A Comprehensive Analysis of Polo-like Kinase 4's Regulation and Role in Centriole Biogenesis

Abstract

Plk4 has been termed a `suicide kinase' because it promotes its own destruction to regulate protein levels. We identified numerous autophosphorylated residues within a region of Plk4 called the Downstream regulatory element (DRE). We find that phosphorylation of a single residue is sufficient for Slimb recruitment and phosphorylation of the surrounding residues builds a high affinity Slimb-binding site. These autophosphorylation events are dependent on Plk4 homodimerization, although the domains that mediate this dimerization are unknown. We show that Plk4 homodimerization is mediated by interactions between the PB1-PB2 cassette. We find that like all Polo kinases, Plk4 encodes a mechanism of kinase autoinhibition. Unlike other Polo kinases, which rely on external inputs for relief of inhibition, Plk4 is self-sufficient in relieving kinase inhibition. This relief of autoinhibition is regulated by PB3 of Plk4 and is dependent on homodimerization, thereby making homodimerization a necessary step in formation of the Slimb phosphodegron on Plk4. Polo Boxes are known as multifunctional domains, and the Polo Boxes of Plk4 are no different. We identified numerous Slimb-mediated ubiquitination sites on PB1 as well as PB2. Furthermore, the PB1-PB2 cassette mediates the interaction between Plk4 and the N-terminus of Asterless. In Drosophila cells, Plk4 requires Asterless for centriolar localization and Asterless overexpression drives centriole amplification in a Plk4 dependent manner. This is a fascinating result as endogenous Plk4 protein levels are undetectable in S2 cells, making it hard to envision a scenario where overexpression of Asterless could shuttle a non-existent Plk4 population to the centriole to initiate duplication. We found that in addition to shuttling Plk4 to the centriole, Asterless stabilizes Plk4, likely protecting Plk4 at the centriole to allow it to `license' the centriole for duplication. Moreover, we show that Asterless encodes two distinct Plk4 binding sites: the previously described N-terminal binding site as well as a novel C-terminal binding site. We found that the interaction between the C-terminal of Asterless and Plk4 is necessary for centriole duplication while the interaction between the N-terminal of Asterless and Plk4 is expendable. Together these findings provide significant insight into Plk4 biology and the mechanisms which limit its activity.