Zigmond S H
Biology Department, University of Pennsylvania, Philadelphia 19104-6018.
Cell Motil Cytoskeleton. 1993;25(4):309-16. doi: 10.1002/cm.970250402.
Cell locomotion depends on polymerization and depolymerization of filamentous actin. Net polymerization at the cell front occurs fast enough to fill the extending lamellipod, and since total F-actin is essentially constant over time, depolymerization must equal polymerization. Indeed, the fastest moving cell types have the highest rates of depolymerization. Accounting for the high rate of depolymerization raises several problems. One is that net depolymerization requires the concentration of G-actin to be low (below the critical concentration), but rapid polymerization (occurring < 1 micron away) requires the concentration of G-actin to be high (well above the critical concentration). This may be accomplished by spatial compartmentalization of factors that favor polymerization or depolymerization, and/or by proteins that bind G-actin and prevent spontaneous polymerization while allowing barbed-end elongation. A second problem is that depolymerization proceeds faster than would seem possible from studies of F-actin in vitro (as calculated from number and lengths of filaments present and in vitro rate constants). Rapid depolymerization may be accomplished by filament cutters or by cytoplasmic components (as yet undiscovered) that increase the rate of depolymerization.
细胞运动依赖于丝状肌动蛋白的聚合和解聚。细胞前端的净聚合发生得足够快，足以填充延伸的片状伪足，并且由于总F-肌动蛋白随时间基本保持恒定，解聚必须等于聚合。事实上，移动速度最快的细胞类型具有最高的解聚速率。解释高解聚速率会引发几个问题。一个问题是净解聚要求G-肌动蛋白的浓度较低（低于临界浓度），但快速聚合（发生在<1微米远处）要求G-肌动蛋白的浓度较高（远高于临界浓度）。这可以通过有利于聚合或解聚的因子的空间分隔，和/或通过结合G-肌动蛋白并防止自发聚合同时允许末端延伸的蛋白质来实现。第二个问题是解聚的速度比从体外F-肌动蛋白研究中得出的速度似乎要快（根据存在的细丝数量和长度以及体外速率常数计算）。快速解聚可能通过细丝切割器或通过增加解聚速率的细胞质成分（尚未发现）来实现。
