C3G localizes to the mother centriole in a cenexin-dependent manner and regulates centrosome duplication and primary cilium length.

C3G (also known as RAPGEF1) plays a role in cell differentiation and is essential for early embryonic development in mice. In this study, we identify C3G as a centrosomal protein that colocalizes with cenexin (also known as ODF2) at the mother centriole in interphase cells. C3G interacts with cenexin through its catalytic domain, and the two proteins show interdependence for localization to the centrosome. C3G depletion causes a decrease in cellular cenexin levels. Centrosomal localization of C3G is lost as myocytes differentiate to form myotubes. Depletion of C3G by CRISPR/Cas9 results in the formation of supernumerary centrioles, whereas overexpression of C3G, or expression of a catalytically active C3G deletion construct, inhibits centrosome duplication. Cilium length is increased in C3G knockout cells, and this phenotype is reverted upon reintroduction of C3G or its catalytic domain alone. Association of C3G with the basal body is dynamic, decreasing upon serum starvation and increasing upon re-entry into the cell cycle. C3G inhibits cilium formation and length, and this inhibition is dependent on C3G catalytic activity. We conclude that C3G regulates centrosome duplication and maintains ciliary homeostasis, properties that could be important for its role in embryonic development.