| Coordination of polarized cell growth with cell division is essential for cells to achieve the correct size and shape. Cell size checkpoints ensure that cell cycle progression does not occur until proper growth has occurred. Signaling networks that control these checkpoints include a number of kinases and phosphatases, and phosphorylation is thought to be a key regulatory mechanism in integrating cell size information with cell cycle events. A group of GTP-binding proteins called the septins, which localize to sites of polar growth, are also thought to be involved. The goal of my research has been to better understand the signaling mechanisms that operate at the two main cell size checkpoints: cell cycle entry in G1, and mitotic entry at the G2/M transition.;At the G2/M transition, I examined the role of the Elm1 kinase in the activation of the Gin4 kinase, and in regulation of the septins. My results suggest that Elm1 phosphorylates Gin4 on threonine 189, which is critical for proper Gin4 function. I discuss key differences between my data and a recent study that attempted to identify Elm1-dependent Gin4 phosphorylation sites in vitro. I also found that Cla4, Gin4 and Elm1 are required for maintenance of phosphorylation of one of the septin proteins, Shs1, as cells enter mitosis, and that the PP2A phosphatase acts in opposition to this phosphorylation.;Finally, I discovered that a regulatory subunit of the PP2A phosphatase complex, Rts1, plays a role in regulating polar cell growth early in the cell cycle. Cells lacking Rts1 delay early cell cycle events, including the accumulation of the G1 cyclin, Cln2, suggesting that Rts1 may be involved in the G1 cell size checkpoint. Our genetic data suggest that Rts1 may act in a novel pathway that regulates polar cell growth by regulating levels of G1 cyclins. We used a systematic screen to identify kinases responsible for phosphorylation of Rts1. We found that cyclin dependent kinase (Cdk1), casein kinase 1, Hrr25 and Kin28 phosphorylate Rts1 in vitro. We also found that phosphorylation of Rts1 appears to be opposed by PP2A-dependent dephosphorylation.;Together, these studies provide new insights into regulatory mechanisms that coordinate polar cell growth with cell cycle progression in G1 and G2/M. |
