Actin Activates a Cryptic Dimerization Potential of the Vinculin Tail Domain

Abstract The tail domain of vinculin (Vt) is an actin binding module containing two regions that interact with F-actin. Although intact Vtpurified from a bacterial expression system is a globular monomer, each actin binding region dimerizes when expressed individually, suggesting the presence of cryptic self-association sites whose exposure is regulated. We show that actin modulates Vt self-association by inducing or stabilizing a conformational change in Vtthat allows dimerization. Chemical cross-linking studies implicate one of the actin binding regions in mediating dimerization in the presence of actin. Actin-induced Vt dimers may play a role in the filament cross-linking activity of this protein. The Vtdimers induced by actin are biochemically distinct from the Vt dimers and higher oligomers induced by acidic phospholipids such as phosphatidylinositol 4,5-bisphosphate, suggesting structural differences in Vt bound to these two ligands that may provide a mechanistic basis for inhibition of F-actin binding by phosphatidylinositol 4,5-bisphosphate. The ability of actin to regulate the dimerization state of an actin binding protein suggests that, rather than serving a passive structural role, actin filaments may directly participate in signal transduction and other cellular events that are known to depend on cytoskeletal integrity.