Classic mechanisms for membrane fusion involve transmembrane proteins that assemble into complexes and dynamically alter their conformation to bend membranes, leading to mixing of membrane lipids (hemifusion) and fusion pore formation. Myomaker and Myomerger govern myoblast fusion and muscle formation but are structurally divergent from traditional fusogenic proteins. Here, we show that Myomaker and Myomerger independently mediate distinct steps in the fusion pathway, where Myomaker is involved in membrane hemifusion and Myomerger is necessary for fusion pore formation. Mechanistically, we demonstrate that Myomerger is required on the cell surface where its ectodomains stress membranes. Moreover, we show that Myomerger drives fusion completion in a heterologous system independent of Myomaker and that a Myomaker-Myomerger physical interaction is not required for function. Collectively, our data identify a stepwise cell fusion mechanism in myoblasts where different proteins are delegated to perform unique membrane functions essential for membrane coalescence.

膜融合的经典机制涉及跨膜蛋白，该膜蛋白组装成复合物并动态改变其构象以弯曲膜，导致膜脂质混合（融合）和融合孔形成。Myomaker和Myomerger控制成肌细胞融合和肌肉形成，但在结构上不同于传统的融合蛋白。在这里，我们显示Myomaker和Myomerger在融合途径中独立地介导了不同的步骤，其中Myomaker参与了膜半融合，而Myomerger对于融合孔的形成是必需的。从机制上讲，我们证明Myomerger是其胞外域应力膜的细胞表面所必需的。而且，我们表明Myomerger在独立于Myomaker的异源系统中驱动融合完成，并且功能不需要Myomaker-Myomerger物理相互作用。总的来说，我们的数据确定了成肌细胞中的逐步细胞融合机制，其中不同的蛋白质被委派来执行膜聚结所必需的独特膜功能。