2.A.58 The Phosphate:Na+ Symporter (PNaS) Family
The PNaS family includes several functionally characterized, sodium-dependent, inorganic phosphate (Pi) transporter (NPT2 or NptA) proteins from mammals. Other animals, including fish and the worm, C. elegans, possess functionally uncharacterized homologues. One closely related bacterial protein, NptA of Vibrio cholerae, resembles the animal proteins (34% identity; 51% similarity to many mammalian Npt2 symporters), but it has close homologues in many bacterial phyla.There are two subfamilies; one includes eukaryotic and prokaryotic proteins, and the other, only bacterial homologues. The bacterial proteins are found in many bacteria including E. coli (543 aas; TC# 2.A.58.2.1; 9 putative TMSs) and Bacillus subtilis (310 aas; TC# 2.A.58.2.5; 8 putative TMSs). The bacterial homologue, NptA of V. cholerae has 10 putative TMSs. The well-characterized mammalian proteins are found in renal (IIa isoform) and intestinal (IIb isoform) brush border membranes and are about 640 amino acyl residues long with 8-12 putative TMSs. The N- and C-termini are in the cytoplasm, and a large hydrophilic loop is localized between TMSs 3 and 4. While IIa isoforms are pH-dependent, IIb isoforms are pH-independent (de la Horra et al., 2000). Members of this family have been reported to have the CNT2 fold (Ferrada and Superti-Furga 2022).
Mammalian porters of the PNaS family may catalyze cotransport of 3 Na+ with 1 inorganic phosphate. In response to parathyroid hormone and dietary inorganic phosphate, the renal cotransporter is rapidly inserted into and retrieved from the renal brush border membrane in a fashion similar to that by which the glucose transporter (Glut4) (TC# 2.A.1.1) is regulated by insulin, and aquaporins 1 and 2 (TC# 1.A.8.1) are regulated by vasopressin (Levi et al., 1999). The renal type IIa PNaS member is a functional monomer (Kohler et al., 2001), but it interacts with PDZ proteins which probably mediate apical sorting, parathyroid hormone-controlled endocytosis and/or lysosomal sorting of internalized transporter (Gisler et al., 2001).
A single organism may have multiple paralogues of the PNaS family. All of these proteins exhibit an internal repeat that probably arose by a tandem intragenic duplication event. Their properties and mechanisms of action have been reviewed (Forster 2019).
The transport reaction catalyzed by the mammalian proteins is:
Pi (out) + 3 Na+ (out) ⇌ Pi (in) + 3 Na+ (in).
Renal Na+-dependent phosphate transport protein 2 (NPT2). Catalyzes 3Na+:1Pi symport (Ghezzi et al., 2009).
NPT2 of Rattus norvegicus (Q60825)
Na+-dependent phosphate transporter, NptA (Lebens et al. 2002).
NptA of Vibrio cholerae
Renal Na+/phosphate electroneutral symporter, NPTIIc, NPT2a or NaPi-IIc; responsible for hypophosphatemic hypercalciuric nephrolithiasis associated with rickets (Stechman et al., 2007). Catalyzes 2Na+:1Pi symport (Ghezzi et al., 2009). Interacts functionally with CFTR (Bakouh et al., 2012). Also interacts with PDZ1 but not PDZ2 of NHERF1 (Mamonova et al. 2015).
SLC34A3 of Homo sapiens
Sodium-dependent phosphate transport protein 2B (Na+:phosphate symporter 2b) (NaPi-2B or NaPi2b) (Sodium/phosphate cotransporter 2B) (Na+/Pi cotransporter 2B) (NaPi-2b) (Solute carrier family 34 member 2). The membrane potential and cation interactions induce large-scale structural rearrangements of the protein (Patti et al. 2016). It may be involved in actively transporting phosphate into cells as the main phosphate transport protein in the intestinal brush border membrane. It may also play a role in the synthesis of surfactant in lung alveoli, and in the pathogenesis of pulmonary alveolar microlithiasis (PAM), a rare, autosomal recessive lung disorder associated with progressive accumulation of calcium phosphate microliths (Saito et al. 2015). NaPi2b may be a possible target for delivery of cytotoxic agents via antibody-drug conjugate models for some patients with lung adenocarcinoma (Heynemann et al. 2021). SLC34A2 promotes cancer proliferation and cell cycle progression by targeting TMPRSS3 in colorectal cancer (Yang et al. 2022).
SLC34A2 of Homo sapiens
Sodium-dependent phosphate transport protein 2A (Sodium-phosphate transport protein 2A; Na+-dependent phosphate cotransporter 2A; NaPiIIa; NaPi-3; NaPi-2a) (Solute carrier family 34 member 1; SLC34A1) of 639 aas and 12 TMSs. Structure-function relationships of the first and fourth predicted extracellular linkers of the type IIa Na+/Pi cotransporter have been stidied (Ehnes et al. 2004). It co-transports 3 Na+ with 1 divalent phosphate anion. There are 3 Na+ binding sites, N1 (binds Na+ or Li+), N2 and N3, (both of which are Na+-specific); residues comprising N1 have been identified (Fenollar-Ferrer et al. 2015).
SLC34A1 of Homo sapiens
Sodium-dependent phosphate transporter, NaPi-IIA. The stoichiometry is probably 3 Na+ to 1 phosphate molecules. Orthologous to 2.A.58.1.4. Predicted to have 12 TMSs with both the N- and C-termini intracellular (Radanovic et al. 2006). The Km for phosphate is 80 μM (Zhifeng et al. 2012).
NaPi-IIB of Sus scrofa
Sodium:phosphate cotransporter of 621 aas and 10 TMSs, NaPi-IIb (SLC34a2). The amino acid sequence showed 79.0 and 70.9% sequence identity to the Astyanax mexicanus and Pundamilia nyererei orthologues, respectively. The protein had eight predicted TMSs, with the amino and carboxy termini intracellular (Chen et al. 2016). Three isoforms have been evaluated with respect to the effects of diatary inorganic phosphate and vitamin D in several tisseues have been determined (Chen et al. 2017).
NaPi-IIb of Tachysurus fulvidraco (Yellow catfish) (Pimelodus fulvidraco)
Sodium-dependent phosphate cotransporterSLC34 Na+:Pi symporter of 350 aas and 10 TMSs.
NptA of Algoriphagus ornithinivorans
Uncharacterized protein of 370 aas and 9 TMSs
UP of Salinarchaeum sp. Harcht-Bsk1
Inorganic phosphate transporter, YjbB. May be able to catalyze both uptake and efflux (Motomura et al. 2011; Ebert et al. 2013).
YjbB of E. coli (P0AF43)
8 TMS NaPi co-transporter with a PhoU C-terminal domain
NaPi transporter of Fusobacterium nucleatum
NaPi cotransporter with 8 TMSs and a C-terminal PhoU domain
NaPi transporter of Natranaeoblius thermophilus
Na+/Pi cotransporter of 616 aas (Ebert et al. 2013).
Phosphate transporter of Dinoroseobacter shibae
YqeW, putative inorganic phosphate up take transporter of 307 aas and 8 TMSs.
YqeW of Bacillus subtilis
Putative Na+-dependent phosphate symporter of 558 aas and 9 TMSs.
PNaS family member of Methanocella arvoryzae
Na/Pi cotransporter family protein of 535 aas and 8 TMSs.
Phosphate transporter of Candidatus Nanohaloarchaeota archaeon