Family 2.A.20 - The Inorganic Phosphate Transporter Family       

Family ID: 52625

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 (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.

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).