2.A.20 The Inorganic Phosphate Transporter (PiT) Family

The proteins of the PiT family are derived from Gram-negative and Gram-positive bacteria, archaea, and eukaryotes. Functionally-characterized members of the family appear to catalyze inorganic phosphate (Pi) or inorganic sulfate uptake either by H+ or Na+ symport. Both PitA and PitB of E. coli probably catalyze metal ion·phosphate:H+ symport, where Mg2+, Ca2+ or Zn2+ (and probably other divalent cations) can complex Pi. The mammalian proteins have been reported to function as viral receptors, but they undoubtedly function as transport proteins as well.  For numerous gammaretroviruses, such as the gibbon ape leukemia virus, woolly monkey virus, feline leukemia virus subgroup B, feline leukemia virus subgroup T, and 10A1 murine leukemia virus, this receptor is the human type III sodium-dependent inorganic phosphate transporter, SLC20A1, also known as PiT1 (Farrell et al. 2009).

The molecular sizes of Pit family members are reported to vary from 354 to 681 residues (10-12 TMSs) with the mammalian and Plasmodium proteins exhibiting the largest sizes. The sulfate permease of B. subtilis, CysP, is of 354 residues with 11 putative TMSs (Mansilla and de Mendoza, 2000).

Phylogenetic grouping of the phosphate transport proteins generally correlates with organismal phylogeny. Thus the fungal, plant, animal and archaeal proteins each cluster separately (Saier et al., 1999). However, the tree exhibits two clusters of bacterial phosphate transport proteins. One bacterial cluster is distant from the eukaryotic proteins while the other cluster is close to the plant proteins. Both clusters include proteins from Gram-negative and Gram-positive bacteria. The sulfate permease, CysP, is distantly related to the phosphate permeases.

Members of the PiT family arose by a tandem internal gene duplication event. Surprisingly, TopPred predicts a 12 TMS topology for the yeast Pho89 protein, but the homologous regions are not predicted to show similar topological features. Thus, for example TMS 1 is homologous to TMS 9, and TMS 4 is predicted to correspond to the loop between TMSs 11 and 12 (Persson et al., 1998, 1999).

The malaria parasite, Plasmodium falciparum, grows within its host erythrocyte and induces an increase in the permeability of the erythrocyte membrane to a range of solutes including Na+ and K+. This results in a progressive increase in the concentration of Na+ in the erythrocyte cytosol. The parasite cytosol has a relatively low Na+ concentration, generating a large inward Na+ gradient across the parasite plasma membrane. Saliba et al. (2006) showed that the parasite exploits the Na+ electrochemical gradient to energize the uptake of inorganic phosphate (Pi) with a stoichiometry of 2Na+:1Pi and with an apparent preference for the monovalent over the divalent form of Pi (see TC #2.A.20.2.5).

The generalized transport reactions possibly catalyzed by members of the PiT family are:

(1) HPO42- (out) + [nH+ or Na+] (out) → HPO42- (in) + [nH+ or Na+] (in)

(2) Me2+ · HPO42- (out) + nH+ (out) → Me2+ · HPO42- (in) + nH+ (in)

(3) SO42- (out) + nH+ (out) → SO42- (in) + nH+ (in)



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Borghese, R., L. Canducci, F. Musiani, M. Cappelletti, S. Ciurli, R.J. Turner, and D. Zannoni. (2016). On the role of a specific insert in acetate permeases (ActP) for tellurite uptake in bacteria: Functional and structural studies. J Inorg Biochem 163: 103-109.

Bøttger, P. and L. Pedersen. (2011). Mapping of the minimal inorganic phosphate transporting unit of human PiT2 suggests a structure universal to PiT-related proteins from all kingdoms of life. BMC Biochem 12: 21.

Bøttger, P., S.E. Hede, M. Grunnet, B. Høyer, D.A. Klaerke, and L. Pedersen. (2006). Characterization of transport mechanisms and determinants critical for Na+-dependent Pi symport of the PiT family paralogs human PiT1 and PiT2. Am. J. Physiol. Cell Physiol. 291: C1377-1387.

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Elías, A.O., M.J. Abarca, R.A. Montes, T.G. Chasteen, J.M. Pérez-Donoso, and C.C. Vásquez. (2012). Tellurite enters Escherichia coli mainly through the PitA phosphate transporter. Microbiologyopen 1: 259-267.

