Orlova A, Prochniewicz E, Egelman E H
Department of Cell Biology and Neuroanatomy, University of Minnesota Medical School, Minneapolis 55455.
J Mol Biol. 1995 Feb 3;245(5):598-607. doi: 10.1006/jmbi.1994.0049.
A large body of biochemical evidence suggests that the F-actin filament can have internal cooperativity. We have observed large cooperative effects on the low-resolution structure of actin filaments under three very different conditions. First, when G-Ca(2+)-actin is polymerized by both Mg2+ and KCl, filaments may be found in two different populations, with two discrete positions seen for subdomain 2. When G-Ca2+ actin is polymerized by only Mg2+, a single F-Mg(2+)-actin population is seen. The structural data suggest that an entire filament exists with subdomain 2 in one state or the other when there is a heterogenous mixture of Mg2+ and Ca(2+)-actin. Second, when actin filaments are nucleated from gelsolin there is a conformational change that can be observed throughout the filament that is consistent with a large shift in the actin C terminus. There must be a large cooperative propagation of this effect throughout the filament from the nucleation point. Third, we have used phalloidin to stabilize F-actin in which two C-terminal residues have been proteolytically removed by trypsin. It has been shown biochemically that this stabilization occurs at substoichiometric amounts of phalloidin. Phalloidin, at either a 1:1 or a 1:20 molar ratio with actin, restores the connectivity between the long-pitch helical strands. F-actin's internal cooperativity will have large implications in vivo, particularly in muscle.
大量生化证据表明，F-肌动蛋白丝可能具有内部协同性。我们在三种截然不同的条件下观察到了肌动蛋白丝低分辨率结构上的巨大协同效应。首先，当G-Ca(2+)-肌动蛋白在Mg2+和KCl的作用下聚合时，细丝可能会出现两种不同的群体，亚结构域2会出现两个离散的位置。当G-Ca2+肌动蛋白仅在Mg2+作用下聚合时，会观察到单一的F-Mg(2+)-肌动蛋白群体。结构数据表明，当存在Mg2+和Ca(2+)-肌动蛋白的异质混合物时，整个细丝的亚结构域2会以一种或另一种状态存在。其次，当肌动蛋白丝从凝溶胶蛋白成核时，会发生构象变化，这种变化可以在整个细丝中观察到，这与肌动蛋白C末端的大幅移动一致。这种效应必须从成核点开始在整个细丝中进行大规模的协同传播。第三，我们使用鬼笔环肽来稳定F-肌动蛋白，其中两个C末端残基已被胰蛋白酶蛋白水解去除。生化研究表明，这种稳定作用在亚化学计量的鬼笔环肽量下就会发生。鬼笔环肽与肌动蛋白的摩尔比为1:1或1:20时，可恢复长间距螺旋链之间的连接性。F-肌动蛋白的内部协同性在体内将具有重大意义，尤其是在肌肉中。
