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, yeast, fungi, plants and animals. Functionally-characterized members of the family appear to catalyze inorganic phosphate (P_i) 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 P_i. The mammalian proteins have been reported to function as viral receptors, but they undoubtedly function as transport proteins as well. The molecular sizes of Pit family members are reported to vary from 354 to 681 residues (10-12 TMSs) with the mammalian 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). Several probable fragments have lower apparent molecular sizes. 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 generalized transport reactions possibly catalyzed by members of the PiT family are: (1) HPO₄^2- (out) + [nH⁺ or Na⁺] (out) Ć HPO₄^2- (in) + [nH⁺ or Na⁺] (in) (2) Me²⁺ · HPO₄^2- (out) + nH⁺ (out) Ć Me²⁺ · HPO₄^2- (in) + nH⁺ (in) (3) SO₄^2- (out) + nH⁺ (out) Ć SO₄^2- (in) + nH⁺ (in).
References:
Daram, P., S. Brunner, C. Rausch, C. Steiner, N. Amrhein and M. Bucher (1999). Pht2;1 encodes a low-affinity phosphate transporter from Arabidopsis. Plant Cell 11: 2153-2166.
Harris, R.M., D.C. Webb, S.M. Howitt and G.B. Cox (2001). Characterization of PitA and PitB from Escherichia coli. J. Bacteriol. 183: 5008-5014.
Mann, B.J., B.J. Bowman, J. Grotelueschen and R.L. Metzenberg (1989). Nucleotide sequence of pho-4, encoding a phosphate-repressible phosphate permease of Neurospora crassa. Gene 83: 281-289.
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: 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.
Persson, B.L., A. Berhe, U. Fristedt, P. Martinez, J. Pattison, J. Petersson and R. Weinander (1998). Phosphate permeases of Saccharomyces cerevisiae. Biochim. Biophys. Acta 1365: 23-30.
Persson, B.L., J. Petersson, U. Fristedt, R. Weinander, A. Berhe and J. Pattison (1999). Phosphate permeases of Saccharomyces cerevisiae: structure, function and regulation. Biochim. Biophys. Acta 1422: 255-272.
Saier, M.H., Jr., B.H. Eng, S. Fard, J. Garg, D.A. Haggerty, W.J. Hutchinson, D.L. Jack, E.C. Lai, H.J. Liu, D.P. Nusinew, A.M. Omar, S.S. Pao, I.T. Paulsen, J.A. Quan, M. Sliwinski, T.-T. Tseng, S. Wachi and G.B. Young (1999). Phylogenetic characterization of novel transport protein families revealed by genome analyses. Biochim. Biophys. Acta 1422: 1-56.
Versaw, W.K. and R.L. Metzenberg (1995). Repressible cation-phosphate symporters in Neurospora crassa. Proc. Natl. Acad. Sci. USA 92: 3884-3887.
Examples:
TC#	Name	Organismal Type	Example
2.A.20.1.1	Low affinity P_i transporter #1, PitA (K_m=2 mM) (Me · P_i:H⁺ symporter)	Bacteria	PitA of E. coli

2.A.20.1.2	Low affinity P_i transporter #2, PitB (K_m=30 mM) (Me · P_i:H⁺ symporter)	Bacteria	PitB of E. coli

2.A.20.2.1	P_i-repressible P_i:Na⁺ symporter	Eukaryotes	Pho4 of Neurospora crassa

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

2.A.20.2.3	Gibbon ape leukemia virus receptor 2/P_i:Na⁺ symporter	Eukaryotes	GLVR2 of Homo sapiens

2.A.20.2.4	Low affinity P_i transporter	Plants	Pht2;1 of Arabidopsis thaliana

2.A.20.3.1	Putative Na⁺-dependent P_i transporter	Archaea	Npt of Methanococcus jannaschii

2.A.20.4.1	Sulfate: H⁺ symporter, SulP	Bacteria	SulP (YlnA) of Bacillus subtilis