Huang M, Yang C, Schafer D A, Cooper J A, Higgs H N, Zigmond S H
Biology Department University of Pennsylvania, Philadelphia, Pennsylvania 19104-6018, USA.
Curr Biol. 1999 Sep 9;9(17):979-82. doi: 10.1016/s0960-9822(99)80428-x.
Each actin filament has a pointed and a barbed end, however, filament elongation occurs primarily at the barbed end. Capping proteins, by binding to the barbed end, can terminate this elongation. The rate of capping depends on the concentration of capping protein [1], and thus, if capping terminates elongation, the length of filaments should vary inversely with the concentration of capping protein. In cell extracts, such as those derived from neutrophils, new actin filaments can be nucleated by addition of GTPgammaS-activated Cdc42 (a small GTPase of the Rho family). To determine whether elongation of these filaments is terminated by capping, we manipulated the concentration of capping protein, the major calcium-independent capping protein in neutrophils, and observed the effects on filament lengths. Depletion of 70% of the capping protein from extracts increased the mean length of filaments elongated from spectrin-actin seeds (very short actin filaments with free barbed ends) but did not increase the mean length of filaments induced by Cdc42. Furthermore, doubling the concentration of capping protein in cell extracts by adding pure capping protein did not decrease the mean length of filaments induced by Cdc42. These results suggest that the barbed ends of Cdc42-induced filaments are protected from capping by capping protein.
每根肌动蛋白丝都有一个 pointed 端和一个 barbed 端，然而，丝的伸长主要发生在 barbed 端。封端蛋白通过与 barbed 端结合，可以终止这种伸长。封端的速率取决于封端蛋白的浓度[1]，因此，如果封端终止伸长，丝的长度应该与封端蛋白的浓度成反比。在细胞提取物中，例如从中性粒细胞中提取的那些，通过添加 GTPγS 激活的 Cdc42（Rho 家族的一种小 GTP 酶）可以使新的肌动蛋白丝成核。为了确定这些丝的伸长是否被封端终止，我们操纵了封端蛋白（中性粒细胞中主要的非钙依赖性封端蛋白）的浓度，并观察了对丝长度的影响。从提取物中去除 70%的封端蛋白增加了从血影蛋白 - 肌动蛋白种子（具有游离 barbed 端的非常短的肌动蛋白丝）伸长的丝的平均长度，但没有增加由 Cdc42 诱导的丝的平均长度。此外，通过添加纯封端蛋白使细胞提取物中封端蛋白浓度加倍并没有降低由 Cdc42 诱导的丝的平均长度。这些结果表明，Cdc42 诱导的丝的 barbed 端受到封端蛋白的保护而不被封端。
