Role of actin in the myosin-linked Ca2+-regulation of ATP-dependent interaction between actin and myosin of a lower eukaryote, Physarum polycephalum

• Published in: Journal of biochemistry (Tokyo) Vol. 110; no. 4; pp. 508 - 513
• Main Authors: Kohama, K, Kohno, T, Okagaki, T, Shimmen, T
• Format: Journal Article
• Language: English
• Published: Oxford Oxford University Press 1991
• Subjects: actin; adenosinetriphosphatase; binding; Biological and medical sciences; calcium; Cell physiology; enzyme activity; Fundamental and applied biological sciences. Psychology; inorganic ions; Molecular and cellular biology; Muscle contraction; myosin; Physarum polycephalum; regulation; Protozoa; Regulation(control); Calcium; Physarum polycephalum; Myosin; Contractile protein; Actins; Molecular interaction; Mycetozoa
• ISSN: 0021-924X; 1756-2651
• DOI: 10.1093/oxfordjournals.jbchem.a123611

Actin-activated ATPase activity of myosin from Physarum polycephalum decreases when it binds Ca2+ and increases when it loses Ca2+. This Ca-inhibition is observed with phosphorylated myosin [Kohama, K. (1990) Trend. Pharmacol. Sci. 11, 433-435]. The activity of dephosphorylated myosin remained at a low level both in the presence and absence of Ca2+, although Ca2+-binding ability was much the same as that of the phosphorylated myosin. The effect of phosphorylation has been studied at a conventional actin concentration, which is comparable with that of myosin by weight. When the concentration of actin was increased by 10 times, the dephosphorylated myosin became actin-activatable in the absence of Ca2+, and Ca-inhibition was recovered. As actin exists quite abundantly in non-muscle cells of Physarum, myosin phosphorylation plays virtually no role in regulating actin-myosin-ATP interaction in vivo. Physiologically the interaction may be regulated by Ca2+ by binding to and subsequent release from myosin. Latex beads coated by either phosphorylated or dephosphorylated myosin moved ATP-dependently on the actin cables of Characeae cells to the same extent in the absence of Ca2+, but the movement was abolished by increasing Ca2+. When the interaction was examined by monitoring the movement of actin filaments on myosin fixed on a coverslip, the movement and Ca-inhibition of the movement were detected with phosphorylated, not dephosphorylated, myosin [Okagaki, T., Higashi-Fujime, S., & Kohama, K. (1989) J. Biochem. 106, 955-957]. The discrepancy between the in vitro motility assays suggests that the former monitors the interaction at a high concentration of actin and the latter, that at a conventional, low concentration of actin. The former assay should be more useful as a monitor of the physiological interaction.