Structural models of the NaPi-II sodium-phosphate cotransporters

Abstract

Progress towards understanding the molecular mechanisms of phosphate homeostasis through sodium-dependent transmembrane uptake has long been stymied by the absence of structural information about the NaPi-II sodium-phosphate transporters. For many other coupled transporters, even those unrelated to NaPi-II, internal repeated elements have been revealed as a key feature that is inherent to their function. Here, we review recent structure prediction studies for NaPi-II transporters. Attempts to identify structural templates for NaPi-II transporters have leveraged the structural repeat perspective to uncover an otherwise obscured relationship with the dicarboxylate-sodium symporters (DASS). This revelation allowed the prediction of three-dimensional structural models of human NaPi-IIa and flounder NaPi-IIb, whose folds were evaluated by comparison with available biochemical data outlining the transmembrane topology and solvent accessibility of various regions of the protein. Using these structural models, binding sites for sodium and phosphate were proposed. The predicted sites were tested and refined based on detailed electrophysiological and biochemical studies and were validated by comparison with subsequently reported structures of transporters belonging to the AbgT family. Comparison with the DASS transporter VcINDY suggested a conformational mechanism involving a large, two-domain structural change, known as an elevator-like mechanism. These structural models provide a foundation for further studies into substrate binding, conformational change, kinetics, and energetics of sodium-phosphate transport. We discuss future opportunities, as well as the challenges that remain.

Structure of the sodium-dependent phosphate transporter reveals insights into human solute carrier SLC20

“…In human, both PiT and NaPi-II are sodium-dependent phosphate transporters. When the Tm PiT-Na/Pi crystal structure was compared with the modeled NaPi-II ( 4 , 31 ), we found that the transmembrane domains of PiT and NaPi-II proteins exhibit similar secondary structure features, especially with regard to a common inverted repeat topology. Both structures are capable of binding three Na ions.…”

“…on November 2, 2020 http://advances.sciencemag.org/ Downloaded from repeated helical hairpins (HP1 and HP2) that perform substrate/Na transport, and they have a stable scaffold domain for oligomerization. Substrate/Na transport in these transporters also operates via an elevator-like mechanism (23,24,31).…”

“…For SLC34 protein, corresponding to functional motifs are “ 161 QSSS” and “ 417 QSSS,” which represent the serine-rich stretches of the HPa-HPb linker ( 4 , 31 ). Structural modeling of NaPi-IIa predicted that the two QSSS motifs are involved in binding Na (Na2 and Na3) and Pi (corresponding to the Na1-Pi-Na2 in Tm PiT) ( 4 , 31 ). Moreover, a conserved aspartic acid is essential for sodium binding, i.e., D327 for Na fore binding in Tm PiT and D224 for Na1 binding by NaPi-IIa/IIb.…”

Solvent accessibility changes in a Na+-dependent C4-dicarboxylate transporter suggest differential substrate effects in a multistep mechanism

“…VcINDY likely undergoes multiple large-and small-scale conformational changes during transport. There are now many examples of structurally characterised transporters that are predicted to employ an elevator-like transport mechanism 24,[29][30][31][32][33][34][35][36][37][38] , which has revealed common features amongst many of them, including; distinct scaffold and transport domains, a broken transmembrane helix containing an intramembrane loop that contributes to the substrate binding site, and two re-entrant hairpin loops that enter the membrane but do not fully span it. While structural differences exist between the predicted elevatortype transporters, they are all predicted (or have been shown) to undergo a substantial vertical translocation of the transport domain, usually accompanied by a pronounced rotation of this domain, to expose the substrate binding site to both sides of the membrane.…”

Phosphate transport: from microperfusion to molecular cloning

“…It is our hope that these articles will provide a useful overview for experts and non-experts alike and stimulate continued research in the P i field. The reviews have been grouped according to five themes: Molecular and Mechanistic Aspects, including interacting proteins [12,13,23,43], Physiological Regulation [26,27,48], Human Mutations/Clinical Aspects and Diseases [4,30], Non-renal Roles for SLC34 Proteins [3,34], and Comparative Aspects [39,52].…”