Centriole growth is not limited by a finite pool of components, but is limited by the Cdk1/Cyclin-dependent phosphorylation of Ana2/STIL

Centrioles duplicate once per cell cycle but it is unclear how daughter centrioles assemble at the right time and place and grow to the right size. Here we show that in early Drosophila embryos the cytoplasmic concentrations of the key centriole assembly proteins Asl, Plk4, Ana2, Sas-6 and Sas-4 are low, but remain constant throughout the assembly process— indicating that none of them are limiting for centriole assembly. The cytoplasmic diffusion rate of Ana2/STIL, however, increased significantly towards the end of S-phase as Cdk/Cyclin activity in the embryo increased. A mutant form of Ana2 that cannot be phosphorylated by Cdk/Cyclins did not exhibit the diffusion rate change, and allowed daughter centrioles to grow for an extended period. Thus, the Cdk/Cyclin-dependent phosphorylation of cytoplasmic Ana2 seems to reduce the efficiency of daughter centriole assembly towards the end of S-phase. This helps to ensure that daughter centrioles stop growing at the correct time, and presumably also helps to explain why centrioles cannot duplicate during mitosis when Cdk/Cyclin activity is high.