The V-ATPase Subunit C Binds to Polymeric F-actin as Well as to Monomeric G-actin and Induces Cross-linking of Actin Filaments

Abstract

Previously, we have shown that the V-ATPase holoenzyme as well as the V 1 complex isolated from the midgut of the tobacco hornworm (Manduca sexta) exhibits the ability of binding to actin filaments via the V 1 subunits B and C (Vitavska, O., Wieczorek, H., and Merzendorfer, H. (2003) J. Biol. Chem. 278, 18499 -18505). Since the recombinant subunit C not only enhances actin binding of the V 1 complex but also can bind separately to F-actin, we analyzed the interaction of recombinant subunit C with actin. We demonstrate that it binds not only to F-actin but also to monomeric G-actin. With dissociation constants of ϳ50 nM, the interaction exhibits a high affinity, and no difference could be observed between binding to ATP-G-actin or ADP-G-actin, respectively. Unlike other proteins such as members of the ADF/cofilin family, which also bind to G-as well as to F-actin, subunit C does not destabilize actin filaments. On the contrary, under conditions where the disassembly of F-actin into G-actin usually occurred, subunit C stabilized F-actin. In addition, it increased the initial rate of actin polymerization in a concentration-dependent manner and was shown to cross-link actin filaments to bundles of varying thickness. Apparently bundling is enabled by the existence of at least two actin-binding sites present in the N-and in the C-terminal halves of subunits C, respectively. Since subunit C has the possibility to dimerize or even to oligomerize, spacing between actin filaments could be variable in size.V-ATPases are ubiquitous and highly conserved proton pumps that acidify specific organelles such as endosomes, lysosomes, or secretory vesicles in every eukaryotic cell (1). They also are found in plasma membranes of many specialized animal cells where they either are involved in pH homeostasis or in membrane energization (2). V-ATPases consist of two complexes, a peripheral V 1 complex whose catalytic part faces the cytosol and a membrane-bound proton-conducting V 0 complex. In the midgut of the tobacco hornworm (Manduca sexta), the V 1 complex of the plasma membrane V-ATPase contains eight different subunits, A-H, whereas the V 0 complex consists of the four different subunits a and c-e (3). Under special physiological conditions V-ATPase activity is down-regulated by reversible dissociation of the V 1 complex from the membrane as was shown in the tobacco hornworm as well as in yeast (4,5).Subunit C appears to be released into the cytoplasm during this process, because the purified V 1 complex lacks most of it (3, 6).Dissociation of subunit C from the V 1 complex and its support of holoenzyme reassembly indicate that this subunit may play a crucial role in the regulation of V-ATPases. Another role, previously detected by us in the M. sexta midgut, is its ability to bind to the actin cytoskeleton (7). In feeding tobacco hornworms, actin filaments co-localize with the V-ATPase at the apical membrane of midgut goblet cells. Like in osteoclasts (8), actin binding occurs via the V 1 subunit B; however, binding to F-actin al...

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