Controls cell movement by regulating actin cytoskeleton homeostasis and filopodium and lamellipodium formation (By similarity). When unphosphorylated, induces cell migration (By similarity). When phosphorylated by MAPK8, induces actin bundles formation and stabilization, thereby reducing actin plasticity, hence restricting cell movement, including neuronal migration (By similarity). May be involved in coupling the protein kinase C and calmodulin signal transduction systems (Probable). Binds to filamentous actin (F-actin), but not to monomeric G-actin, independently of its phosphorylation status (By similarity). Interacts with calmodulin (PubMed:1516135). Associates with the membrane via the insertion of the N-terminal N-myristoyl chain and the partial insertion of the effector domain. Association of the effector domain with membranes may be regulated by Ca(2+)/calmodulin. Colocalizes with F-actin at the leading edge of migrating cells. Up-regulated in alveolar macrophages in response to bacterial lipopolysaccharide (LPS). Phosphorylated (PubMed:1516135). Phosphorylation at Ser-120 and Thr-182 is non-redundantly catalyzed by MAPK8 in vivo (By similarity). Phosphorylation at Thr-148 is preferentially catalyzed by MAPK8 in vivo, but this modification can also be catalyzed by other kinases in the absence of MAPK8 (By similarity). May be phosphorylated by protein kinase C, which disrupts the interaction with calmodulin (PubMed:1516135). Belongs to the MARCKS family.