The spectrin-based membrane skeleton is a ubiquitous membrane-associated two-dimensional cytoskeleton underneath the lipid membrane of metazoan cells. Mutations of skeleton proteins impair the mechanical strength and functions of the membrane, leading to several different types of human diseases. Here, we report the cryo-EM structures of the native spectrin-actin junctional complex (from porcine erythrocytes), which is a specialized short F-actin acting as the central organizational unit of the membrane skeleton. While an α-/β-adducin hetero-tetramer binds to the barbed end of F-actin as a flexible cap, tropomodulin and SH3BGRL2 together create an absolute cap at the pointed end. The junctional complex is strengthened by ring-like structures of dematin in the middle actin layers and by patterned periodic interactions with tropomyosin over its entire length. This work serves as a structural framework for understanding the assembly and dynamics of membrane skeleton and offers insights into mechanisms of various ubiquitous F-actin-binding factors in other F-actin systems.

基于血影蛋白的膜骨架是后生动物细胞脂质膜下普遍存在的膜相关二维细胞骨架。骨架蛋白的突变会损害膜的机械强度和功能，从而导致几种不同类型的人类疾病。在这里，我们报告了天然血影蛋白-肌动蛋白连接复合物（来自猪红细胞）的冷冻电镜结构，这是一种专门的短 F-肌动蛋白，充当膜骨架的中央组织单位。虽然 α-/β-内收蛋白异源四聚体作为柔性帽与 F-肌动蛋白的倒刺端结合，但 tropomodulin和 SH3BGRL2 一起在尖端形成了一个绝对帽。dematin 的环状结构加强了连接复合体在中间肌动蛋白层中，并通过在其整个长度上与原肌球蛋白的模式化周期性相互作用。这项工作作为理解膜骨架的组装和动力学的结构框架，并提供对其他 F-肌动蛋白系统中各种普遍存在的 F-肌动蛋白结合因子机制的见解。