Normal bone remodeling requires precise control over the rates of bone formation by osteoblasts and degradation by osteoclasts. Bone degradation entails the activation of osteoclasts to a highly polarized state, with formation of a specialized ruffled membrane at their bone attachment site that confers the ability to resorb bone. The osteoclast ruffled membrane is bounded by a ring of actin filaments and associated proteins on the cytoplasmic face of the plasma membrane, localized to the zone on the cell surface that is adherent to bone (2-4). The adherence zone forms a tightly sealed extracellular compartment that is acidified by densely packed proton-transporting V-ATPases 1 in the ruffled membrane (1, 5); acidification of this compartment promotes dissolution of bone mineral and degradation of bone matrix protein by cysteine proteinases secreted by the osteoclast (6 -8).In osteoclasts actively resorbing bone, most of the cellular V-ATPase is polarized to the ruffled membrane (1). Polarization of V-ATPases to discrete plasmalemmal domains in epithelial cells is dependent on the microtubules in the cytoskeleton (9). The involvement of actin filaments in polarization is not yet clear, although actin filament binding to V-ATPasedense vesicles has been observed in proton-transporting epithelial cells of toad urinary bladder (10).When cultured in medium containing 1,25-dihydroxyvitamin D 3 (1,25-(OH) 2 D 3 ), mouse marrow cells develop into osteoclasts (11, 12) that contain most of the V-ATPase in the cultures (13). Osteoclasts from 1,25-(OH) 2 D 3 -treated cultures plated on bone or dentine slices undergo a transformation to the activated phenotype, in which actin rings form, V-ATPase is polarized to the ruffled membrane, and bone is resorbed (1, 2, 5). In contrast, osteoclasts from identical cultures placed on glass coverslips fail to form ruffled membranes and do not have physiologically detectable plasma membrane V-ATPase (14, 15).Recent studies suggest that V-ATPase is associated with the cytoskeleton in osteoclasts cultured in vitro (16, 17). V-ATPase was found in a detergent-insoluble (cytoskeletal) fraction of osteoclast-containing bone marrow cultures from normal mice (16). In contrast, in osteoclasts from osteosclerotic (oc Ϫ /oc Ϫ ) mice, which are unable to form ruffled membranes, the amount of V-ATPase in the detergent-insoluble fraction was reduced (16), prompting speculation that V-ATPase binding to the cytoskeleton might be involved in ruffled membrane formation. In addition, genetic studies from yeast point toward an association between V-ATPase and the actin cytoskeleton (18). These authors (18) postulated that alterations in the actin cytoskeleton and cytoskeletal processes caused by mutation of V-ATPase subunits are the result of indirect effects resulting, perhaps, from changes in intracellular pH affecting cytoskeletal organization.Here, we examine the interaction between the Triton-insoluble cytoskeleton and V-ATPase in mouse marrow osteoclasts. We show that V-ATPase binds actin filaments dir...