Has a dual function in spindle-assembly checkpoint signaling and in promoting the establishment of correct kinetochore-microtubule (K-MT) attachments. Plays a role in the kinetochore-mediated mitotic checkpoint, which delays anaphase onset until all chromosomes have attached properly to the mitotic spindle. Required to establish and maintain spindle checkpoint arrest in eggs. Found in complex with bub1 or bub1b (phosphorylated). Component of a complex along with bub1b, mad2 and cdc20 located at the kinetochore. Interacts with znf207/BuGZ. Aurora B controls the kinetochore localization. Kinetochore localization starts in early prophase and maintains the localization until the metaphase-anaphase transition. Diffusively localized to the nucleus during interphase and telophase. Maternal transcripts localize exclusively to animal blastomeres. Active expression (mRNA level) in whole oocytes during early stages (Dumont I-II). Transcripts accumulate in the animal half during the latest stages of oogenesis (Dumont IV-VI). Localized in the perinuclear cytoplasm in the full-grown oocyte. Close association with chromosome movement in dividing blastomeres. After gastrulation zygotic expression is observed in the whole ectodermal region, but enhanced in the dorsoanterior region. In the tadpole, expression is observed in the central nervous system, eyes and branchial arches. Belongs to the WD repeat BUB3 family. Truncated N-terminus.