Catalytic domain of the Serine/Threonine kinases, Spindle assembly checkpoint proteins Bub1 and BubR1
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. This subfamily is composed of Bub1 (Budding uninhibited by benzimidazoles 1), BubR1, and similar proteins. They contain an N-terminal Bub1/Mad3 homology domain essential for Cdc20 binding and a C-terminal kinase domain. Bub1 and BubR1 are involved in SAC, a surveillance system that delays metaphase to anaphase transition by blocking the activity of APC/C (the anaphase promoting complex) until all chromosomes achieve proper attachments to the mitotic spindle, to avoid chromosome missegregation. Impaired SAC leads to genomic instabilities and tumor development. Bub1 and BubR1 facilitate the localization of SAC proteins to kinetochores and regulate kinetochore-microtubule (K-MT) attachments. Repression studies of Bub1 and BubR1 show that they exert an additive effect in misalignment phenotypes and may function cooperatively or in parallel pathways in regulating K-MT attachments. The Bub1/BubR1 subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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