One possibility is that cyclin-dependent kinases act to directly phosphorylate proteins involved in specific cell cycle events, such as nuclear lamins, components of the DNA replication machinery, or microtubule-associated proteins. Our data demonstrated that proper activation of Gin4 requires Elm1 activity and the absence of Gin4 abolishes Elm1-dependent Shs1 modification in vivo. A new bud emerges from the mother cell during interphase and grows in a polar manner with actin localized at the bud tip. This signaling network appears to require the function of a Clb2-binding protein called Nap1, the Cdc42 GTPase, and two protein kinases called Gin4 and Cla4. This switch in the pattern of bud growth is not necessary for viability, and mutations that disrupt the switch can be easily identified because they cause cells to have highly elongated buds and to form colonies with an unusual morphology 1 , 5. Members of a family of proteins called cyclin-dependent kinases control the events of the eukaryotic cell cycle 33 , Model illustrating pathways leading to the regulation of Gin4 and Shs1. Alternatively, cyclin-dependent kinases may activate intricate signaling pathways involving multiple additional kinases that are ultimately responsible for phosphorylating the many proteins involved in cell cycle events. Unlike Gin4 and Kcc4, Hsl1 functions downstream of septin assembly and plays a critical role in Swe1 regulation.
We have used a combination of genetics and biochemistry to identify proteins that are required for the mitotic switch in the pattern of bud growth 1 , 8 , 21 , Thus, whether the Cdcdependent Shs1 phosphorylation regulates Shs1 sumoylation and whether this modification promotes the Shs1-Gin4 interaction critical for the Elm1-dependent Gin4 activation are intriguing questions that remain to be further investigated. Our data demonstrated that proper activation of Gin4 requires Elm1 activity and the absence of Gin4 abolishes Elm1-dependent Shs1 modification in vivo. Similarly, Clb2-Cdc28 did not activate Elm1 kinase activity under the various conditions tested Fig. The Cla4 kinase is also hyperphosphorylated during mitosis, and hyperphosphorylation requires the activity of Clb2, Cdc28, Nap1, and the GTP-bound form of Cdc42, but not Gin4 Interestingly, Elm1 was shown to be required for the Gin4-Cdc11 interaction 13 , Cooper and Prakash Chudalayandi for helpful advice. These experiments have provided evidence for the existence of an intricate signaling network that includes a Clb2-binding protein called Nap1, the Cdc42 GTPase, members of the septin family, and two protein kinases called Cla4 and Gin4. It has been shown that loss of CLA4 leads to the hypophosphorylation and inactivation of Gin4 in vivo Elm1 may activate Kcc4 but likely in S phase. Taken together, these results are consistent with the model that Elm1 activates Gin4 and that this step is critical for Gin4-dependent Shs1 regulation in vivo. However, our in vitro results showed that the level of Elm1-dependent Gin4 activation in the presence of Shs1 was similar to that in the absence of Shs1 compare lane 5 with lane 7 in Fig. Although cellular division by budding does not occur in all organisms, many of the proteins that function in the pathway that controls bud growth during mitosis are highly conserved, suggesting that similar pathways are used by other organisms to control mitotic events. The Gin4 kinase binds tightly to Nap1 and is activated by hyperphosphorylation during mitosis. We are also grateful to J. In support of this view, inhibition of mitotic Cdc28 activity greatly diminished the level of Shs1 sumoylation in vivo Fig. The mitosis-specific activation of Gin4 is dependent in vivo upon the function Nap1, Cla4, the GTP-bound form of Cdc42, and the septins 1 , Cells carrying a deletion of the ELM1 gene undergo a prolonged mitotic delay, fail to negatively regulate polar bud growth during mitosis, and show defects in septin organization. These findings raise the possibility that Cdc28 may contribute to the Shs1 modification through a direct phosphorylation. The hyperphosphorylated form of Cla4 appears to be responsible for relaying the signal to activate the Gin4 kinase Thus, although the modification of Shs1 in G1 is not fully understood, clearly Cdc28 activity contributes to the mitotic Shs1 modification. Unlike Gin4 and Kcc4, Hsl1 functions downstream of septin assembly and plays a critical role in Swe1 regulation. Previous studies have suggested that Elm1 may function to negatively regulate the Swe1 kinase. In this study, we demonstrate that the Elm1 kinase also plays a role in the control of bud growth during mitosis. Solid arrows indicate direct biochemical steps. The events leading to activation of cyclin-dependent kinase complexes are well understood; however, little is known about the molecular pathways that are initiated by cyclin-dependent kinases to control specific cell cycle events.
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