![]() Cells lacking either Hsl1p or Hsl7p fail to target Swe1p to the neck and fail to degrade Swe1p, suggesting a link between Swe1p neck localization and its degradation ( McMillan et al., 1999a Longtine et al., 2000). After bud emergence, a subset of the Swe1p is targeted to the bud side of the mother-bud neck in a septin-dependent manner ( Longtine et al., 2000). When it is first synthesized during late G1, Swe1p accumulates predominantly in the nucleus. Finally, Swe1p degradation requires Hsl1p, a Nim1-family protein kinase, and Hsl7p, a methyltransferase that binds directly to both Swe1p and Hsl1p ( Ma et al., 1996 Barral et al., 1999 McMillan et al., 1999a Shulewitz et al., 1999 Lee et al., 2000 Cid et al., 2001). ![]() Swe1p degradation also requires the presence of active Clb1p-4p/Cdc28p complexes, suggesting the presence of a feedback loop whereby Swe1p-dependent inhibition of Cdc28p causes stabilization and consequent accumulation of Swe1p ( Sia et al., 1998). Met30p is an F-box protein that forms part of an SCF ubiquitin ligase, and Swe1p degradation is blocked in temperature-sensitive met30 mutants, suggesting that SCF Met30 directs Swe1p ubiquitination and consequent destruction ( Kaiser et al., 1998). Several factors have been shown to participate in targeting Swe1p for degradation in unstressed cells. This response involves at least two pathways: one blocks degradation of Swe1p ( Sia et al., 1998), whereas another appears to inhibit Mih1p, the Cdc25-family phosphatase that dephosphorylates Cdc28p at Y19 ( Harrison et al., 2001). Cdc28p Y19 phosphorylation occurs at low basal levels in unstressed cells, but rises dramatically in response to actin perturbation ( Harrison et al., 2001). Swe1p blocks entry into mitosis through inhibitory phosphorylation of a conserved tyrosine residue, Y19, in the cyclin-dependent kinase Cdc28p ( Booher et al., 1993). The cell cycle arrest is enacted by Swe1p, the sole Wee1-family kinase in S. This in turn triggers cell cycle arrest in G2 through the morphogenesis checkpoint ( Lew and Reed, 1995 McMillan et al., 1998). Bud formation requires a polarized actin cytoskeleton, so that during some stress responses bud formation is temporarily halted. Stress responses often involve transient depolarization of the actin cytoskeleton ( Chowdhury et al., 1992 Delley and Hall, 1999) in addition to changes in gene expression ( Gasch et al., 2000). Yeast cells deploy a panoply of stress responses to adapt to changes in environmental conditions. The other region did not appear to affect interactions with known Swe1p regulators, suggesting that other as-yet-unknown regulators exist. One of these regions mediates interaction of Swe1p with Hsl7p, showing that the Swe1p-Hsl7p interaction is critical for Swe1p neck targeting and degradation. Third, a screen for functional but nondegradable mutants of SWE1 identified two small regions of Swe1p that are key to its degradation. Second, cyclin/Cdc28p does not influence Swe1p neck targeting, but can directly phosphorylate Swe1p, suggesting that it acts downstream of neck targeting in the Swe1p degradation pathway. First, we show here that Met30p (and by implication SCF Met30) is not, in fact, required for Swe1p degradation. The basis for regulation of Swe1p degradation by the morphogenesis checkpoint remains unclear, and in order to elucidate that regulation we have dissected the Swe1p degradation pathway in more detail, yielding several novel findings. In addition, Swe1p degradation is stimulated by its substrate, cyclin/Cdc28p, and Swe1p is thought to be a target of the ubiquitin ligase SCF Met30 acting with the ubiquitin-conjugating enzyme Cdc34p. Swe1p degradation involves the relocalization of Swe1p from the nucleus to the mother-bud neck, and neck targeting requires the Swe1p-interacting protein Hsl7p. The Swe1p stabilization promotes cell cycle arrest through Swe1p-mediated inhibitory phosphorylation of Cdc28p until the cells can recover from the perturbation and resume bud formation. However, Swe1p degradation is halted by the morphogenesis checkpoint, which responds to insults that perturb bud formation. Swe1p, the sole Wee1-family kinase in Saccharomyces cerevisiae, is synthesized during late G1 and is then degraded as cells proceed through the cell cycle.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |