CCDC15 localizes to the centriole inner scaffold and regulates centriole integrity and ciliogenesis

Centrioles are evolutionarily conserved microtubule-based organelles critical to form centrosomes and cilia, which act as microtubule-organizing, signaling and motility centers. Biogenesis and maintenance of centrioles with proper number, size and architecture are crucial for their functions during development and physiology. Consequently, their deregulation causes developmental disorders and cancer. Although centriole number control has been extensively studied, less is known about how centrioles are maintained as stable structures with conserved size and architecture over successive cell divisions and upon ciliary and flagellar motility. Here, we addressed this question by identifying and characterizing new components of the centriole inner scaffold, a recently discovered centriolar sub-compartment critical for centriole size control and integrity. To this end, we generated proximity interactomes of Centrin-2 and POC5 and used them to define CCDC15 as a new centriolar protein that co-localizes and interacts with known inner scaffold proteins. Ultrastructure expansion microscopy analysis of CCDC15-depleted cells revealed its functions in centriole length control and integrity, resulting in defective ciliogenesis and Hedgehog signaling. Loss-of-function experiments also defined CCDC15 as a dual regulator for the recruitment of the inner scaffold protein POC1B and the distal SFI1/Centrin complex to the centrioles. Together, our findings uncovered new players and mechanisms of centriole architectural integrity and thereby, provide insights into diseases linked to centriolar defects.