We have shown that mutations in genes encoding kinetochore components cause a mitotic delay that is dependent on the checkpoint. We have used kinetochore mutants to identify new genes, including the regulatory subunit of PP2A, required for checkpoint function. We have shown that the source of the checkpoint signal is within the kinetochore and that it functions in the absence of DNA replication. This means that unreplicated chromosomes can signal for a mitotic arrest and that the source of the signal is dependent on kinetochore assembly and function but independent of the presence or absence of tension between sister chromatids. This suggests a simple model where kinetochores signal for a mitotic arrest when microtubules are unbound.
Microtubules bind to the kinetochore and shut off the signal.
We are continuing our collaboration with the Gorbsky lab to study two other conserved genes that appear to function with p55CDC in checkpoint control. The spindle checkpoint is a fundamental regulatory mechanism that assures genomic stability. Genomic instability is common among transformed cells and it is reasonable to assume that mitotic checkpoints become compromised during the transformation process. Some transforming oncogenes directly abrogate spindle checkpoint function, which presumably results in genomic instability. Checkpoint genes from yeast to humans are have been identified on the basis of conserved sequences suggesting that there are fundamental aspects of the checkpoint mechanism shared by all cells.