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Feng, H., X. Li, D. Sun, Y. Chen, G. Xu, Y. Cao, and L.Q. Ma. (2021). Expressing Phosphate Transporter PvPht2;1 Enhances P Transport to the Chloroplasts and Increases Arsenic Tolerance in. Environ Sci Technol. [Epub: Ahead of Print]

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Jackson, R.J., M.R. Binet, L.J. Lee, R. Ma, A.I. Graham, C.W. McLeod, and R.K. Poole. (2008). Expression of the PitA phosphate/metal transporter of Escherichia coli is responsive to zinc and inorganic phosphate levels. FEMS Microbiol. Lett. 289: 219-224.

Kim, S., T.D. Lieberman, and R. Kishony. (2014). Alternating antibiotic treatments constrain evolutionary paths to multidrug resistance. Proc. Natl. Acad. Sci. USA 111: 14494-14499.

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Li, S.H., B.B. Xia, C. Zhang, J. Cao, and L.H. Bai. (2012). Cloning and characterization of a phosphate transporter gene in Dunaliella salina. J Basic Microbiol 52: 429-436.

Li, Y., X. Lin, M. Zhu, J. Li, Z. Yuan, and H. Xu. (2020). Whole‑exome sequencing identifies a novel mutation of SLC20A2 (c.C1849T) as a possible cause of hereditary multiple exostoses in a Chinese family. Mol Med Rep 22: 2469-2477.

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Mansilla, M.C. and D. de Mendoza. (2000). The Bacillus subtilis cysP gene encodes a novel sulphate permease related to the inorganic phosphate transporter (Pit) family. Microbiology 146(Pt4): 815-821.

Martinez, P. and B.L. Persson (1998). Identification, cloning and characterization of a derepressible Na+-coupled phosphate transporter in Saccharomyces cerevisiae. Mol. Gen. Genet. 258: 628-638.

McCarthy, S., C. Ai, G. Wheaton, R. Tevatia, V. Eckrich, R. Kelly, and P. Blum. (2014). Role of an archaeal PitA transporter in the copper and arsenic resistance of Metallosphaera sedula, an extreme thermoacidophile. J. Bacteriol. 196: 3562-3570.

Młodzińska, E. and M. Zboińska. (2016). Phosphate Uptake and Allocation - A Closer Look at Arabidopsis thaliana L. and Oryza sativa L. Front Plant Sci 7: 1198.

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TC#NameOrganismal TypeExample

Low affinity Pi or phosphate-zinc complex uptake transporter #1, PitA (Km=2 μM) (Me·Pi:H+ symporter) (Beard et al. 2000; Jackson et al. 2008). Also transports tellurite (TeO32-) slowly (Elías et al. 2012; Borghese et al. 2016).


PitA of E. coli (P0AFJ7)


Low affinity Pi transporter #2, PitB (Km=30 μM) PitB, like PitA is also a Me·Pi:H+ symporter (Borghese et al. 2016)


PitB of E. coli


Probable low-affinity inorganic phosphate transporter, Pit


Pit of Mycobacterium bovis

2.A.20.1.4Probable low-affinity inorganic phosphate transporterBacteria

Pit of Rhizobium meliloti


Putative low affinity Pi transporter PitH1, Sco4138 (Santos-Beneit et al. 2008).


PitH1 of Streptomyces coelicolor.


Putative low affinity Pi transporter PitH2, Sco1845 (Santos-Beneit et al., 2008).


PitH2 of Streptomyces coelicolor.


Low affinity inorganic phosphate uptake porter of 335 aas, PitA (Kim et al. 2014; Mechler L, ... Bertram R, personal communication).


PitA of Staphylococcus aureus


TC#NameOrganismal TypeExample
2.A.20.2.1Pi-repressible Pi:Na+ symporter Eukaryotes Pho4 of Neurospora crassa

Phosphate:sodium symporter of 576 aas, Pho89 (Ahn et al. 2009).


Pho89 of Komagataella pastoris (Yeast) (Pichia pastoris)


Phosphate transporter of 601 aas and 12 TMSs. Pho4.


Pho4 of Trypanosoma cruzi


Putative phosphate:sodium symporter of 752 aas and 12 - 14 TMSs.

PNaS family member of Lutibacter sp.


Putative sodium/phosphate symporter of 534 aas and 12 TMSs.

