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The B1 Subunit of the HATPase Is a PDZ Domain-binding Protein COLOCALIZATION WITH NHE-RF IN RENAL B-INTERCALATED CELLS*

The 56-kDa B1 subunit of the vacuolar HATPase has a C-terminal DTAL amino acid motif typical of PDZbinding proteins that associate with the PDZ protein, NHE-RF (Na/H exchanger regulatory factor). This B1 isoform is amplified in renal intercalated cells, which play a role in distal urinary acid-base transport. In contrast, proximal tubules express the B2 isoform that lacks the C-terminal PDZ-binding motif. Both the B1 56-kDa subunit and the 31-kDa (E) subunit of the HATPase are pulled down by glutathione S-transferase NHE-RF bound to GSH-Sepharose beads. These subunits associate in vivo as part of the cytoplasmic V1 portion of the HATPase, and the E subunit was co-immunoprecipitated from rat kidney cytosol with NHE-RF antibodies. The interaction of HATPase subunits with NHE-RF was inhibited by a peptide derived from the C terminus of the B1 but not the B2 isoform. NHE-RF colocalized with HATPase in either the apical or the basolateral region of B-type intercalated cells, whereas NHE-RF staining was undetectable in A-intercalated cells. In proximal tubules, NHE-RF was located in the apical brush border. In contrast, HATPase was concentrated in a distinct membrane domain at the base of the brush border, from which NHE-RF was absent, consistent with the expression of the truncated B2 subunit isoform in this tubule segment. The colocalization of NHE-RF and HATPase in Bbut not A-intercalated cells suggests a role in generating, maintaining, or modulating the variable HATPase polarity that characterizes the B-cell phenotype.