Figure 7.

Model relating the recruitment and dynamics of SAS-4 and SAS-6 to ultrastructural steps in the duplication cycle. (A) SAS-4 (red) is recruited to the central tube as it forms during S phase. Between late S phase and early prophase, centriolar SAS-4 is in dynamic equilibrium with the cytoplasmic pool but its levels remain constant. During late prophase, centriolar SAS-4 is stably incorporated into the outer centriole wall in a step that requires γ-tubulin and cell cycle progression into mitosis and likely corresponds to assembly of the centriolar microtubules. (B) Two models to explain the recruitment and subsequent reduction in the amount of SAS-6 at the site of new centriole assembly. In the first model (top), newly recruited SAS-6 is strictly associated with the daughter centriole. In this model, an SAS-6–containing structure forms during S phase and is subsequently reduced in size by half in a step that normally occurs in late prophase but does not require cell cycle progression into mitosis. In the second model (bottom), SAS-6 is recruited to the parent centriole before central tube formation. Subsequently, a portion of this SAS-6 is incorporated into the central tube of the daughter centriole. Assembly of the central tube triggers loss of the SAS-6 associated with the parent centriole that was not incorporated into the daughter.