Bub1p kinase activates the Saccharomyces cerevisiae spindle assembly checkpoint

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• The spindle assembly checkpoint (SAC) ensures accurate chromosome segregation during cell division.
• BUB1 is a key protein kinase involved in SAC function, preventing premature anaphase onset.
• Spindle damage typically triggers the SAC, but mechanisms for its aberrant activation are less understood.

Purpose of the Study:

• To investigate the function of a dominant BUB1 allele (BUB1-5) in Saccharomyces cerevisiae.
• To determine if BUB1 can activate the SAC in the absence of detectable spindle damage.
• To elucidate the relationship between Bub1p and Mps1p kinases in SAC signaling.

Main Methods:

• Genetic analysis of a dominant BUB1 allele (BUB1-5) in yeast.
• Cell cycle analysis using DNA content and Clb2p levels.
• Assessment of checkpoint protein phosphorylation (e.g., Mad1p).
• Gene dependency studies involving other SAC components (BUB2, BUB3, MAD1, MAD2, MAD3).

Main Results:

• Overexpression of BUB1-5 induced a mitotic delay in yeast with intact spindles.
• The BUB1-5-induced delay occurred post-spindle assembly but before anaphase.
• Cells exhibited G2 DNA content and elevated Clb2p levels during the delay.
• Hyperphosphorylated Mad1p did not accumulate, unlike in typical spindle damage responses.
• The BUB1-5 delay required other SAC genes and was interdependent with MPS1 overexpression.

Conclusions:

• Bub1p can activate the SAC independently of overt spindle damage.
• Bub1p and Mps1p kinases likely function together in an early step of generating the spindle damage signal.
• This study reveals a novel mechanism for SAC activation and highlights the cooperative roles of key kinases.