The fertilized embryo supports both the meiotic and the first mitotic spindles. These differ in many respects, yet both spindles form in the same cytoplasm within a short time of one another. This requires careful regulation of components specific to each spindle type. Previously we demonstrated that the MEI-1/katanin microtubule-severing complex is required for meiosis but must be inactivated prior to the first mitotic cleavage. Downregulation of MEI-1 depends on MEL-26, which binds MEI-1 to target it for degradation by the CUL-3 E3 ubiquitin ligase complex. We have found that a second protein degradation pathway, involving the anaphase promoting complex (APC), acts in parallel to MEL-26 to inactivate MEI-1.In the absence of MEL-26, ectopic MEI-1 persists into mitosis, resulting in lethality. While
mel-26(null) is completely lethal at 25C, 20% of embryos hatch at 15C and lower levels of ectopic MEI-1 accumulate. Therefore a second pathway partly suffices to degrade MEI-1 with the slower cell-cycle at 15C. We examined genetic interactions between other protein degradation pathways that are active during fertilization to determine what acts in parallel to
mel-26. Ubiquitination often requires phosphorylation, and MBK-2 kinase is required to degrade MEI-1 and cytoplasmic fate determinants. However,
mbk-2 enhances the incomplete lethality of
mel-26(null) at 15C. This argues that MEL-26 mediated degradation does not require MBK-2 and instead that the two pathways act in parallel. Likewise, APC mutants also enhance
mel-26(null), implying that APC and MEL-26 act in parallel. The genetic interactions are blocked by tubulin mutations that partially suppress ectopic MEI-1 activity, demonstrating that enhanced lethality is caused by MEI-1 misregulation. In addition, enhancement between
mel-26 and apc or
mbk-2 correlates with increased ectopic MEI-1 expression. Thus the embryo has redundant mechanisms to eliminate MEI-1 activity after completion of meiosis.Although MEL-26 mRNA is present maternally, MEL-26 protein does not accumulate to appreciable levels until it is needed to degrade MEI-1 after meiosis. Therefore, the question of what blocks MEI-1 activity after meiosis is now rephrased to ask what prevents significant MEL-26 accumulation before mitosis. We are testing candidate genes to determine how MEL-26 accumulation is inhibited prior to this time.