Mast, a conserved microtubule-associated protein required for bipolar mitotic spindle organization

Through mutational analysis in Drosophila, we have identified the gene mult iple asters (mast), which encodes a new 165 kDa protein, mast mutant neurob lasts are highly polyploid and show severe mitotic abnormalities including the formation of mono- and multi-polar spindles organized by an irregular n umber of microtubule-organizing centres of abnormal size and shape. The mas t gene product is evolutionarily conserved since homologues were identified from yeast to man, revealing a novel protein family. Antibodies against Ma st and analysis of tissue culture cells expressing an enhanced green fluore scent protein-Mast fusion protein show that during mitosis, this protein lo calizes to centrosomes, the mitotic spindle, centromeres and spindle midzon e. Microtubule-binding assays indicate that Mast is a microtubule-associate d protein displaying strong affinity for polymerized microtubules. The defe cts observed in the mutant alleles and the intracellular localization of th e protein suggest that Mast plays an essential role in centrosome separatio n and organization of the bipolar mitotic spindle.