Conserved Molecular Interactions in Centriole-to-Centrosome Conversion

Centrioles are required to assemble both centrosomes and cilia and their dysfunction is associated with multiple inherited diseases and cancer1,2. Centriole duplication and maturation are tightly regulated; the centriole pair of each centrosome disengages as cells exit mitosis allowing new pro-centrioles to assemble in G1-S and elongate during S and G2. Although the newly assembled centriole reaches its full length in early mitosis, it cannot duplicate or organize pericentriolar material (PCM) until it has passed through mitosis3,4. The molecular basis of this conversion from centriole to centrosome has remained mysterious. Understanding the conversion requires knowledge of which centriolar components are loaded during mitosis, their spatial organization and the timing and dependencies of their recruitment. Recent developments in super resolution microscopy have proved invaluable in analyzing spatial relationships between centrosome components5-10. In Drosophila, Ana2:STIL and Sas-6 first appear at the site of pro-centriole formation9,11 (Zone I, Fig. 2a). Ana2:STIL is phosphorylated by Polo-like kinase 4 (Plk4)11,12, master regulator of centriole duplication13-16, thus recruits Sas-611,12 which can self-assemble into nine dimers to form the nine-fold symmetrical, proximally-located cartwheel17,18. Bld10:Cep135, also in Zone I9, is critical for cartwheel formation in Chlamydomonas and Paramecium19,20 and its human counterpart connects Sas-6 to CPAP and the centriole wall21. Drosophila Sas-4 is part of the Zone II cylinder and Sas-4:CPAP is considered to promote polymerization of centriolar microtubules22-24. The distal cap comprising CP110 and its partners constitutes Zone V that, together with proteins of Zones I and II, form the core of both mother and daughter centrioles. Zone III proteins, Asterless (Asl), Plk4 and Dplp (Drosophila pericentrin-like protein), associate with mother centriole throughout the cell cycle but have not yet assembled onto the daughter in interphase. Thus far Asl is the sole factor known to recruit Plk4 to the centrosome in Drosophila and hence is key for centriole duplication25,26. Zone IV proteins (Polo, Spd-2, Centrosomin and γ-tubulin) also do not associate with daughter centriole, but accumulate robustly around the mother upon mitotic entry. Figure 2 Cep135, Ana1 and Asl are extended molecules that span from inner to outer centriole Here we have determined which proteins known to be required for Drosophila centriole duplication27,28 are loaded onto the daughter late in interphase and during mitosis. This has revealed a conserved architectural network of Cep135-Ana1:Cep295-Asl:Cep152 that is key for centriole-to-centrosome conversion in both Drosophila and human. Our findings thus account for the final stages in the assembly of the daughter centriole that convert it into a mature mother able to duplicate.