1.A.31 The Annexin (Annexin) Family
The annexins are a structurally conserved family of proteins characterized by reversible Ca2+-dependent intracellular membrane/phospholipid binding. Membrane association is critical for their proposed functions which include vesicle trafficking, membrane repair, membrane fusion and ion channel formation (McNeil et al., 2006). High-resolution crystal structures of the soluble forms of several annexins are available. These include hydra annexin XII and human annexin V. A low resolution structure is available for the membrane-bound Annexin 5 trimer (Oling et al., 2000). These proteins bind to surfaces of phosphatidylserine-containing phospholipid bilayers either in the presence of Ca2+ or under conditions of low pH (pH 5-6). Then they undergo major conformational changes involving three states: (1) soluble state (monomer) → (2) peripheral membrane-associated state (trimer) → (3) integral transmembrane channel state (hexamer). This last state requires major conformational changes with the formation of a putative polytopic, amphipathic channel. Ca2+ induces dimer, trimer and hexamer formation as well as phospholipid association. A helix-loop-helix structure in the soluble form is believed to be converted into one of the continuous transmembrane α-helices. Reviakine 2018 have examined avalable data on annexin-lipid interactions regarding two lines of inquiry: the well-characterized peripheral assembly of annexins at membranes, and their putative transmembrane insertion.
All annexins display a conserved core domain consisting of four homologous repeats, each of about 70 residues. Two of these repeat units may comprise a single Ca2+/phospholipid binding site. Some annexins (e.g., Annexin VI) are twice as large as others (e.g., Annexin X) because of an intragenic duplication. The ion channel properties of the integral membrane forms of annexins have been amply documented. However, it is not clear that channel activity explains all of their biological properties. For example, Annexin VI has been reported to modulate maxichloride channel currents as well as K+ and Ca2+ currents in different cell types, possibly by regulating the activities of other channels (Riquelme et al., 2004). Annexins are also called Lipocortins, Synexins, Endonexins and Calpactins. Most are 310-350 residues long.
Annexins comprise a large family derived from animals with many paralogues in any one. They are found throughout the eukaryotic kingdoms. They have been subdivided into three groups: (1) tetradcore with short amino termini; (2) tetradcore with long amino termini; (3) octadcore with short amino termini. The core is a 34 kDa C-terminal domain of 4 repeats except for annexin VI which has 8 repeats. Each repeat is 70aas with an 'endonexin fold' with its identifying GXGTDE sequence. Each repeat forms a compact α-helical domain consisting of 5 α-helicies wound in a right-handed superhelix. The four domains are arranged in a flat cylindrical array with the hydrophilic channel in the center of the molecule. Ca2+ is preferred over other divalent cations, but both cations and anions can be transported.
Medicago truncatula annexin 1 (AnnMt1) participates in nodulation (Nod factor signaling) and mycorrhization in legume plants. AnnMt1 mediates non-selective membrane permeabilization to cations with conductances ranging from 16 pS to 329 pS (Kodavali et al. 2013). In agreement with other structurally determined annexins, homology modeling of AnnMt1 suggests that most of the functional determinants are on the convex surface of the protein.
Annexins are soluble proteins that bind acidic phospholipids such as phosphatidylserine in a calcium-dependent manner. Observations of specific ion conductances in annexin-bound membranes is controversial (Reviakine 2018). The controversy considering the well-characterized peripheral assembly of the annexins at membranes vs. their transmembrane insertion and mediation of lipid rearrangements is the subject of a review (Reviakine 2018).
At least one transport reaction catalyzed by annexin channels is:
Ions (in) ions (out)
Annexin A1 (McNeil et al., 2006)
Animals, plants, fungi, eukaryotic protists
Annexin A1 of Homo sapiens (346 aas; P04083)
Annexin 2 or Annexin A2 (ANXA2) of 339 aas. Forms a tetrameric complex with the S100A10 protein and binds the C-terminus of the AHNAK protein via the N-terminus of annexin 2 (De Seranno et al., 2006). Direct translocation of Annexin 2 to the cell surface occurs by pore-formation. External annexin A2 acts as a plasminogen receptor, able to stimulate fibrinolysis and cell migration (Pompa et al. 2017).
Annexin 2 of Homo sapiens
Non-selective cation channel-forming annexin 1 of 313 aas, Ann1 (Kodavali et al. 2013).
Ann1 of Medicago truncatula
Annexxin of 369 aas. Schistosomiasis, a major parasitic disease of humans, is second only to malaria in its global impact. The disease is caused by digenean trematodes that infest the vasculature of their human hosts. These flukes are limited externally by a body wall composed of a syncytial epithelium, the apical surface membrane, a parasitism-adapted dual membrane complex. Annexins are important for the stability of this apical membrane system. Leow et al. 2013 presented the first structural and immunobiochemical characterization of an annexin from Schistosoma mansoni. The crystal structures of annexin B22 (4MDV and 4MDU) in the apo and Ca2+ bound forms confirmed the presence of the previously predicted α-helical segment in the II/III linker and revealed a covalently linked head-to-head dimer. The dimeric arrangement revealed a non-canonical membrane binding site and a probable binding groove opposite the binding site. Annexin B22 expression correlated with life stages of the parasite that possess the syncytial tegument layer, and ultrastructural localization by immuno-electron microscopy confirmed the occurrence of annexins in the tegument of S. mansoni.
Annexin B22 of Schistosoma mansoni
Annexin A5 of 320 aas. Annexin A5 (ANXA5), a Ca2+ and phospholipid binding protein, interacts with the N-terminal leucine-rich repeats of polycystin-1 (TC# 1.A.5.1.2). This interaction is direct and specific, and involves a conserved sequence of the ANXA5 N-terminal domain (Markoff et al. 2007).
Annexin A5 of Homo sapiens
Annexin D1 (Anx23; Ann1; AnnAT1; AtoxY; Oxy5) of 317 aas. It has a peroxidase activity and may act to
counteract oxidative stress (Gorecka et al. 2005). May also mediate regulated, targeted
secretion of Golgi-derived vesicles during seedling development (Clark et al. 2005). Can transport Ca2+ (Demidchik et al. 2018).
Annexin 1D of Arabidopsis thaliana (Mouse-ear cress)
Annexin XII, Annexin 12, Annexin-12, AnnexinB12 of 316 aas. It is a calcium- and phospholipid-binding protein, phosphorylated by PKC. The x-ray structure of the heximer has been solved (Luecke et al. 1995). A reversible transition occurs between the surface trimer and membrane-inserted monomer (Ladokhin and Haigler 2005).
Annexin-12 of Hydra vulgaris