Myosin regulatory subunit that plays an important role in regulation of both smooth muscle and nonmuscle cell contractile activity via its phosphorylation. Implicated in cytokinesis, receptor capping, and cell locomotion (PubMed:11942626, PubMed:2526655). In myoblasts, may regulate PIEZO1-dependent cortical actomyosin assembly involved in myotube formation (By similarity). Myosin is a hexamer of 2 heavy chains and 4 light chains: interacts with myosin heavy chain MYO19. Colocalizes with F-actin, MYH9 and PIEZO1 at the actomyosin cortex in myoblasts. Smooth muscle tissues and in some, but not all, nonmuscle cells. Phosphorylation increases the actin-activated myosin ATPase activity and thereby regulates the contractile activity. It is required to generate the driving force in the migration of the cells but not necessary for localization of myosin-2 at the leading edge. Phosphorylation is required for myotube formation. The disease may be caused by variants affecting the gene represented in this entry. This chain binds calcium.