After fertilization, dramatic changes of the Caenorhabditis elegans cytoskeleton occur in the transition from meiosis to mitosis that require precise regulation. The MEI-1/MEI-2 katanin microtubule-severing complex is required for meiotic spindle formation but must be inactivated within roughly 15 minutes to allow formation of the mitotic spindle. This tightly regulated process is dependent on the function of several partially redundant pathways. In order to clarify how genes of these pathways function relative to one another, we developed a robust antibody-staining assay to measure anti-MEI-1 protein levels during the early embryonic divisions. Using this assay, we show that the anaphase-promoting complex, APC/C, the DYRK homolog MBK-2 and the CUL-2 (cullin) E3 ubiquitin ligase all act in the same pathway to degrade MEI-1. MEL-26, a substrate adaptor for a CUL-3-based ubiquitin ligase acts in a parallel pathway. During meiosis, when MEI-1 is active, CUL-2 does not degrade MEI-1 and instead acts to keep MEL-26/CUL-3 activity in check. These results suggest that CUL-2 switches from degrading MEL-26 in meiosis to degrading MEI-1 in mitosis. To confirm this, we set out to identify the substrate adaptors for these two complexes. We tested known CUL-2 substrate recognition subunits and candidate subunits from a computational search and identified three novel proteins acting in this pathway. In an independent screen for genes acting in parallel to MEL-26, we identified a HECT -domain E3 ubiquitin ligase, HECD-1. Further genetic testing suggests that HECD-1 is acting both as a meiotic activator and a mitotic inhibitor of MEI-1. However, using the quantitative antibody-staining assay, HECD-1 does not affect the levels of MEI-1. Our results further highlight the importance of having multiple redundant pathways to regulate a key developmental switch.
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