Catalytic domain of the Serine/Threonine kinase, Vertebrate Spindle assembly checkpoint protein Bub1
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. Bub1 (Budding uninhibited by benzimidazoles 1) contains an N-terminal Bub1/Mad3 homology domain essential for Cdc20 binding, a GLEBS motif for Bub3/kinetochore binding, and a C-terminal kinase domain. It is 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. Bub1 contributes to the inhibition of APC/C by phosphorylating its crucial cofactor, Cdc20, rendering it unable to activate APC/C. In addition, Bub1 facilitates the localization to kinetochores of other SAC and motor proteins including Mad1, Mad2, BubR1, and Plk1. It acts as the master organizer of the functional inner centromere. Bub1 also play roles in protecting sister chromatid cohesion and normal metaphase congression. The Bub1 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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