Metaphase arrest with centromere separation in polo mutants of Drosophila

The Drosophila gene polo encodes a conserved protein kinase known to be req uired to organize spindle poles and for cytokinesis. Here we report two str ongly hypomorphic mutations of polo that arrest cells of the larval brain a t a point in metaphase when the majority of sister kinetochores have separa ted by between 20-50% of the total spindle length in intact cells. In contr ast, analysis of sister chromatid separation in squashed preparations of ce lls indicates that some 83% of sisters remain attached. This suggests the s eparation seen in intact cells requires the tension produced by a functiona l spindle. The point of arrest corresponds to the spindle integrity checkpo int; Bub1 protein and the 3F3/2 epitope are present on the separated kineto chores and the arrest is suppressed by a bub1 mutation. The mutant mitotic spindles are anastral and have assembled upon centrosomes that are associat ed with Centrosomin and the abnormal spindle protein (Asp), but neither wit h gamma -tubulin nor CP190. We discuss roles for Polo kinase in recruiting centrosomal proteins and in regulating progression through the metaphase-an aphase checkpoint.