Peltz G, Kuczmarski E R, Spudich J A
J Cell Biol. 1981 Apr;89(1):104-8. doi: 10.1083/jcb.89.1.104.
Chymotrypsin cleaves Dictyostelium myosin in half, splitting the heavy chain (210,000 daltons) into two fragments of 105,000 daltons each. One of the two major fragments is soluble at low ionic strength and has a native molecular weight of 130,000. As judged by SDS polyacrylamide gel electrophoresis, this soluble fragment consists of the two intact myosin light chains of 18,000 and 16,000 daltons and a 105,000-dalton polypeptide derived from the myosin heavy chain. The soluble fragment retains actin-activated ATPase activity and the ability to bind to actin in an ATP-dissociable fashion. The maximal velocity of the actin-activated ATPase activity of the soluble fragment is 80% of that of uncleaved myosin, although its apparent Km for actin is 12-fold greater than that of myosin. In addition to the major soluble 105,000-dalton fragment discussed above, chymotryptic cleavage of the Dictyostelium myosin also generates fragments that are insoluble at low ionic strength. The major insoluble fragment is 105,000 daltons on an SDS polyacrylamide gel and forms thick filaments that are devoid of myosin heads. A less prevalent insoluble fragment has a molecular weight of 83,000 and is probably a subfragment of the insoluble 105,000-dalton fragment. The heavy chain of myosin is phosphorylated in vivo and the phosphorylation site has been localized to the insoluble fragments, which derive from the tail portion of the myosin molecule.
胰凝乳蛋白酶将盘基网柄菌肌球蛋白切成两半，使重链（210,000道尔顿）分成两个各为105,000道尔顿的片段。两个主要片段之一在低离子强度下可溶，其天然分子量为130,000。通过SDS聚丙烯酰胺凝胶电泳判断，这个可溶片段由两条完整的分别为18,000和16,000道尔顿的肌球蛋白轻链以及一个源自肌球蛋白重链的105,000道尔顿的多肽组成。该可溶片段保留了肌动蛋白激活的ATP酶活性以及以ATP可解离方式与肌动蛋白结合的能力。可溶片段的肌动蛋白激活的ATP酶活性的最大速度是未切割肌球蛋白的80%，尽管其对肌动蛋白的表观Km值比肌球蛋白大12倍。除了上述主要的105,000道尔顿可溶片段外，盘基网柄菌肌球蛋白的胰凝乳蛋白酶切割还产生在低离子强度下不溶的片段。主要的不溶片段在SDS聚丙烯酰胺凝胶上为105,000道尔顿，并形成没有肌球蛋白头部的粗丝。一个不太常见的不溶片段分子量为83,000，可能是105,000道尔顿不溶片段的一个亚片段。肌球蛋白的重链在体内被磷酸化，磷酸化位点已定位到源自肌球蛋白分子尾部的不溶片段上。
