POLAR ORGANIZATION OF GAMMA-TUBULIN IN ACENTRIOLAR MITOTIC SPINDLES OF DROSOPHILA-MELANOGASTER CELLS

The spindle pole localization of gamma-tubulin was compared in wild ty pe and acentriolar cultured Drosophila cells using polyclonal antibodi es specifically raised against the carboxy terminal amino acid sequenc e of Drosophila gamma-tubulin-1 (-KSEDSRSVTSAGS), During interphase, g amma-tubulin was present in the centrosome of wild type cells and accu mulated around this organelle in a cell cycle dependent manner, In con trast, no such structure was observed in acentriolar cells, Wild type mitoses were homogeneously composed of biconical spindles, with two ce ntrosome-associated gamma-tubulin spots at the poles, The mitotic appa ratuses observed in the acentriolar cells were heterogeneous; multipol ar mitoses, bipolar mitoses with a barrel-shaped spindle and bipolar m itoses with biconical spindles were observed, In acentriolar cells, ga mma-tubulin accumulation at mitotic poles was dependent on spindle mic rotubule integrity, Most acentriolar spindles presented a dispersed ga mma-tubulin labeling at the poles. Only well polarized and biconical a centriolar spindles showed a strong gamma-tubulin polar spot, Finally, acentriolar mitotic poles were-not organized around true centrosomes. In contrast to wild type cells, in acentriolar cells the Bx63 centros ome-associated antigen was absent and the gamma-tubulin containing mat erial dispersed readily following microtubule disassembly. These obser vations confirm that gamma-tubulin plays an essential role in the nucl eation of microtubules even in the absence of mitotic polar organelles , In addition the data suggest that the mechanisms involved in the bip olarization of wild type and acentriolar mitoses are different, and th at centrioles play a role in the spatial organization of the nucleatin g material containing gamma-tubulin.