Choe Han, Burtnick Leslie D, Mejillano Marisan, Yin Helen L, Robinson Robert C, Choe Senyon
Structural Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, CA 92186-5800, USA.
J Mol Biol. 2002 Dec 6;324(4):691-702. doi: 10.1016/s0022-2836(02)01131-2.
Gelsolin participates in the reorganization of the actin cytoskeleton that is required during such phenomena as cell movement, cytokinesis, and apoptosis. It consists of six structurally similar domains, G1-G6, which are arranged at resting intracellular levels of calcium ion so as to obscure the three actin-binding surfaces. Elevation of Ca(2+) concentrations releases latches within the constrained structure and produces large shifts in the relative positioning of the domains, permitting gelsolin to bind to and sever actin filaments. How Ca(2+) is able to activate gelsolin has been a major question concerning the function of this protein. We present the improved structure of the C-terminal half of gelsolin bound to monomeric actin at 3.0 A resolution. Two classes of Ca(2+)-binding site are evident on gelsolin: type 1 sites share coordination of Ca(2+) with actin, while type 2 sites are wholly contained within gelsolin. This structure of the complex reveals the locations of two novel metal ion-binding sites in domains G5 and G6, respectively. We identify both as type 2 sites. The absolute conservation of the type 2 calcium-ligating residues across the six domains of gelsolin suggests that this site exists in each of the domains. In total, gelsolin has the potential to bind eight calcium ions, two type 1 and six type 2. The function of the type 2 sites is to facilitate structural rearrangements within gelsolin as part of the activation and actin-binding and severing processes. We propose the novel type 2 site in G6 to be the critical site that initiates overall activation of gelsolin by releasing the tail latch that locks calcium-free gelsolin in a conformation unable to bind actin.
凝溶胶蛋白参与肌动蛋白细胞骨架的重组，这一过程在细胞运动、胞质分裂和细胞凋亡等现象中是必需的。它由六个结构相似的结构域G1 - G6组成，在细胞内钙离子处于静息水平时，这些结构域的排列会掩盖三个肌动蛋白结合表面。钙离子浓度升高会释放受限结构内的锁扣，并使结构域的相对位置发生大幅变化，从而使凝溶胶蛋白能够结合并切断肌动蛋白丝。钙离子如何激活凝溶胶蛋白一直是关于该蛋白功能的一个主要问题。我们展示了分辨率为3.0埃的与单体肌动蛋白结合的凝溶胶蛋白C端一半的优化结构。在凝溶胶蛋白上可明显看到两类钙离子结合位点：1型位点与肌动蛋白共享钙离子配位，而2型位点完全包含在凝溶胶蛋白内。该复合物的这种结构揭示了分别位于结构域G5和G6中的两个新的金属离子结合位点。我们将两者都鉴定为2型位点。凝溶胶蛋白六个结构域中2型钙离子连接残基的绝对保守性表明每个结构域中都存在该位点。总的来说，凝溶胶蛋白有潜力结合八个钙离子，两个1型和六个2型。2型位点的功能是促进凝溶胶蛋白内的结构重排，作为激活以及肌动蛋白结合和切断过程的一部分。我们提出G6中的新型2型位点是通过释放将无钙凝溶胶蛋白锁定在无法结合肌动蛋白构象的尾部锁扣来启动凝溶胶蛋白整体激活的关键位点。
