UC Santa Cruz

Cell cycle progression is dependent upon cell growth. Cells must, therefore, translate growth into a proportional signal that can be used to determine when sufficient growth has occurred for cell cycle progression. In budding yeast, a protein kinase called Gin4 is required for normal control of cell growth and undergoes gradual hyperphosphorylation that is dependent upon and proportional to added membrane growth in metaphase within the mitotic phase of the cell cycle. These observation suggest that growth-dependent phosphorylation of Gin4 could play a role in mechanisms that measure cell growth in mitosis. However, the molecular mechanisms that drive growth-dependent hyperphosphorylation of Gin4 are poorly understood. Here, we utilized a combination of biochemical reconstitution and genetic analysis to define the molecular mechanisms that drive growth dependent phosphorylation of Gin4. Our working model is that lipid vesicles that drive plasma membrane growth drive also deliver key signaling molecules that drive Gin4 hyperphosphorylation, which would suggest a simple mechanistic explanation for how growth-dependent Gin4 phosphorylation signals are generated. Gin4 has a KA1 domain that binds phosphatidylserine lipid and is required for growth-dependent phosphorylation of Gin4. Our work rules out a simple model in which phosphatidylserine delivered to sites of membrane growth binds and activates Gin4 to undergo autophosphorylation. Rather, the data suggest that phosphatidylserine recruits Gin4 to sites of membrane growth, where it is phosphorylated and activated by Elm1 in a Yck1-dependent manner. In this manner, Gin4 phosphorylation signals are generated to drive inhibition of the mitotic inhibitor Swe1 to promote the metaphase to anaphase transition, allowing cells to complete division.