Bailin G
Experientia. 1984 Nov 15;40(11):1185-8. doi: 10.1007/BF01946645.
In smooth muscle the Mr 20,000 light chain of myosin is phosphorylated by a calmodulin-dependent protein kinase. It consists of 2 subunits: calmodulin, an acidic protein of Mr 17,000 that binds 4 moles of Ca2+; and a larger protein of Mr circa 130,000. Activation of the kinase is dependent upon their association in the presence of Ca2+. Cyclic AMP-dependent protein kinase phosphorylation of the myosin light chain kinase occurs at 2 sites. It decreases the affinity of the kinase for calmodulin and a reduction in the rate of light chain phosphorylation occurs. The kinase has an overall asymmetric shape composed of a globular head and tail region for the skeletal muscle enzyme. Trypsin digestion of this kinase releases a fragment of Mr 36,000 from the globular region that contains the catalytic and calmodulin binding sites. Chymotrypsin digestion of the kinase from smooth muscle generates a fragment of Mr 80,000 that does not contain the calmodulin binding or cyclic AMP-dependent protein kinase phosphorylation sites. It is a Ca2+-independent form of the kinase that phosphorylates the light chain of myosin. These structural features indicate a regulatory role for the kinase in smooth muscle phosphorylation and contraction.
在平滑肌中，肌球蛋白的20,000道尔顿轻链由一种钙调蛋白依赖性蛋白激酶磷酸化。它由2个亚基组成：钙调蛋白，一种17,000道尔顿的酸性蛋白，可结合4摩尔Ca2+；以及一个约130,000道尔顿的较大蛋白。激酶的激活取决于它们在Ca2+存在下的结合。肌球蛋白轻链激酶的环磷酸腺苷依赖性蛋白激酶磷酸化发生在2个位点。它降低了激酶对钙调蛋白的亲和力，并导致轻链磷酸化速率降低。该激酶整体呈不对称形状，由骨骼肌酶的球状头部和尾部区域组成。用胰蛋白酶消化这种激酶会从包含催化和钙调蛋白结合位点的球状区域释放出一个36,000道尔顿的片段。用胰凝乳蛋白酶消化平滑肌中的激酶会产生一个80,000道尔顿的片段，该片段不包含钙调蛋白结合或环磷酸腺苷依赖性蛋白激酶磷酸化位点。它是一种不依赖Ca2+的激酶形式，可使肌球蛋白轻链磷酸化。这些结构特征表明该激酶在平滑肌磷酸化和收缩中起调节作用。
