From START to FINISH : the influence of osmotic stress on the cell cycle

Peer reviewedPublisher PD

The Anaphase-Promoting Complex or Cyclosome Supports Cell Survival in Response to Endoplasmic Reticulum Stress

The anaphase-promoting complex or cyclosome (APC/C) is a multi-subunit ubiquitin ligase that regulates exit from mitosis and G1 phase of the cell cycle. Although the regulation and function of APC/CCdh1 in the unperturbed cell cycle is well studied, little is known of its role in non-genotoxic stress responses. Here, we demonstrate the role of APC/CCdh1 (APC/C activated by Cdh1 protein) in cellular protection from endoplasmic reticulum (ER) stress. Activation of APC/CCdh1 under ER stress conditions is evidenced by Cdh1-dependent degradation of its substrates. Importantly, the activity of APC/CCdh1 maintains the ER stress checkpoint, as depletion of Cdh1 by RNAi impairs cell cycle arrest and accelerates cell death following ER stress. Our findings identify APC/CCdh1 as a regulator of cell cycle checkpoint and cell survival in response to proteotoxic insults

Arsenite-induced Cdc25C degradation is through the KEN-box and ubiquitin–proteasome pathway

Arsenite is a known human carcinogen that induces tumorigenesis through either a genotoxic or an epigenetic mechanism. In this study, the effect of arsenite on cell cycle regulation and the mechanisms that contribute to this effect were investigated. Treatment of the cells with arsenite suppressed cell proliferation and reduced cell viability in a dose- or time-dependent manner. Analysis of cell cycle profile and cell cycle regulatory proteins indicated that arsenite arrested the cell cycle at G(2)/M phase, partially through induction of cell division cycle 25 (Cdc25) isoform C (Cdc25C) degradation via ubiquitin–proteasome pathways. Mutation of the putative KEN box within the region 151 to 157 of human Cdc25C or treatment of the cells with a peptide competitor encompassing the KEN box partially inhibited arsenite-induced ubiquitination of Cdc25C. Thus, these results indicate that the regulated ubiquitination of Cdc25C may be involved in the arsenite-induced proteolytic down-regulation of Cdc25C activity in the G(2)/M phase of the cell cycle and suggest a link between cell cycle and the carcinogenic effects of arsenite

Molecular mechanism of APC/C activation by mitotic phosphorylation

In eukaryotes, the anaphase-promoting complex/cyclosome (APC/C) regulates the ubiquitin-dependent proteolysis of specific cell cycle proteins to coordinate chromosome segregation in mitosis and entry into G1 (refs 1,2). The APC/C’s catalytic activity and ability to specify the destruction of particular proteins at different phases of the cell cycle are controlled by its interaction with two structurally related coactivator subunits (Cdc20 and Cdh1). Coactivators recognize substrate degrons3, and enhance the APC/C’s affinity for its cognate E2 (refs 4–6). During mitosis, cyclin-dependent kinase and polo kinase control Cdc20 and Cdh1-mediated activation of the APC/C. Hyper-phosphorylation of APC/C subunits, notably Apc1 and Apc3, is required for Cdc20 to activate the APC/C7–12, whereas phosphorylation of Cdh1 prevents its association with the APC/C9,13,14. Since both coactivators associate with the APC/C through their common C box15 and IR (Ile-Arg) tail motifs16,17, the mechanism underlying this differential regulation is unclear, as is the role of specific APC/C phosphorylation sites. Here, using cryo-electron microscopy (cryo-EM) and biochemical analysis, we define the molecular basis of how APC/C phosphorylation allows for its control by Cdc20. An auto-inhibitory (AI) segment of Apc1 acts as a molecular switch that in apo unphosphorylated APC/C interacts with the C-box binding site and obstructs engagement of Cdc20. Phosphorylation of the AI segment displaces it from the C-box binding site. Efficient phosphorylation of the AI segment, and thus relief of auto-inhibition, requires the recruitment of Cdk-cyclin-Cks to a hyper-phosphorylated loop of Apc3. We also find that the small molecule inhibitor, tosyl-L-arginine methyl ester (TAME), preferentially suppresses APC/C(Cdc20) rather than APC/C(Cdh1), and interacts with both the C-box and IR-tail binding sites. Our results reveal the mechanism for the regulation of mitotic APC/C by phosphorylation and provide a rationale for the development of selective inhibitors of this state