A novel role for subunit C in mediating binding of the H+-V-ATPase to the actin cytoskeleton

Primary proton transport by V-ATPases is regulated via the reversible dissociation of the V1V0 holoenzyme into its V-1 and V-0 subcomplexes. Laser scanning microscopy of different tissues from the tobacco hornworm revealed co-localization of the holoenzyme and F-actin close to the apical membranes of the epithelial cells. In midgut goblet cells, no co-localization was observed under conditions where the V-1 complex detaches from the apical membrane. Binding studies, however, demonstrated that both the V-1 complex and the holoenzyme interact with F-actin, the latter with an apparently higher affinity. To identify F-actin binding subunits, we performed overlay blots that revealed two V-1 subunits as binding partners, namely subunit B, resembling the situation in the osteoclast V-ATPase (Holliday, L. S., Lu, M., Lee, B. S., Nelson, R. D., Solivan, S., Zhang, L., and Gluck, S. L. (2000) J. Biol. Chem. 275, 32331-32337), but, in addition, subunit C, which gets released during reversible dissociation of the holoenzyme. Overlay blots and co-pelleting assays showed that the recombinant subunit Calso binds to F-actin. When the V-1 complex was reconstituted with recombinant subunit C, enhanced binding to F-actin was observed. Thus, subunit C may function as an anchor protein regulating the linkage between V-ATPase and the actin-based cytoskeleton.