Nagi G. Ayad

The anaphase promoting complex induces substrate degradation during neuronal differentiation

The anaphase promoting complex (APC) is an E3 ubiquitin ligase required for the metaphase-to-anaphase transition and mitotic exit. However, APC also plays roles in G1, where it is regulated by Cdh1, and APC activity has also been detected in differentiated and non-proliferating cells, suggesting that it may play roles outside the cell cycle. Here, we report that disrupting APCCdh1 activity inhibits neurite outgrowth of both PC12 pheochromocytoma cells and primary cerebellar granule cells. APCCdh1 activity dramatically increases as PC12 cells differentiate in response to nerve growth factor. Furthermore, a key target degraded by APCCdh1 following nerve growth factor treatment is the F-box protein Skp2, and APCCdh1-mediated destruction of Skp2 is essential for proper terminal differentiation of neuronal precursors.

Redundant Ubiquitin Ligase Activities Regulate Wee1 Degradation and Mitotic Entry

The irreversible nature of mitotic entry is due to the activation of mitosis specific kinases such as cdk1/cyclin B. Cdk1/cyclin B induces activation of mitosis by promoting phosphatases while suppressing inhibitory factors such as the tyrosine kinase wee1. Since wee1 keeps cdk1/cyclin B inactive during the S and G2 phases, its activity must be down-regulated for mitotic progression to occur. One mechanism of suppressing wee1 activity is ubiquitin-dependent proteolysis. Cdk1/cyclin B1 phosphorylates wee1, targeting it for recognition by ubiquitin ligases and subsequent proteasomal degradation. One of the ubiquitin ligases promoting wee1 destruction during mitosis is the SCFβ-trcp complex. We demonstrate that this complex, and a second SCF complex containing the F-box protein Tome-1, regulate wee1 degradation during the S and G2 phases of the cell cycle. Therefore, redundant ubiquitin ligase activities promote efficient mitotic entry of eukaryotic cells.

Activation Domain-dependent Degradation of Somatic Wee1 Kinase

Cell cycle progression is dependent upon coordinate regulation of kinase and proteolytic pathways. Inhibitors of cell cycle transitions are degraded to allow progression into the subsequent cell cycle phase. For example, the tyrosine kinase and Cdk1 inhibitor Wee1 is degraded during G2 and mitosis to allow mitotic progression. Previous studies suggested that the N terminus of Wee1 directs Wee1 destruction. Using a chemical mutagenesis strategy, we report that multiple regions of Wee1 control its destruction. Most notably, we find that the activation domain of the Wee1 kinase is also required for its degradation. Mutations in this domain inhibit Wee1 degradation in somatic cell extracts and in cells without affecting the overall Wee1 structure or kinase activity. More broadly, these findings suggest that kinase activation domains may be previously unappreciated sites of recognition by the ubiquitin proteasome pathway.

CDKs Give Cdc6 a License to Drive into S Phase

The accumulation of Cdc6 promotes the initiation of DNA replication. In this issue of Cell, Mailand and Diffley (2005) show that phosphorylation of Cdc6 by cyclin-dependent kinases prevents its destruction by the anaphase promoting complex (APC). This simple mechanism explains how the APC simultaneously spares Cdc6 while targeting for destruction suppressors of DNA replication during the transition from quiescence to cell cycle reentry.