The core structure of motile cilia and flagella, the axoneme, is built from a stable population of doublet microtubules. This unique stability is brought about, at least in part, by a network of Microtubule Inner Proteins (MIPs) that are bound to the luminal side of the microtubule walls. Rib72A and Rib72B were identified as MIPs in the motile cilia of the protist Tetrahymena thermophila. Loss of these proteins leads to ciliary defects and loss of additional MIPs. We performed mass spectrometry coupled with proteomic analysis and bioinformatics to identify the MIPs lost in RIB72A/B knockout Tetrahymena axonemes. We identified a number of candidate MIPs and pursued one, Fap115, for functional characterization. We find that loss of Fap115 results in disrupted cell swimming and aberrant ciliary beating. Cryo-electron tomography reveals that Fap115 localizes to MIP6a in the A-tubule of the doublet microtubules. Overall, our results highlight the complex relationship between MIPs, ciliary structure, and ciliary function.

运动纤毛和鞭毛的核心结构，即轴丝，是由稳定的双峰微管群构成的。这种独特的稳定性至少部分是由与微管壁的腔侧结合的微管内蛋白 (MIP)网络带来的。Rib72A 和 Rib72B 被鉴定为原生生物嗜热四膜虫的活动纤毛中的 MIP 。这些蛋白质的丢失会导致纤毛缺陷和额外 MIP 的丢失。我们进行了质谱结合蛋白质组学分析和生物信息学，以识别在RIB72A/B敲除四膜虫中丢失的 MIP轴丝。我们确定了许多候选 MIP，并采用 Fap115 进行功能表征。我们发现 Fap115 的缺失会导致细胞游动中断和纤毛跳动异常。冷冻电子断层扫描显示 Fap115 定位于双峰微管的 A 小管中的 MIP6a。总体而言，我们的结果突出了 MIP、纤毛结构和纤毛功能之间的复杂关系。