The main force generators in eukaryotic cilia and flagella are axonemal outer dynein arms (ODAs). During ciliogenesis, these ~1.8-megadalton complexes are assembled in the cytoplasm and targeted to cilia by an unknown mechanism. Here, we used the ciliate Tetrahymena to identify two factors (Q22YU3 and Q22MS1) that bind ODAs in the cytoplasm and are required for ODA delivery to cilia. Q22YU3, which we named Shulin, locked the ODA motor domains into a closed conformation and inhibited motor activity. Cryo–electron microscopy revealed how Shulin stabilized this compact form of ODAs by binding to the dynein tails. Our findings provide a molecular explanation for how newly assembled dyneins are packaged for delivery to the cilia.

真核纤毛和鞭毛中的主要力发生器是轴丝外动力蛋白臂 (ODA)。在纤毛发生过程中，这些约 1.8 兆道尔顿的复合物在细胞质中组装，并通过未知机制靶向纤毛。在这里，我们使用纤毛四膜虫来识别两个因子（Q22YU3 和 Q22MS1），它们在细胞质中结合 ODA，并且是 ODA 传递到纤毛所必需的。我们将其命名为 Shulin 的 Q22YU3 将 ODA 运动域锁定为闭合构象并抑制运动活动。冷冻电子显微镜揭示了 Shulin 如何通过与动力蛋白尾部结合来稳定这种紧凑形式的 ODA。我们的研究结果为新组装的动力蛋白如何包装以递送至纤毛提供了分子解释。