Ubiquitin-dependent degradation of proteins by the 26S proteasome plays a major role in many biological processes including cellular division, proliferation and differentiation. Protein targeting by the ubiquitin system is a multi-step process, whereby ubiquitin is first activated by an ubiquitin-activating enzyme (E1), then passed to an ubiquitin-conjugating enzyme (E2) and is terminally transferred to the target protein by the ubiquitin-ligase (E3). Several reports have recently highlighted the importance of this process during division and development in C. elegans embryo. For example, inactivation of the CUL-3-based E3-ligase leads to several defects during division of the one-cell stage C. elegans embryos, including hypercontractility during pronuclear migration and mitosis, defects in microtubule-dependent processes and delay in DNA replication. We identified the microtubule-severing protein MEI-1 as one of the key substrates of the CUL-3 complex (Kurz et al., 2002; Pintard et al., 2003a). MEI-1 is required for the assembly of the meiotic spindle, but its degradation prior to mitosis is essential for the assembly of a functional mitotic spindle. Interestingly, while inactivation of
mei-1 in
cul-3(-) embryos suppresses defects in microtubule-dependent processes, it does not suppress other defects. This suggests the existence of other substrates. In the CUL-3 complex, the BTB-containing protein MEL-26 directly interacts with CUL-3 and functions as a substrate-specific adaptor in the complex targeting MEI-1 for degradation (Pintard et al., 2003b). Similar to the case for F-box proteins, that display substrate specificity in the CUL-1-based ligases, MEL-26 is actively degraded via an autocatalytic mechanism by the CUL-3 complex. Indeed, in conditions in which the function of the CUL-3 complex is compromised, MEL-26 strongly accumulates at the cortex and the cleavage furrow where it binds the actin-binding protein POD-1. Importantly, inactivation of
mel-26 in
cul-3(-) embryos suppresses the hypercontractility. These results suggest that MEL-26 induces cortical activities through its interaction with POD-1 and accomplishes this function independently of its role in MEI-1 degradation. Kurz, T., Pintard, L., Willis, J.H., Hamill, D.R., Gonczy, P., Peter, M. and Bowerman, B. (2002) Cytoskeletal regulation by the Nedd8 ubiquitin-like protein modification pathway. Science, 295, 1294-1298. Pintard, L., Kurz, T., Glaser, S., Willis, J.H., Peter, M. and Bowerman, B. (2003a) Neddylation and Deneddylation of CUL-3 Is Required to Target MEI-1/Katanin for Degradation at the Meiosis-to-Mitosis Transition in C. elegans. Curr Biol, 13, 911-921. Pintard, L., Willis, J.H., Willems, A., Johnson, J.L., Srayko, M., Kurz, T., Glaser, S., Mains, P.E., Tyers, M., Bowerman, B. and Peter, M. (2003b) The BTB protein MEL-26 is a substrate-specific adaptor of the CUL-3 ubiquitin-ligase. Nature, 425, 311-316.