Serine/threonine-protein kinase essential for spindle-assembly checkpoint signaling. Plays a key role in the recruitment of the checkpoint proteins bub-3, mdf-1 and mdf-2 to unattached kinetochores (PubMed:18936247, PubMed:19109417, PubMed:24567362, PubMed:25987605). mdf-1 recruitment is independent of bub-1 kinase activity (PubMed:24567362). Has a role in the correct kinetochore localization of the spindly-like protein spdl-1 (PubMed:18765790). In addition, during meiotic anaphase I, controls the recruitment of hcp-1/2 and klp-19 to the ring-shaped domain formed between chromosomes (PubMed:20729837). Involved in chromosome alignment, chromosome homolog segregation and spindle assembly (PubMed:20729837, PubMed:25987605). In association with bub-3 at the kinetochore region of chromosomes, promotes the onset on anaphase independently from spindle checkpoint signaling and promotes the formation of stable end-on bipolar attachments of chromosomes (PubMed:25987605). Plays a role in nuclear envelope breakdown (PubMed:19109417). Required maternally during embryogenesis and in the zygote for the postembryonic development of several tissues including ventral cord neurons, gonad, intestine and seam cells (PubMed:19526056). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts (via kinase domain) with mdf-1 (via coiled coil domain); the interaction recruits mdf-1 to unattached kinetochores during mitosis and between homologous chromosomes in early anaphase of meiosis I (PubMed:24567362). May interact with bub-3; for localization at the kinetochore and the onset of anaphase (PubMed:24567362, PubMed:25987605). Localizes to kinetochores during prophase and metaphase. In metaphase, localizes also to a matrix-like structure around the spindle (PubMed:11402065). In anaphase, localizes only to unattached kinetochores which is kln-1-dependent (PubMed:11402065, PubMed:24567362). Localizes also in a ring-shaped domain between kinetochores, during anaphase of meiosis I which is air-2 and kln-1-dependent and between sister chromatids during meiosis II, which is kln-1-independent (PubMed:20729837). Localization to the kinetochore is dependent on bub-3 and on knl-2 (PubMed:25987605, PubMed:17339379). Expressed throughout embryogenesis mainly in dividing cells. Embryonic lethal (PubMed:25987605). RNAi-mediated knockdown in oocytes results in aberrant homologous chromosome segregation during meiosis due to severe pre-anaphase spindle defects characterized by disorganized microtubule bundles (PubMed:20729837). Delays in the processes leading up to exit from mitosis including delayed activation of separase, which results in a delay in the separation of sister chromatids, delayed cytokinesis onset and delayed chromosome decondensation (PubMed:25987605). These culminate in a delay in the onset of anaphase with a 50% increase in the interval between nuclear envelope breakdown and the onset of anaphase during embryogenesis (PubMed:25987605). Reduced bub-3 localization at the kinetochore region of chromosomes in embryos, but without a reduction in bub-3 protein (PubMed:25987605). In addition there is reduced localization of the spindly-like protein spdl-1 to kinetochores (PubMed:18765790). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. BUB1 subfamily.