Galkin Vitold E, Orlova Albina, Fattoum Abdellatif, Walsh Michael P, Egelman Edward H
Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, VA 22908-0733, USA.
J Mol Biol. 2006 Jun 2;359(2):478-85. doi: 10.1016/j.jmb.2006.03.044. Epub 2006 Apr 3.
Many actin-binding proteins have been observed to have a modular architecture. One of the most abundant modules is the calponin-homology (CH) domain, found as tandem repeats in proteins that cross-link actin filaments (such as fimbrin, spectrin and alpha-actinin) or link the actin cytoskeleton to intermediate filaments (such as plectin). In proteins such as the eponymous calponin, IQGAP1, and Scp1, a single CH-domain exists, but there has been some controversy over whether this domain binds to actin filaments. A previous three-dimensional reconstruction of the calponin-F-actin complex has led to the conclusion that the visualized portion of calponin bound to actin belongs to its amino-terminal homology (CH) domain. We show, using a calponin fragment lacking the CH-domain, that this domain is not bound to F-actin, and cannot be positioning calponin on F-actin as hypothesized. Further, using classification methods, we show a multiplicity in cooperative modes of binding of calponin to F-actin, similar to what has been observed for other actin-binding proteins such as tropomyosin and cofilin. Our results suggest that the form and function of the structurally conserved CH-domain found in many other actin-binding proteins have diverged. This has broad implications for inferring function from the presence of structurally conserved domains.
许多肌动蛋白结合蛋白已被观察到具有模块化结构。最丰富的模块之一是钙调蛋白同源（CH）结构域，在交联肌动蛋白丝的蛋白质（如丝束蛋白、血影蛋白和α-辅肌动蛋白）或连接肌动蛋白细胞骨架与中间丝的蛋白质（如网蛋白）中以串联重复形式存在。在诸如同名的钙调蛋白、IQGAP1和Scp1等蛋白质中，存在单个CH结构域，但关于该结构域是否与肌动蛋白丝结合一直存在一些争议。先前对钙调蛋白 - F - 肌动蛋白复合物的三维重建得出结论，与肌动蛋白结合的钙调蛋白可视化部分属于其氨基末端同源（CH）结构域。我们使用缺乏CH结构域的钙调蛋白片段表明，该结构域不与F - 肌动蛋白结合，并且不能如假设的那样将钙调蛋白定位在F - 肌动蛋白上。此外，使用分类方法，我们展示了钙调蛋白与F - 肌动蛋白结合的协同模式具有多样性，这与对其他肌动蛋白结合蛋白（如原肌球蛋白和丝切蛋白）所观察到的情况类似。我们的结果表明，在许多其他肌动蛋白结合蛋白中发现的结构保守的CH结构域的形式和功能已经发生了分化。这对于从结构保守结构域的存在推断功能具有广泛的意义。
