Emerging regulatory mechanisms in ubiquitin-dependent cell cycle control

Abstract

The covalent modification of proteins with ubiquitin is required for accurate cell division in all eukaryotes. Ubiquitylation depends on an enzymatic cascade, in which E3 enzymes recruit specific substrates for modification. Among ~600 human E3s, the SCF (Skp1-cullin1-F-box) and the APC/C (anaphase-promoting complex/cyclosome) are known for driving the degradation of cell cycle regulators to accomplish irreversible cell cycle transitions. The cell cycle machinery reciprocally regulates the SCF and APC/C through various mechanisms, including the modification of these E3s or the binding of specific inhibitors. Recent studies have provided new insight into the intricate relationship between ubiquitylation and the cell division apparatus as they revealed roles for atypical ubiquitin chains, new mechanisms of substrate and E3 regulation, as well as extensive crosstalk between ubiquitylation enzymes. Here, we review these emerging regulatory mechanisms of ubiquitin-dependent cell cycle control and discuss how their manipulation might provide therapeutic benefits in the future.

Fig. 1.

Diversity of ubiquitin chain topologies with roles in cell cycle control. Ubiquitin chains of different topologies have distinct functional consequences. When linked through K48 of ubiquitin, ubiquitin chains trigger proteasomal degradation. K48-linked chains are important, for example, at the G1–S transition, when they trigger the degradation of CDK2 inhibitors to promote S-phase entry. K11-linked ubiquitin chains are assembled by the APC/C and also trigger degradation by the 26S proteasome. The K11-linked chains are important during mitosis, when they promote the metaphase-anaphase transition. By contrast, both K63- and M1-linked ubiquitin chains act in a non-proteolytic manner and regulate complex formation or kinase activation. In this way, M1-linked ubiquitin chains drive activation of the NF-κB transcription factor, which leads to the synthesis of important cell cycle regulators. K63-linked chains are assembled, for example, at sites of DNA damage, and they are required for establishing a G2–M checkpoint that inhibits cell division in the face of DNA damage.

Fig. 2.

Mechanisms of ubiquitin-dependent feedback regulation. Different mechanisms allow E3 enzymes to destabilize active cell cycle regulators. These include the involvement of essential cofactors, such as PCNA, autophosphorylation, as observed in the case of PLK4, and coupled activation and degradation, as observed for spindle assembly factors (SAFs). In all cases, the inactive cell cycle regulator is not recognized by the E3, and hence, it is spared from ubiquitylation and subsequent degradation.

Fig. 3.

Coordination of APC/C and SCF activities in cells experiencing spindle damage. Spindle damage, as induced by anti-tubulin chemotherapeutics such as taxol, activates the spindle assembly checkpoint (SAC), which in turn inhibits the APC/C. If cells can sustain APC/C inhibition, the caspase inhibitor MCL1 is phosphorylated and targeted for ubiquitylation by SCF^FBW7. Degradation of MCL1 leads to cell death. Cancer cells can bypass this protective mechanism by impeding SAC function, mutating FBW7, or overexpressing the DUB USP9X. These cells can eventually downregulate CDK1 activity and slip out of mitosis without undergoing apoptosis.