Munc-18-2 regulates exocytosis of H+-ATPase in rat inner medullary collecting duct cells

1 Renal Section, Boston University Medical Center, and Departments of 2 Medicine, 3 Physiology, and 4 Pathology, Boston University School of Medicine, Boston, Massachusetts 02118 Submitted 31 December 2003 ; accepted in final form 3 July 2004 Exocytic insertion of H + -ATPase into the apical membrane of inner medullary collecting duct (IMCD) cells is dependent on a soluble N -ethylmaleimide-sensitive factor (NSF) attachment protein target receptor (SNARE) complex. In this study we determined the role of Munc-18 in regulation of IMCD cell exocytosis of H + -ATPase. We compared the effect of acute cell acidification (the stimulus for IMCD exocytosis) on the interaction of syntaxin 1A with Munc-18-2 and the 31-kDa subunit of H + -ATPase. Immunoprecipitation revealed that cell acidification decreased green fluorescent protein (GFP)-syntaxin 1A and Munc-18-2 interaction by 49 ± 7% and increased the interaction between GFP-syntaxin 1A and H + -ATPase by 170 ± 23%. Apical membrane Munc-18-2 decreased by 27.5 ± 4.6% and H + -ATPase increased by 246 ± 22%, whereas GP-135, an apical membrane marker, did not increase. Pretreatment of IMCD cells with a PKC inhibitor (GO-6983) diminished the previously described changes in Munc-18-2-syntaxin 1A interaction and redistribution of H + -ATPase. In a pull-down assay of H + -ATPase by glutathione S -transferase (GST)-syntaxin 1A bound to beads, preincubation of beads with an approximately twofold excess of His-Munc-18-2 decreased H + -ATPase pulled down by 64 ± 16%. IMCD cells that overexpress Munc-18-2 had a reduced rate of proton transport compared with control cells. We conclude that Munc-18-2 must dissociate from the syntaxin 1A protein for the exocytosis of H + -ATPase to occur. This dissociation leads to a conformational change in syntaxin 1A, allowing it to interact with H + -ATPase, synaptosome-associated protein (SNAP)-23, and vesicle-associated membrane protein (VAMP), forming the SNARE complex that leads to the docking and fusion of H + -ATPase vesicles. soluble N -ethylmaleimide-sensitive factor attachment protein target receptor; cell pH; acid secretion Address for reprint requests and other correspondence: J. H. Schwartz, Evans Biomedical Research Center, 650 Albany St., Boston, MA 02118-2908 (E-mail: jhsch{at}bu.edu )