Required for oocyte-to-zygote transition in which it phosphorylates oocyte proteins, including mei-1, oma-1, oma-2, mex-5, and mex-6, modifying their activity and/or stability following meiosis. Through phosphorylation of P granule components including meg-1, promotes the disassembly of zygotic P granules in the anterior cytoplasm during zygote polarization, and thus plays a role in P granule distribution and segregation in early stage embryos following meiosis (By similarity). Functions in both spindle positioning and in the posterior localization of cytoplasmic determinants, including pie-1, pos-1, and pgl-1, in early embryos. Involved in the asymmetric distribution of plk-1 at the 2-cell embryonic stage. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Activated during oocyte maturation by phosphorylation on Ser-361 by cdk-1. The pseudotyrosine phosphatases egg-4 and egg-5 sequester activated mbk-2 until the meiotic divisions and inhibit mbk-2 kinase activity directly, using a mixed-inhibition mechanism that does not involve tyrosine dephosphorylation (By similarity). Part of a complex, consisting of pseudophosphatases egg-3, egg-4, egg-5 and kinase mbk-2. Interacts (via Tyr-618 and Tyr-620) with egg-4 (via tyrosine-protein phosphatase domain) and egg-5 (via tyrosine-protein phosphatase domain); mbk-2 tyrosine phosphorylation enhances the interaction. The interaction inhibits mbk-2 kinase activity and is required for mbk-2 oocyte cortex localization. Interacts (via N-terminus) with egg-3 (via tyrosine-protein phosphatase domain); the interaction does not affect mbk-2 kinase activity, is enhanced by mbk-2 tyrosine phosphorylation status and requires prior binding of mbk-2 to egg-4 and egg-5. Maintained at the cortex by the cortical anchor egg-3 before meiotic divisions. During anaphase of meiosis I, egg-3 translocates into the cytoplasm on vesicles and is slowly degraded, releasing mbk-2 (By similarity). Autophosphorylated. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. MNB/DYRK subfamily.