Brenner S L, Korn E D
J Biol Chem. 1980 Feb 10;255(3):841-4.
Substoichiometric concentrations of cytochalasin D inhibited the rate of polymerization of actin in 0.5 mM MgCl2, increased its critical concentration and lowered its steady state viscosity. Stoichiometric concentrations of cytochalasin D in 0.5 mM MgCl2 and even substoichiometric concentrations of cytochalasin D in 30 mM KCl, however, accelerated the rate of actin polymerization, although still lowering the final steady state viscosity. Cytochalasin B, at all concentrations in 0.5 mM MgCl2 or in 30 mM KCl, accelerated the rate of polymerization and lowered the final steady state viscosity. In 0.5 mM MgCl2, cytochalasin D uncoupled the actin ATPase activity from actin polymerization, increasing the ATPase rate by at least 20 times while inhibiting polymerization. Cytochalasin B had a very much lower stimulating effect. Neither cytochalasin D nor B affected the actin ATPase activity in 30 mM KCl. The properties of cytochalasin E were intermediate between those of cytochalasin D and B. Cytochalasin D also stimulated the ATPase activity of monomeric actin in the absence of MgCl2 and KCl and, to a much greater extent, stimulated the ATPase activity of monomeric actin below its critical concentration in 0.5 mM MgCl2. Both above and below its critical concentration and in the presence and absence of cytochalasin D, the initial rate of actin ATPase activity, when little or no polymerization had occurred, was directly proportional to the actin concentration and, therefore, apparently was independent of actin-actin interactions. To rationalize all these data, a working model has been proposed in which the first step of actin polymerization is the conversion of monomeric actin-bound ATP, A . ATP, to monomeric actin-bound ADP and Pi, A* . ADP . Pi, which, like the preferred growing end of an actin filament, can bind cytochalasins.
亚化学计量浓度的细胞松弛素D在0.5 mM MgCl₂中抑制肌动蛋白的聚合速率，增加其临界浓度并降低其稳态粘度。然而，在0.5 mM MgCl₂中化学计量浓度的细胞松弛素D，甚至在30 mM KCl中亚化学计量浓度的细胞松弛素D，尽管仍会降低最终的稳态粘度，但却加速了肌动蛋白的聚合速率。在0.5 mM MgCl₂或30 mM KCl中，所有浓度的细胞松弛素B都加速了聚合速率并降低了最终的稳态粘度。在0.5 mM MgCl₂中，细胞松弛素D使肌动蛋白ATP酶活性与肌动蛋白聚合解偶联，在抑制聚合的同时将ATP酶速率提高了至少20倍。细胞松弛素B的刺激作用要低得多。在30 mM KCl中，细胞松弛素D和B均不影响肌动蛋白ATP酶活性。细胞松弛素E的特性介于细胞松弛素D和B之间。细胞松弛素D在不存在MgCl₂和KCl的情况下也刺激单体肌动蛋白的ATP酶活性，并且在0.5 mM MgCl₂中，在低于其临界浓度时更大程度地刺激单体肌动蛋白的ATP酶活性。在其临界浓度之上和之下以及在存在和不存在细胞松弛素D的情况下，当几乎没有或没有发生聚合时，肌动蛋白ATP酶活性的初始速率与肌动蛋白浓度成正比，因此显然与肌动蛋白-肌动蛋白相互作用无关。为了合理解释所有这些数据，提出了一个工作模型，其中肌动蛋白聚合的第一步是将单体肌动蛋白结合的ATP（A·ATP）转化为单体肌动蛋白结合的ADP和Pi（A*·ADP·Pi），后者如同肌动蛋白丝的优选生长末端一样，能够结合细胞松弛素。
