Protein evolution: Keeping BUBR1 intact

Abstract

ET TY /P H O TO D IS C ORIGINAL RESEARCH PAPER Suijkerbuijk, S. J. E. et al. The vertebrate mitotic checkpoint protein BUBR1 is an unusual pseudokinase. Dev. Cell 22, 1321–1329 (2012) FURTHER READING Adrain, C. & Freeman, M. New lives for old: evolution of pseudoenzyme function illustrated by iRhoms. Nature Rev. Mol. Cell Biol. 11 Jul 2012 (doi:10.1038/nrm3392) Enzymes can be unpredictable. Although many enzymes have been deemed catalytically ‘dead’ owing to mutations in key residues, some retain activity through a different mode of catalysis. Here, Kops and colleagues extend the enzyme repertoire again by showing that the mitotic kinase BUBR1 uses key residues in its catalytic domain not for catalysis but to provide structural integrity to the protein. BUBR1 has an integral role in promoting correct chromosome segregation during mitosis. As part of the mitotic checkpoint complex, it prevents mitotic progression until all kinetochores are correctly attached to the spindle. BUBR1 is similar in domain architecture to BUB1 kinase; however, in most non-vertebrate model organisms BUBR1 lacks the kinase domain that is present in the vertebrate protein. To understand this discrepancy, the authors carried out phylogenetic analysis of BUB1 and BUBR1 homologues and found evidence of nine independent gene duplications, which were followed by parallel subfunctionalization that gave rise to two proteins with distinct functions. For homologues of BUBR1, this subfunctionalization involved widespread removal of the kinase domain across eukaryotic evolution. A key question has been whether vertebrate BUBR1 does in fact act as a kinase during mitosis. Analysis of the sequence of the human protein indicated that BUBR1 has diverged from BUB1 and other kinases through evolution, having extensive modifications in its kinase motifs that are incompatible with catalytic activity. Interestingly, although human BUBR1 still carries the three essential catalytic residues that are present in conventional kinases, BUBR1 showed no detectable phosphorylation activity in vitro. To examine whether this result is physiologically relevant, the authors used BUBR1 mutants that were predicted to be catalytically inactive and assessed their ability to drive mitotic progression. Mutation of the catalytic residues Lys795 or Asp911 reduced checkpoint activity and accelerated mitotic exit. However, because mutation of the catalytic residue Asp882 or removal of the entire kinase domain had no effect on BUBR1 function, the authors postulated that the effect of Lys795 or Asp911 mutation might be catalysis independent. This suggests that BUBR1 is indeed a pseudokinase but requires its catalytic residues for another purpose. Indeed, Lys795 mutation was found to affect the structural integrity of the protein: for example, BUBR1 protein levels were reduced to a greater extent by the more drastic mutation Lys795Ala than by the milder Lys795Arg mutation; and the Lys795Ala mutant had compromised thermal stability compared with the wild-type protein. Further analysis revealed that mutation of the nearby residue Val793, which has a role in ATP binding, also affects protein stability, suggesting that BUBR1 instability resulting from mutation of Lys795 may be due to reduced ATP binding. So, BUBR1 is an unusual pseudokinase that seems to be catalytically inactive despite retaining its catalytic residues. Instead, its kinase domain has been diverted towards the control of protein stability. Rachel David P R OT E I N E VO L U T I O N

DOI: 10.1038/nrm3399

Cite this paper

@article{David2012ProteinEK, title={Protein evolution: Keeping BUBR1 intact}, author={Rachel V David}, journal={Nature Reviews Molecular Cell Biology}, year={2012}, volume={13}, pages={478-479} }