Mutations in the essential spindle checkpoint gene bub1 cause chromosome missegregation and fail to block apoptosis in Drosophila

We have characterized the Drosophila mitotic checkpoint control protein Bub 1 and obtained mutations in the bub1 gene. Drosophila Bub1 localizes strong ly to the centromere/kinetochore of mitotic and meiotic chromosomes that ha ve not yet reached the metaphase plate. Animals homozygous for P-element-in duced, near-null mutations of bub1 die during late larval/pupal stages due to severe mitotic abnormalities indicative of a bypass of checkpoint functi on. These abnormalities include accelerated exit from metaphase and chromos ome missegregation and fragmentation. Chromosome fragmentation possibly lea ds to the significantly elevated levels of apoptosis seen in mutants. We have also investigated the relationship between Bub1 and other kinetocho re components. We show that Bub1 kinase activity is not required for phosph orylation of 3F3/2 epitopes at prophase/prometaphase, but is needed for 3F3 /2 dephosphorylation at metaphase. Neither 3F3/2 dephosphorylation nor loss of Bub1 from the kinetochore is a prerequisite for anaphase entry. Bub1's localization to the kinetochore does not depend on the products of the gene s zw10, rod, polo, or fizzy, indicating that the kinetochore is constructed from several independent subassemblies.