Effects of Ca2+ on the conformation and enzymatic activity of smooth muscle myosin.
The influence of Ca2+ on the enzymatic and physical properties of smooth muscle myosin was studied. The actin-activated ATPase activity of phosphorylated gizzard myosin and heavy meromyosin is higher in the presence of Ca2+ than in its absence, but this effect is found only at lower MgCl2 concentrations. As the MgCl2 concentration is increased, Ca2+ sensitivity is decreased. The concentration of Ca2+ necessary to activate ATPase activity is higher than that required to saturate calmodulin. The similarity of the pCa dependence of ATPase activity and of Ca2+ binding to myosin and the competition by Mg2+ indicate that these effects involved the Ca2+-Mg2+ binding sites of gizzard myosin. For the actin dependence of ATPase activity of phosphorylated myosin at low concentrations of MgCl2, both Vmax and Ka are influenced by Ca2+. The formation of small polymers by phosphorylated myosin in the presence of Ca2+ could account for the alteration in the affinity for actin. For the actin dependence of phosphorylated heavy meromyosin at low MgCl2 concentrations, Ca2+ induces only an increase in Vmax. To detect alterations in physical properties, two techniques were used: viscosity and limited papain hydrolysis. For dephosphorylated myosin, 6 S or 10 S, Ca2+-dependent effects are not detected using either technique. However, for phosphorylated myosin the decrease in viscosity corresponding to the 6 S to 10 S transition is shifted to lower KCl concentrations by the presence of Ca2+. In addition, a Ca2+ dependence of proteolysis rates is observed with phosphorylated myosin but only at low ionic strength, i.e. under conditions where myosin assumes the folded conformation.
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D000199:Actins / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology*; D004789:Enzyme Activation / Q000187:drug effects; D005895:Gizzard, Avian / Q000032:analysis; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D009130:Muscle, Smooth / Q000032:analysis*; D015879:Myosin Subfragments / Q000378:metabolism; D009218:Myosins / Q000378:metabolism*; D010206:Papain / Q000378:metabolism; D010766:Phosphorylation; D011487:Protein Conformation / Q000187:drug effects; D014335:Tropomyosin / Q000494:pharmacology; D014422:Turkeys
D000199:Actins; D015879:Myosin Subfragments; D014335:Tropomyosin; D015636:Magnesium Chloride; D010206:Papain; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D009218:Myosins; D008274:Magnesium; D002118:Calcium