MARCKS is a substrate for protein kinase C. It is localized to the plasma membrane and is an actin filament crosslinking protein. Phosphorylation by protein kinase C or binding to calcium-calmodulin inhibits its association with actin and with the plasma membrane, leading to its presence in the cytoplasm. MARCKS is a filamentous actin crosslinking protein, with activity that is inhibited by PKC-mediated phosphorylation and by binding to calcium-calmodulin. MARCKS may be a regulated crossbridge between actin and the plasma membrane, and modulation of the actin crosslinking activity of the MARCKS protein by calmodulin and phosphorylation represents a potential convergence of the calcium-calmodulin and protein kinase C signal transduction pathways in the regulation of the actin cytoskeleton.