Phosphorylation and activation of Bub1 on unattached chromosomes facilitate the spindle checkpoint.
The spindle checkpoint inhibits anaphase until all kinetochores have attached properly to spindle microtubules. The protein kinase Bub1 is an essential checkpoint component that resides at kinetochores during mitosis. It is shown herein that Xenopus Bub1 becomes hyperphosphorylated and the kinase is activated on unattached chromosomes. MAP kinase (MAPK) contributes to this phosphorylation, as inhibiting MAPK or altering MAPK consensus sites in Bub1 to alanine or valine (Bub1(5AV)) abolishes the phosphorylation and activation on chromosomes. Both Bub1 and Bub1(5AV) support the checkpoint under an optimal condition for spindle checkpoint activation. However, Bub1, but not Bub1(5AV), supports the checkpoint at a relatively low concentration of nuclei or the microtubule inhibitor nocodazole. Similar to Bub1(5AV), Bub1 without the kinase domain (Bub1(deltaKD)) is also partially compromised in its checkpoint function and in its ability to recruit other checkpoint proteins to kinetochores. This study suggests that activation of Bub1 at kinetochores enhances the efficiency of the spindle checkpoint and is probably important in maintaining the checkpoint toward late prometaphase when the cell contains only a few or a single unattached kinetochore.
Chen
Rey-Huei
RH
Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY, USA. rhchan@imb.sinica.edu.tw
D000818:Animals; D002453:Cell Cycle; D018797:Cell Cycle Proteins; D002875:Chromosomes / Q000378:metabolism*; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D018386:Kinetochores / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D010063:Ovum / Q000166:cytology / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases; D008941:Spindle Apparatus / Q000378:metabolism*; D014982:Xenopus laevis
D018797:Cell Cycle Proteins; D011494:Protein Kinases; C090464:Bub1 spindle checkpoint protein; D017346:Protein Serine-Threonine Kinases; D048049:Extracellular Signal-Regulated MAP Kinases