Modulation of Electrogenic Sodium Bicarbonate Transport

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Sodium bicarbonate cotransporters contribute to intracellular pH (pHi) regulation and the transepithelial transport of sodium and bicarbonate in several tissues. The recent cloning, functional expression, and immunolocalization of electrogenic and electroneutral sodium bicarbonate cotransport (NBC) proteins provides an opportunity to investigate the molecular mechanisms responsible for modulating their function. The electrogenic sodium bicarbonate cotransporter kNBC1 is the main pathway for proximal tubule basolateral bicarbonate effiux. Loss of function mutations in the NBC1 gene cause a severe form of autosomal recessive proximal renal tubular acidosis. There is currently a paucity of information regarding both the structural motifs responsible for the electrogenicity of kNBC 1, and the biologically important protein interactions that modulate its function. In recent studies, we have demonstrated that PKA-dependent phosphorylation of the C-terminal Ser982 residue altered the electrogenicity of kNBC1 by shifting its HCO3-:Na + stoichiometry from 3:1 to 2:1. In the region adjacent to Ser982, structural analysis reveals a charged region with aspartic acid residues that could potentially play an important role in this regard. We hypothesized that the phosphorylation state of Ser982 determines whether this negatively charged region in the kNBC1 C-terminus will interact electrostatically either with one bicarbonate binding site in the transporter (2:1 mode), or a putative binding protein (3:1 mode). On this basis we screened a human kidney cDNA library in a yeast two-hybrid assay using the C-terminus of kNBC 1 as bait, and isolated the enzyme aspartoacylase. Aspartoacylase has several N-terminal basic residues which could mediate its interaction electrostatically with the C-terminus of kNBC1. Aspartoacylase was localized to the basolateral membrane of proximal tubule cells, and co-immunoprecipitated with kNBC 1 from kidney. PKA-dependent phosphorylation of kNBC1-Ser982 prevented the interaction between the proteins. Furthermore, the function of kNBC1 was significantly greater in cells co-transfected with kNBC 1 and aspartoacylase in the presence of N-acetylaspartate. We will use the mPCT cell line as a model system for achieving the goals of this proposal. Successful completion of this project will enhance our understanding of the mechanisms responsible for regulating H+/base transporters.