Ca-inhibitory myosins: their structure and function.

Actin-myosin-ATPase interaction of Physarum is under inhibitory Ca-control. Ca-binding to myosin inhibits the interaction, while Ca-release from myosin removes inhibition. However, this myosin-linked control system alone is not sufficient to account for in vivo contraction. A Ca-dependent inhibitory factor, whose active principle appears to be identical with or similar to Ca-Lc of myosin, augments myosin-linked control by interacting with actin. This novel regulatory system may not be specific for the actomyosin system in Physarum plasmodia, because some actomyosin-related processes, i.e., secretion and motility, are subject to inhibitory Ca-control. Accumulation of examples for the inhibitory Ca-control in higher and lower eukaryotes will clarify its phylogenic significance.