Click here to close Hello! We notice that you are using Internet Explorer, which is not supported by Xenbase and may cause the site to display incorrectly. We suggest using a current version of Chrome, FireFox, or Safari.
J Membr Biol 2005 Aug 01;2063:227-38. doi: 10.1007/s00232-005-0796-x.
Show Gene links Show Anatomy links

Functionally important residues in the predicted 3(rd) transmembrane domain of the type IIa sodium-phosphate co-transporter (NaPi-IIa).

Virkki LV , Forster IC , Bacconi A , Biber J , Murer H .

The type IIa Na(+)/P(i), cotransporter (NaPi-IIa) mediates electrogenic transport of three Na(+) and one divalent P(i) ion (and one net positive charge) across the cell membrane. Sequence comparison of electrogenic NaPi-IIa and IIb isoforms with the electroneutral NaPi-IIc isoform pointed to the third transmembrane domain (TMD-3) as a possibly significant determinant of substrate binding. To elucidate the role of TMD-3 in the topology and mechanism underlying NaPi-IIa function we subjected it to cysteine scanning mutagenesis. The constructs were expressed in Xenopus oocytes and P(i) transport kinetics were assayed by electrophysiology and radiotracer uptake. Cys substitution resulted in only marginally altered kinetics of P(i) transport in those mutants providing sufficient current for analysis. Only one site, at the extracellular end of TMD-3, appeared to be accessible to methanethiosulfonate reagents. However, additional mutations carried out at D224 (replaced by E, G or N) and N227 (replaced by D or Q) resulted in markedly altered voltage and substrate dependencies of the P(i)-dependent currents. Replacing Asp-224 (highly conserved in electrogenic a and b isoforms) with Gly (the residue found in the electroneutral c isoform) resulted in a mutant that mediated electroneutral Na(+)-dependent P(i) transport. Since electrogenic NaPi-II transports 3 Na(+)/transport cycle, whereas electroneutral NaPi-IIc only transports 2, we speculate that this loss of electrogenicity might result from the loss of one of the three Na(+) binding sites in NaPi-IIa.

PubMed ID: 16456717
Article link: J Membr Biol

Species referenced: Xenopus laevis
Genes referenced: ttn

References [+] :
Bacconi, Renouncing electroneutrality is not free of charge: switching on electrogenicity in a Na+-coupled phosphate cotransporter. 2005, Pubmed, Xenbase