Actin kinase phosphorylates actin of fragmin—actin complex, resulting in the inactivation of the nucleation and capping activities of the complex. Fragmin—actin complex was prepared by a new purification procedure. Incubation with ATP caused inactivation of the purified complex and phosphorylation of actin of fragmin—actin complex. The detailed analysis of the complex by SDS-gel electrophoresis showed that actin kinase was co-purified with the fragmin—actin complex. Formation of such an association between actin kinase and substrate suggests that the kinase is localized on the fragmin—actin complex to efficiently regulate actin cytoskeletons.
