The Drosophila protein Asp is involved in microtubule organization during spindle formation and cytokinesis

Abnormal spindle (Asp) is a 220-kD microtubule-associated protein from Dros ophila that has been suggested to be involved in microtubule nucleation fro m the centrosome. Here, we show that Asp is enriched at the poles of meioti c and mitotic spindles and localizes to the minus ends of central spindle m icrotubules. Localization to these structures is independent of a functiona l centrosome. Moreover, colchicine treatment disrupts Asp localization to t he centrosome, indicating that Asp is not an integral centrosomal protein. In both meiotic and mitotic divisions of asp mutants, microtubule nucleatio n occurs from the centrosome, and gamma -tubulin localizes correctly. Howev er, spindle pole focusing and organization are severely affected. By examin ing cells that carry mutations both in asp and in aster-less, a gene requir ed for centrosome function, we have determined the role of Asp in the absen ce of centrosomes. Phenotypic analysis of these double mutants shows that A sp is required for the aggregation of microtubules into focused spindle pol es, reinforcing the conclusion that its function at the spindle poles is in dependent of any putative role in microtubule nucleation. Our data also sug gest that Asp has a role in the formation of the central spindle. The inabi lity of asp mutants to correctly organize the central spindle leads to disr uption of the contractile ring machinery and failure in cytokinesis.