Keiser T, Schiller A, Wegner A
Biochemistry. 1986 Aug 26;25(17):4899-906. doi: 10.1021/bi00365a026.
The nonlinear increase of the elongation rate of actin filaments above the critical monomer concentration was investigated by nucleated polymerization of actin. Significant deviations from linearity were observed when actin was polymerized in the presence of magnesium ions. When magnesium ions were replaced by potassium or calcium ions, no deviations from linearity could be detected. The nonlinearity was analyzed by two simple assembly mechanisms. In the first model, if the ATP hydrolysis by polymeric actin is approximately as fast as the incorporation of monomers into filaments, terminal subunits of lengthening filaments are expected to carry to some extent ADP. As ADP-containing subunits dissociate from the ends of actin filaments faster than ATP-containing subunits, the rate of elongation of actin filaments would be nonlinearly correlated with the monomer concentration. In the second model (conformational change model), actin monomers and filament subunits were assumed to occur in two conformations. The association and dissociation rates of actin molecules in the two conformations were thought to be different. The equilibrium distribution between the two conformations was assumed to be different for monomers and filament subunits. The ATP hydrolysis was thought to lag behind polymerization and conformational change. As under the experimental conditions the rate of ATP hydrolysis by polymeric actin was independent of the concentration of filament ends, the observed nonlinear increase of the rate of elongation with the monomer concentration above the critical monomer concentration was unlikely to be caused by ATP hydrolysis at the terminal subunits. The conformational change model turned out to be the simplest assembly mechanism by which all available experimental data could be explained.
通过肌动蛋白的成核聚合反应，研究了肌动蛋白丝伸长率在临界单体浓度以上的非线性增加情况。当肌动蛋白在镁离子存在下聚合时，观察到明显的线性偏差。当镁离子被钾离子或钙离子取代时，未检测到线性偏差。通过两种简单的组装机制对非线性进行了分析。在第一个模型中，如果聚合的肌动蛋白水解ATP的速度与单体掺入丝中的速度大致相同，那么延长丝的末端亚基预计在一定程度上携带ADP。由于含ADP的亚基比含ATP的亚基更快地从肌动蛋白丝末端解离，肌动蛋白丝的伸长率将与单体浓度呈非线性相关。在第二个模型（构象变化模型）中，假定肌动蛋白单体和丝亚基存在两种构象。认为这两种构象中肌动蛋白分子的缔合和解离速率不同。假定单体和丝亚基在两种构象之间的平衡分布不同。认为ATP水解滞后于聚合和构象变化。由于在实验条件下，聚合的肌动蛋白水解ATP的速率与丝末端的浓度无关，因此在临界单体浓度以上观察到的伸长率随单体浓度的非线性增加不太可能是由末端亚基处的ATP水解引起的。结果表明，构象变化模型是能够解释所有现有实验数据的最简单的组装机制。
