whole body energy metabolism (Iena and Lebeck 2018).
Aqp7 of Homo sapiens
Glycerol uptake facilitator of 393 aas
Glycerol transporter of Cordyceps militaris (Caterpillar fungus)
Aquaporin/glycerol facilitator of 294 aas and 6 TMSs. May play a role in freeze tolerance (Hirota et al. 2015).
Aqp-9 of Xenopus tropicalis