Phosphorylation-dependent conformational changes induce a switch in the actin-binding function of MARCKS

Phosphorylation of myristoylated alanine-rich protein kinase C substrate (M ARCKS) by protein kinase C eliminates actin filament cross-linking activity , but residual filament binding activity docks phosphorylated MARCKS on fil amentous actin. The postulated actin-binding region of MARCKS, which includ es a Ca2+-calmodulin-binding site, has been portrayed with alpha-helical st ructure, analogous to other calmodulin-binding domains. Previous speculatio n suggested that MARCKS may dimerize to form the two functional actin-bindi ng sites requisite for cross-linking activity. Contrary to these hypotheses , we show that MARCKS peptide with actin-cross-linking activity has an exte nded structure in aqueous solution but assumes a more compact structure upo n phosphorylation. We hypothesize that structural changes in the MARCKS pep tide induced by phosphorylation create a dynamic structure that, on average , has only one actin-binding site. Moreover, independent of the state of ph osphorylation, this peptide is monomeric rather than dimeric, implying that two distinct actin-binding sites are responsible for the actin-crosslinkin g activity of unphosphorylated MARCKS. These studies uniquely elucidate the mechanism by which phosphorylation of MARCKS induces structural changes an d suggest how these structural changes determine biological activity.