In-cell structural insight into the asymmetric assembly of central apparatus in mammalian sperm axoneme

In motile cilia or flagella, the axoneme typically exhibits a "9+2" configuration, with the central apparatus (CA) consisting of extensively modified microtubules C1 and C2 to regulate ciliary motility. How C1 and C2 are interconnected asymmetrically remains unknown due to the lack of complete structural model of CA. Here, we utilized the in situ cryo-electron tomography approach to solve the in-cell structure of intact mouse sperm CA at sub-nanometer resolution and built its near-complete atomic model with the aid of AlphaFold2. We identified 39 different CA-associated proteins with 8 never reported from isolated specimen. We assigned the long chain-like ASH-containing proteins CFAP47 and HYDIN responsible for connecting C1 and C2. Sperm from Cfap47-knockout mice displayed a hollowing bridge in the CA structure, correlating with its reduced progressive motility. Our findings elucidate the molecular mechanisms of CA components in ciliary motility, and the implications of their mutations in human ciliopathies.