Chalovich J M
East Carolina University, School of Medicine, Department of Biochemistry, Greenville, NC 27858.
Cell Biophys. 1988 Jan-Jun;12:73-85. doi: 10.1007/BF02918351.
Smooth muscle contraction is regulated by phosphorylation of myosin and also possibly by the actin associated protein, caldesmon. The properties of caldesmon are discussed and compared with those of tropomyosin-troponin, the well characterized actin-based regulatory system of striated muscle. Caldesmon functions quite differently from tropomyosin-troponin. Under relaxing conditions tropomyosin-troponin does not affect the binding of myosin subfragment-1 to actin. In contrast, caldesmon strongly inhibits the binding of subfragment-1 to actin in the presence of ATP. This inhibition of binding parallels the decrease in ATPase activity that occurs as the caldesmon concentration is increased. Caldesmon has the opposite effect on the two headed myosin subfragment, heavy meromyosin. The apparent binding of skeletal heavy meromyosin increases slightly as the caldesmon concentration is increased, although the rate of ATP hydrolysis is inhibited. It is suggested that in the presence of caldesmon, myosin.ATP does not bind to the productive actin binding site but interacts with a distinct site on actin-caldesmon. This could lead to both an inhibition of ATP hydrolysis and an increase in resting stiffness of relaxed smooth muscle.
平滑肌收缩受肌球蛋白磷酸化调节，也可能受肌动蛋白相关蛋白钙调蛋白调节。本文讨论了钙调蛋白的特性，并将其与原肌球蛋白 - 肌钙蛋白的特性进行了比较，原肌球蛋白 - 肌钙蛋白是横纹肌中特征明确的基于肌动蛋白的调节系统。钙调蛋白的功能与原肌球蛋白 - 肌钙蛋白有很大不同。在舒张条件下，原肌球蛋白 - 肌钙蛋白不影响肌球蛋白亚片段 -1 与肌动蛋白的结合。相反，在 ATP 存在的情况下，钙调蛋白强烈抑制亚片段 -1 与肌动蛋白的结合。这种结合抑制与随着钙调蛋白浓度增加而发生的 ATP 酶活性降低平行。钙调蛋白对双头肌球蛋白亚片段重酶解肌球蛋白有相反的作用。随着钙调蛋白浓度增加，骨骼肌重酶解肌球蛋白的表观结合略有增加，尽管 ATP 水解速率受到抑制。有人提出，在钙调蛋白存在的情况下，肌球蛋白·ATP 不与有效的肌动蛋白结合位点结合，而是与肌动蛋白 - 钙调蛋白上的一个不同位点相互作用。这可能导致 ATP 水解受到抑制，以及舒张的平滑肌静息硬度增加。
