Polarized cell growth: double grip by CDK1

Cyclin-dependent kinase-mediated phosphorylation of the exocyst subunit Exo84 in late G1 phase suppresses exocytic secretion and cell growth in yeast

In eukaryotic cells, the growth rate is strictly regulated for proper progression of the cell cycle. In the budding yeast Saccharomyces cerevisiae, it was previously shown that cell growth dramatically slows down when the cells start budding at the G₁/S transition. However, the molecular mechanism for this G₁/S-associated growth arrest is unclear. In this study, using exocytic secretion, cyclin-dependent kinase (CDK) assay, immunoprecipitation, and microscopy, we demonstrate that the exocyst subunit Exo84, which is known to be phosphorylated in mitosis, can also be phosphorylated directly by Cdk1 in the late G₁ phase. Of note, we found that the Cdk1-mediated Exo84 phosphorylation impairs exocytic secretion in the late G₁ phase. Using conditional cdc mutants and phosphodeficient and phosphomimetic exo84 mutants, we further observed that Cdk1-phosphoryated Exo84 inhibits the exocyst complex assembly, exocytic secretion, and cell growth, which may be important for proper execution of the G₁/S-phase transition before commitment to a complete cell cycle. Our results suggest that the direct Cdk1-mediated regulation of the exocyst complex critically contributes to the coordination of cell growth and cell cycle progression.