B‐50/GAP‐43 Binds to Actin Filaments Without Affecting Actin Polymerization and Filament Organization

: To investigate a possible function of the nervous tissuespecific protein kinase C substrate B‐50/GAP‐43 in regulati of the dynamics of the submembranous cytoskeleton. we studii the interaction between purified 6–50 and actin. Both the phosphorylated and dephosphorylated forms of 8–50 cosedi‐mented with filamentous actin (F‐actin) in a Ca2+‐independent manner. Neither 6–50 nor phospho‐6–50 had any effect on the kinetics of actin polymerization and on the critical concentration at steady state, as measured using pyrenylated actin. tight scattering of F‐actin samples was not increased in the presence of 550, suggesting that 550 does not bundle actin filaments. The number of actin filaments, determined by [3H]cytochalasin B binding, was not affected by either phospho‐ or dephospho‐B‐50, indicating that 550 has neither a severing nor a capping effect. These observations were confirmed by electron microscopic evaluation of negatively stained F‐actin samples, which did not reveal any structural changes in the actin meshwork on addition of 6–50, We conclude that 6–50 is an actin‐binding protein that does not directly affect actin dynamics.