Cilia are plasma membrane protrusions that act as cellular antennae and propellers in eukaryotes. To achieve their sensory and motile functions, cilia maintain protein and lipid compositions that are distinct from those of the cell body. The transition zone (TZ) is a specialized region located at the ciliary base, which functions as a barrier separating the interior and exterior of cilia. The TZ comprises a number of transmembrane and soluble proteins. MKS1, B9D1/MKS9, and B9D2/MKS10 are soluble TZ proteins that are encoded by causative genes of Meckel syndrome (MKS), and have a B9 domain (B9D) in common. We here demonstrate the interaction mode of these B9D proteins to be MKS1–B9D2–B9D1, and demonstrate their interdependent localization to the TZ. Phenotypic analyses of MKS1-knockout (KO) and B9D2-KO cells show that the B9D proteins are involved in, although not essential, for normal cilia biogenesis. Rescue experiments of these KO cells show that formation of the B9D protein complex is crucial for creating a diffusion barrier for ciliary membrane proteins.

纤毛是质膜突起，在真核生物中充当细胞天线和螺旋桨。为了实现感觉和运动功能，纤毛维持与细胞体不同的蛋白质和脂质组成。过渡区（TZ）是位于纤毛基部的一个特殊区域，充当分隔纤毛内部和外部的屏障。 TZ 包含许多跨膜蛋白和可溶性蛋白。 MKS1、B9D1/MKS9 和 B9D2/MKS10 是可溶性 TZ 蛋白，由梅克尔综合征 (MKS) 致病基因编码，并具有共同的 B9 结构域 (B9D)。我们在这里证明这些 B9D 蛋白的相互作用模式是 MKS1–B9D2–B9D1，并证明它们相互依赖地定位到 TZ。 MKS1敲除 (KO) 和B9D2 -KO 细胞的表型分析表明，B9D 蛋白参与正常纤毛生物发生，尽管不是必需的。这些 KO 细胞的拯救实验表明，B9D 蛋白复合物的形成对于为纤毛膜蛋白创建扩散屏障至关重要。