Na+:Phosphate symporter of Emiliania huxleyi virus 84


Phosphate uptake transporter of 869 aas and 10 TMSs, 6 N-terminal and 4 C-terminal. Toxoplasma expresses one Pi transporter harboring two  PHO4 binding domains (N- and C-terminal) that typify the PiT Family. This transporter named TgPiT, localizes to the plasma membrane, the inward buds of the endosomal organelles termed VAC, and many cytoplasmic vesicles (Asady et al. 2020). It catalyzes Pi:Na+ = 1:2 and iselectrogenic. Upon Pi limitation in the medium, TgPiT is more abundant at the plasma membrane. ΔTgPiT parasites accumulate 4-times more acidocalcisomes, storage organelles for phosphate, and exhibit many traits that differ from the wild type organims including poor virulence (Asady et al. 2020).

TgPiT of Toxoplasma gondii

2.A.20.2.2High affinity Pi:Na+ symporter Eukaryotes Pho89 (YBR296c) of Saccharomyces cerevisiae

Gibbon ape leukemia virus receptor 2/sodium-dependent phosphate uptake transporter, Pi:Na+ symporter, PiT2, PiT-2, GLVR2, SLC20A2. Mapping of the minimal transporting unit suggested a structure universal to all PiT-related proteins (Battger and Pedersen, 2011).The protein can transport Pi in the absence of Na+, and mutations allow Na+ transport in the absence of Pi. Transmembrane amino acids E(55) and E(575) appear to be responsible for linking Pi import to Na+symport (Bøttger et al. 2006). SLC20A2 variants cause the loss of Pi transport activity in mammalian cells (Sekine et al. 2019). Mutation of SLC20A2 seems to cause hereditary multiple exostoses (Li et al. 2020). It has been associated with Primary Familial Brain Calcification (PFBC) (Wang et al. 2012).


SLC20A2 of Homo sapiens


Low affinity housekeeping Pi transporter, PHT2, Pht2;1, Pht2,1, PT2-1 of 587 aas (Młodzińska and Zboińska 2016). The cucumber (Cucumis sativus) ortholog is involved in cucumber growth and metabolism; PT2-1 transcript levels in roots were high when grown in low (limiting) Pi-containing media, but low when grown in high Pi media (Naureen et al. 2018). Expressing Pht2;1 of Pteris vittata enhances phosphate transport in chloroplasts and increases arsenic tolerance in Arabidopsis thaliana (Feng et al. 2021).


Pht2;1 of Arabidopsis thaliana


The Pi:Na+ symporter, PfPit (669 aas; Saliba et al., 2006)

Malaria parasite

PfPit of Plasmodium falciparum (Q7YUD6)


The Pi:(Na+)2 symporter, GLVR1, PiT1 or PiT-1 (Ravera et al., 2007). Mutating the conserved loop region between TMSs 2 and 3 alterred the binding properties of the transporter for Na+/Li+ and phosphate/arenate (Ravera et al. 2013).


Pit1 of Xenopus tropicalis (Q5BL44)


Na+-dependent phosphate transporter 1 (PiT-1). Gibbon ape leukemia virus receptor 1 (GLVR-1), Slc20A1 (KM=24μM).  The protein has an experimentally tested 12 TMS topology (Farrell et al. 2009).  A knock-out mutation of the mouse orthologue results in abnormal endocytosis in the yolk sac visceral endoderm and embyonic death at the 12.5 day stage (Wallingford and Giachelli 2014). PiT1 levels are elevanted in somatotroph adenomas and are positively associated with tumor size, invasive behavior and tumor recurrence in somatotroph adenomas. It may be associated with the activation of the Wnt/betacatenin signaling pathway (Li et al. 2019).


SLC20A1 of Homo sapiens


Na+:Pi transporter, SPT1 (Li et al., 2011)


SPT1 of Dunaliella viridis (A7U4W2)

2.A.20.2.9Putative phosphate permease HP_1491BacteriaHP_1491 of Helicobacter pylori

TC#NameOrganismal TypeExample
2.A.20.3.1Putative Na+-dependent Pi transporter Archaea Npt of Methanococcus jannaschii

Low affinity high velocity phosphate transporter, PitA if 320 aas (McCarthy et al. 2014).


PitA of Metallosphaera cuprina


Low affinity phosphate transporter, PitA of 328 aas


PitA of Sulfolobus solfataricus


Low affinity phosphate carrier, PitA of 309 aas.


PitA of Archaeoglobus profundus


TC#NameOrganismal TypeExample

Sulfate:H+ symporter, CysP (Aguilar-Barajas et al., 2011; Mansilla and de Mendoza, 2000)


CysP (YlnA) of Bacillus subtilis