The transition from egg to embryo occurs in the absence of transcription yet requires significant changes in gene activity. In C. elegans, this transition is marked by two important changes: 1) a switch from meiosis to mitosis, and 2) the development of molecular asymmetries along the anterior/posterior axis.We have identified a new gene called minibrain-kinase-2 (
mbk-2; formerly
pom-1), that is required for both of these changes. Our results suggest that the primary defect in
mbk-2 mutants is a failure to degrade several maternal proteins.
mbk-2 mutants fail to degrade the meiosis-specific katanin subunits MEI-1 and MEI-2 at the meiosis-to-mitosis transition, and the first mitotic division fails.
mbk-2 is also required for timely degradation of OMA-1, a regulator of oocyte maturation, and for the degradation of germ plasm components in anterior cells. Finally,
mbk-2 is required for asymmetric segregation of germline determinants in the zygote, implicating regulated protein degradation in this process.MBK-2 distribution changes dramatically after fertilization during the transition from meiosis I to meiosis II. We have found that this change coincides with activation of
mbk-2-dependent protein degradation. Mutants arrested in meiosis I do not relocalize MBK-2, and like
mbk-2 mutants, do not degrade MEI-1, do not degrade OMA-1, and do not segregate P granules.
mbk-2 mutants, however, complete the meiotic divisions normally. We conclude that
mbk-2, while not required for meiosis, is activated by the meiotic divisions and serves to coordinate maternal protein degradation during the egg-to-embryo transition.MBK-2 belongs to the Dual-Specificity Yak1-Related Kinase (DYRK) family, which is conserved from yeast to humans. Intriguingly, DYRKs have been implicated in the regulation of cell polarity and protein degradation in other systems, suggesting that the function of MBK-2-related kinases may have been conserved throughout evolution.We thank Valerie Reinke, Stuart Kim, Bill Raich, Oliver Hobert, Thimo Kurz, Bruce Bowerman, Edward Kipreos, and Rueyling Lin for reagents and strains used in this work.