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 (PubMed:16289132, PubMed:16338136, PubMed:17869113, PubMed:18854162, PubMed:18199581, PubMed:19879842). 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 (PubMed:25535836). Functions in both spindle positioning and in the posterior localization of cytoplasmic determinants, including pie-1, pos-1, and pgl-1, in early embryos (PubMed:14697358). Involved in the asymmetric distribution of plk-1 at the 2-cell embryonic stage (PubMed:18199581). 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-362 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. The presence of egg-4 in the assay increases the KM of mbk-2 for mei-1. Part of a complex, consisting of pseudophosphatases egg-3, egg-4, egg-5 and kinase mbk-2; this complex is required for the oocyte-to-zygote transition (PubMed:19879842). Interacts (via Tyr-619 and Tyr-621) with egg-4 (via tyrosine-protein phosphatase domain) and egg-5 (via tyrosine-protein phosphatase domain); mbk-2 tyrosine phosphorylation enhances the interaction (PubMed:19879842). 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 (PubMed:17869113, PubMed:19879842). 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. In L1 larvae, expressed widely in the nervous system, including head neurons and the ventral nerve cord. In adult animals, continues to be expressed in the nervous system and is also expressed in body wall muscle. Expressed both maternally and zygotically. Autophosphorylated. Maternal-effect embryonic lethality due to defects in spindle positioning and cytokinesis in the early embryo (PubMed:12618396, PubMed:14697358). Microtubules are fragmented and disordered (PubMed:14697358). Abolishes phosphorylation of RNA-binding protein oma-1 in embryos (PubMed:16289132). RNAi-mediated knockdown causes an increase in taf-4 nuclear localization in the one-cell embryo (PubMed:18854162). RNAi-mediated knockdown causes a loss in plk-1 asymmetric localization in 2-cell stage embryo without affecting mex-5 polarization (PubMed:18199581). RNAi-mediated knockdown disrupts P granule distribution in zygotes after meiosis whereby zygotic P granules fail to disassemble in the anterior cytoplasm during zygote polarization, however new P granules assemble and accumulate in the posterior cytoplasm as in wild-type (PubMed:25535836). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. MNB/DYRK subfamily.