Differentially accessible Cdc4 phospho-degrons regulate Ctf19CCAN kinetochore subunit stability in mitosis

Abstract

Kinetochores are multi-subunit protein assemblies that link chromosomes to microtubules of the mitotic and meiotic spindle. How effective, yet strictly centromere-dependent kinetochore assembly is coupled to cell cycle progression is incompletely understood. Here, by combining comprehensive phosphorylation analysis of native Ctf19CCAN subunits with biochemical and functional assays in the model system budding yeast, we demonstrate that Cdk1 phosphorylation activates phospho-degrons on the essential subunit Ame1CENP-U which are recognized by the E3 ubiquitin ligase complex SCF-Cdc4. Gradual phosphorylation of degron motifs culminates in M-Phase and targets the protein for degradation. Binding of the Mtw1 complex shields the proximal phospho-degron, protecting kinetochore-bound Ame1 from the degradation machinery. Artificially increasing degron strength partially suppresses the temperature-sensitivity of a cdc4 mutant, while overexpression of Ame1-Okp1 is toxic to cells, demonstrating the physiological importance of this mechanism. We propose that phospho-regulated clearance of excess CCAN subunits protects against ectopic kinetochore assembly and contributes to mitotic checkpoint silencing. Our results suggest a novel strategy for how phospho-degrons can be used to regulate the assembly of multi-subunit complexes.