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Myosin II regulatory light chains have an important role in the organization and function of the contractile machinery at cytokinesis. Two recent reports provide new insights into these important proteins.
Loss of sister-chromatid cohesion triggers chromosome segregation. Several recent reports show that the protease Esp1 cleaves the cohesin subunit Scc1/Mcd1 to induce sister-chromatid segregation in yeast and vertebrates. This finding indicates that cohesin cleavage may control sister-chromatid separation in all eukaryotes.
Recent work has revealed an evolutionarily conserved trio of proteins that regulate cell polarity in epithelial cells, embryonic blastomeres and neural precursors. This common cell-polarity mechanism is used in cell-specific ways, as highlighted by the recent finding that at least two different types of asymmetric division are observed in Drosophila neural precursors.
Recent studies have shown that under conditions of Golgi breakdown, Golgi glycosylation enzymes recycle to the endoplasmic reticulum (ER), whereas Golgi matrix proteins are retained in a set of cytoplasmic membranes. These findings have implications for the role of the ER in Golgi reassembly.
ATM, the product of the gene that is mutated in the human genetic disorder ataxia telangiectasia (A-T), responds to DNA damage by phosphorylating several key substrates that are involved in both the sensing of damage and the activation of cell-cycle checkpoints. The unexpected activation of ATM kinase in response to insulin supports a more general signalling role for this enzyme.
Moving proteins between cellular locations is proving to be a tightly regulated process, and provides an important mechanism for controlling protein function. The tumour-suppressor protein p53 has been shown to use microtubules to aid nuclear localization, a finding that raises interesting questions about the action of microtubule-disrupting chemotherapeutic drugs.
The catenin p120 protein functions as a traffic light at the crossroads between cell–cell adhesion and cell migration. It can influence the choice between sessile and migratory behaviour by interfering with the function of RHO proteins.
Regulated ubiquitin/proteasome-dependent processing controls transcription by triggering the release of dormant transcriptional activators from the endoplasmic reticulum. This intersection of transcription and proteolysis has important implications for gene control, proteasome–substrate interactions and signal integration.
Combined efforts of developmental biologists and cancer geneticists have outlined the canonical Wnt/Wingless (Wg) signalling pathway as we currently know it. At the heart of the pathway is the molecule β-catenin/Armadillo. Recent work has shown that the tumour suppressor adenomatous polyposis coli (APC) acts to shuttle β-catenin from the nucleus to the cytoplasm.
Protein kinases play an integral role in the regulation of virtually every major cellular and developmental process, and an understanding of how protein kinases function in vivo is therefore of central importance to cell biology. Newly developed protein-kinase inhibitors provide powerful tools for understanding the functions of kinases in vivo.
