Behavior of delta-tubulin during spindle formation in Xenopus oocytes: requirement of cytoplasmic dynein-dependent translocation.

Vertebrate oocytes do not contain centrosomes and therefore form an acentrosomal spindle during oocyte maturation. gamma-Tubulin is known to be essential for nucleation of microtubules at centrosomes, but little is known about the behaviour and role of gamma-tubulin during spindle formation in oocytes. We first observed sequential localization of gamma-tubulin during spindle formation in Xenopus oocytes. gamma-Tubulin assembled in the basal regions of the germinal vesicle (GV) at the onset of germinal vesicle breakdown (GVBD) and remained on the microtubule-organizing centre (MTOC) until a complex of the MTOC and transient-microtubule array (TMA) reached the oocyte surface. Prior to bipolar spindle formation, oocytes formed an aggregation of microtubules and gamma-tubulin was concentrated at the centre of the aggregation. At the late stage of bipolar spindle formation, gamma-tubulin accumulated at each pole. Anti-dynein antibody disrupted the localization of gamma-tubulin, indicating that the translocation described above is dependent on dynein activity. We finally revealed that XMAP215, a microtubule-associated protein cooperating with gamma-tubulin for the assembly of microtubules, but not gamma-tubulin, was phosphorylated during oocyte maturation. These results suggest that gamma-tubulin is translocated by dynein to regulate microtubule organization leading to spindle formation and that modification of the molecules that cooperate with gamma-tubulin, but not gamma-tubulin itself, is important for microtubule reorganization.