title: Functional characterization of nonsynonymous single nucleotide polymorphisms in the electrogenic Na+-HCO3- cotransporter NBCe1A.
abstract: The electrogenic Na(+)-HCO(3)(-) cotransporter NBCe1 encoded by SLC4A4 plays essential roles in the regulation of intracellular/extracellular pH. Homozygous mutations in NBCe1 cause proximal renal tubular acidosis associated with ocular abnormalities. In the present study, we tried to perform functional characterization of the four nonsynonymous single nucleotide polymorphisms (SNPs), E122G, S356Y, K558R, and N640I in NBCe1A. Functional analysis in Xenopus oocytes revealed that while the K558R variant had a significantly reduced transport activity corresponding to 47% of the wild-type activity, the remaining variants E122G, S356Y, and N640I did not change the NBCe1A activity. Apparent Na(+) affinity of K558R was not different from that of wild-type NBCe1A. Immunohistological analyses in HEK293 cells and MDCK cells indicated that none of these SNPs changed the trafficking behaviors of NBCe1A. Functional analysis in HEK293 cells also revealed that only the K558R variant had a reduced transport activity, corresponding to 41-47% of the wild-type activity. From these results, we conclude that among four SNPs, only the K558R variant, which is predicted to lie in transmembrane segment 5, significantly reduces the NBCe1A activity without changing the trafficking behavior or the apparent extracellular Na(+) affinity.
authors.affiliation: Department of Internal Medicine, Faculty of Medicine, University of Tokyo, 7-3-1 Bunkyo-ku, Hongo, Tokyo, 113-0033, Japan.
authors.forename: Osamu
authors.initials: O
authors.lastname: Yamazaki
authors.forename: Hideomi
authors.initials: H
authors.lastname: Yamada
authors.forename: Masashi
authors.initials: M
authors.lastname: Suzuki
authors.forename: Shoko
authors.initials: S
authors.lastname: Horita
authors.forename: Ayumi
authors.initials: A
authors.lastname: Shirai
authors.forename: Motonobu
authors.initials: M
authors.lastname: Nakamura
authors.forename: George
authors.initials: G
authors.lastname: Seki
authors.forename: Toshiro
authors.initials: T
authors.lastname: Fujita
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D004285:Dogs; D057809:HEK293 Cells; D006801:Humans; D009865:Oocytes / Q000502:physiology; D020641:Polymorphism, Single Nucleotide; D027982:Sodium-Bicarbonate Symporters / Q000235:genetics* / Q000502:physiology; D014982:Xenopus laevis
chemical_list: C498898:SLC4A4 protein, human; D027982:Sodium-Bicarbonate Symporters