Distinct Dgrip84 Isoforms Correlate with Distinct γ-Tubulins inDrosophila

Abstract

γ-Tubulin is an indispensable component of the animal centrosome and is required for proper microtubule organization. Within the cell, γ-tubulin exists in a multiprotein complex containing between two (some yeasts) and six or more (metazoa) additional highly conserved proteins named gamma ring proteins (Grips) or gamma complex proteins (GCPs). γ-Tubulin containing complexes isolated from Xenopus eggs or Drosophila embryos appear ring-shaped and have therefore been named the γ-tubulin ring complex (γTuRC). Curiously, many organisms (including humans) have two distinct γ-tubulin genes. In Drosophila, where the two γ-tubulin isotypes have been studied most extensively, the γ-tubulin genes are developmentally regulated: the “maternal” γ-tubulin isotype (named γTub37CD according to its location on the genetic map) is expressed in the ovary and is deposited in the egg, where it is thought to orchestrate the meiotic and early embryonic cleavages. The second γ-tubulin isotype (γTub23C) is ubiquitously expressed and persists in most of the cells of the adult fly. In those rare cases where both γ-tubulins coexist in the same cell, they show distinct subcellular distributions and cell-cycle-dependent changes: γTub37CD mainly localizes to the centrosome, where its levels vary only slightly with the cell cycle. In contrast, the level of γTub23C at the centrosome increases at the beginning of mitosis, and γTub23C also associates with spindle pole microtubules. Here, we show that γTub23C forms discrete complexes that closely resemble the complexes formed by γTub37CD. Surprisingly, however, γTub23C associates with a distinct, longer splice variant of Dgrip84. This may reflect a role for Dgrip84 in regulating the activity and/or the location of the γ-tubulin complexes formed with γTub37CD and γTub23C.