title: Dependence on Ca2+ and tropomyosin of the actin-activated ATPase activity of phosphorylated gizzard myosin in the presence of low concentrations of Mg2+.
abstract: Ca2+ and tropomyosin are required for activation of ATPase activity of phosphorylated gizzard myosin by gizzard actin at less than 1 mM Mg2+, relatively low Ca2+ concentrations (1 microM), producing half-maximal activation. At higher concentrations, Mg2+ will replace Ca2+, 4 mM Mg2+ increasing activity to the same extent as does Ca2+ and abolishing the Ca2+ dependence. Above about 1 mM Mg2+, tropomyosin is no longer required for activation by actin, activity being dependent on Ca2+ between 1 and 4 mM Mg2+, but independent of [Ca2+] above 4 mM Mg2+. Phosphorylation of the 20,000-Da light chain of gizzard myosin is required for activation of ATPase activity by actin from chicken gizzard or rabbit skeletal muscle at all concentrations of Mg2+ employed. The effect of adding or removing Ca2+ is fully reversible and cannot be attributed either to irreversible inactivation of actin or myosin or to dephosphorylation. After preincubating in the absence of Ca2+, activity is restored either by adding micromolar concentrations of this cation or by raising the concentration of Mg2+ to 8 mM. Similarly, the inhibition found in the absence of tropomyosin is fully reversed by subsequent addition of this protein. Replacing gizzard actin with skeletal actin alters the pattern of activation by Ca2+ at concentrations of Mg2+ less than 1 mM. Full activation is obtained with or without Ca2+ in the presence of tropomyosin, while in its absence Ca2+ is required but produces  only partial activation. Without tropomyosin, the range of Mg2+ concentrations over which activity is Ca2+-dependent is restricted to lower values with skeletal than with gizzard actin. The activity of skeletal muscle myosin is activated by the gizzard actin-tropomyosin complex without Ca2+, although Ca2+ slightly increases activity. The Ca2+ sensitivity of reconstituted gizzard actomyosin is partially retained by hybrid actomyosin containing gizzard myosin and skeletal actin, but less Ca2+ dependence is retained in the hybrid containing skeletal myosin and gizzard actin.
mesh_terms: D000199:Actins / Q000494:pharmacology*; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002645:Chickens; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation / Q000187:drug effects; D005895:Gizzard, Avian / Q000032:analysis; D008274:Magnesium / Q000494:pharmacology*; D009218:Myosins / Q000378:metabolism*; D010766:Phosphorylation; D014335:Tropomyosin / Q000494:pharmacology*
chemical_list: D000199:Actins; D014335:Tropomyosin; D000251:Adenosine Triphosphatases; D009218:Myosins; D008274:Magnesium; D002118:Calcium