Maruta H, Isenberg G
J Biol Chem. 1983 Aug 25;258(16):10151-8.
Cap 42 (a + b), a Ca2+-dependent, actin-binding and phosphorylatable protein consisting of two distinct subunits a and b of 42,000 Da in Physarum polycephalum, has been identified as a new F-actin-capping protein. It capped or bound to the fast growing ends of actin filaments and blocked actin polymerization at this end. The capping activity residing in subunit a and its Ca2+-dependency were regulated by phosphorylation of subunit b; subunit a required Ca2+ for its capping activity when subunit b was phosphorylated, whereas this activity became Ca2+ independent when subunit b was dephosphorylated. Subunit b contained at least two phosphorylatable threonine residues and probably three additional phosphorylation sites. Like cytochalasins and other F-actin-capping proteins, Cap 42 (a + b) was able to induce a rapid depolymerization of actin filaments at the slow growing end, and also to nucleate actin polymerization. However, unlike Physarum fragmin, Cap 42 (a + b) had no severing activity leading to the fragmentation of actin filaments. Our results indicate that Cap 42 (a + b) is the first Ca2+-dependent F-actin-capping phosphoprotein whose phosphorylation regulates its actin-binding and vice versa. A possible mechanism of the capping action of Cap 42 (a + b) in vitro and also its conceivable role in the regulation of the Ca2+/actin-dependent cytoplasmic streaming in plasmodia are discussed.
42（a + b）帽蛋白是多头绒泡菌中一种依赖钙离子、能与肌动蛋白结合且可磷酸化的蛋白质，由两个不同的亚基a和b组成，分子量均为42,000道尔顿，已被鉴定为一种新的F - 肌动蛋白封端蛋白。它封端或结合在肌动蛋白丝的快速生长末端，并在此末端阻止肌动蛋白聚合。位于亚基a中的封端活性及其对钙离子的依赖性受亚基b磷酸化的调节；当亚基b磷酸化时，亚基a的封端活性需要钙离子，而当亚基b去磷酸化时，该活性变得不依赖钙离子。亚基b含有至少两个可磷酸化的苏氨酸残基，可能还有另外三个磷酸化位点。与细胞松弛素和其他F - 肌动蛋白封端蛋白一样，42（a + b）帽蛋白能够在肌动蛋白丝的缓慢生长末端诱导其快速解聚，也能引发肌动蛋白聚合。然而，与多头绒泡菌肌动蛋白结合蛋白不同的是，42（a + b）帽蛋白没有导致肌动蛋白丝断裂的切断活性。我们的结果表明，42（a + b）帽蛋白是首个依赖钙离子的F - 肌动蛋白封端磷蛋白，其磷酸化调节其与肌动蛋白的结合，反之亦然。文中讨论了42（a + b）帽蛋白在体外封端作用的可能机制及其在调节疟原质体中钙离子/肌动蛋白依赖性细胞质流动中可能发挥的作用。
