Calcium control of actin-activated myosin adenosine triphosphatase from Dictyostelium discoideum.

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Abstract
A protein fraction from the cellular slime mold D. discoideum confers Ca2+-sensitivity on the activation of purified myosin ATPase (ATP phosphohydrolase, EC 3.6.1.3) from Dictyostelium by purified Dictyostelium actin, i.e., the fraction inhibits the actomyosin ATPase activity in the absence of Ca2+ but not in the presence of Ca2+. This Ca2+-sensitizing factor affects only the actin-activated myosin ATPase and not the enzyme activity of myosin alone. The Ca2+-sensitivity is conserved when muscle actin replaces Dictyostelium actin, but is lost when muscle myosin replaces Dictyostelium myosin. The factor appears to be a protein since it is nondialyzable, is heat labile, and can be precipitated with ammonium sulfate. The factor was purified 70-fold on an actin-affinity column.