The barbed and pointed ends of the actin filament (F-actin) are the sites of growth/shrinkage and the targets of capping proteins that block subunit exchange, including CapZ at the barbed end and tropomodulin at the pointed end. We describe cryo-electron microscopy structures of the free and capped ends of F-actin. Terminal subunits at the free barbed end adopt a “flat” F-actin conformation. CapZ binds with minor changes to the barbed end but major changes to itself. In contrast, subunits at the free pointed end adopt a “twisted” G-actin conformation. Tropomodulin binding forces the second subunit into an F-actin conformation. The structures reveal how the ends differ from the middle in F-actin and how these differences control subunit addition/dissociation, capping, and interactions with end-binding proteins.

中文翻译：

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肌动蛋白丝的自由端和加帽端的结构

肌动蛋白丝 (F-肌动蛋白) 的带刺和尖端是生长/收缩的位点，也是阻止亚基交换的加帽蛋白的目标，包括带刺端的 CapZ 和尖端的原调节蛋白。我们描述了 F-肌动蛋白自由端和加帽端的冷冻电子显微镜结构。自由刺端的末端亚基采用“扁平”F-肌动蛋白构象。CapZ 的结合对带倒刺的末端进行了微小的改变，但对其自身进行了重大的改变。相比之下，自由尖端的亚基采用“扭曲”的 G 肌动蛋白构象。原调节蛋白结合迫使第二个亚基形成 F-肌动蛋白构象。这些结构揭示了 F-肌动蛋白末端与中间的差异，以及这些差异如何控制亚基添加/解离、加帽以及与末端结合蛋白的相互作用。