The γ-tubulin ring complex (γTuRC) is important for microtubule nucleation from the centrosome. In addition to γ-tubulin, the Drosophila γTuRC contains at least six subunits, three of which [Drosophila gamma ring proteins (Dgrips) 75/d75p, 84, and 91] have been characterized previously. Dgrips84 and 91 are present in both the small γ-tubulin complex (γTuSC) and the γTuRC, while the remaining subunits are found only in the γTuRC. To study γTuRC assembly and function, we first reconstituted γTuSC using the baculovirus expression system. Using the reconstituted γTuSC, we showed for the first time that this subcomplex of the γTuRC has microtubule binding and capping activities. Next, we characterized two new γTuRC subunits, Dgrips128 and 163, and showed that they are centrosomal proteins. Sequence comparisons among all known γTuRC subunits revealed two novel sequence motifs, which we named grip motifs 1 and 2. We found that Dgrips128 and 163 can each interact with γTuSC. However, this interaction is insufficient for γTuRC assembly.

Chromosomes are dynamic structures that are reorganized during the cell cycle to optimize them for distinct functions. SMC and non-SMC condensin proteins associate into complexes that have been implicated in the process of chromosome condensation. The roles of the individual non-SMC subunits of the complex are poorly understood, and mutations in the CAP-G subunit have not been described in metazoans. Here we elucidate a role for dCAP-G in chromosome condensation and cohesion in Drosophila. We illustrate the requirement of dCAP-G for condensation during prophase and prometaphase; however, we find that alternate mechanisms ensure that replicated chromosomes are condensed prior to metaphase. In contrast, dCAP-G is essential for chromosome condensation in metaphase of single, unreplicated sister chromatids, suggesting that there is an interplay between replicated chromatids and the condensin complex. In the dcap-g mutants, defects in sister-chromatid separation are also observed. Chromatid arms fail to resolve in prophase and are unable to separate at anaphase, whereas sister centromeres show aberrant separation in metaphase and successfully move to spindle poles at anaphase. We also identified a role for dCAP-G during interphase in regulating heterochromatic gene expression.

Production of haploid gametes relies on the specially regulated meiotic cell cycle. Analyses of the role of essential mitotic regulators in meiosis have been hampered by a shortage of appropriate alleles in metazoans. We characterized female-sterile alleles of the condensin complex component dcap-g and used them to define roles for condensin in Drosophila female meiosis. In mitosis, the condensin complex is required for sister-chromatid resolution and contributes to chromosome condensation. In meiosis, we demonstrate a role for dcap-g in disassembly of the synaptonemal complex and for proper retention of the chromosomes in a metaphase I-arrested state. The chromosomal passenger complex also is known to have mitotic roles in chromosome condensation and is required in some systems for localization of the condensin complex. We used the QA26 allele of passenger component incenp to investigate the role of the passenger complex in oocyte meiosis. Strikingly, in incenp QA26 mutants maintenance of the synaptonemal complex is disrupted. In contrast to the dcap-g mutants, the incenp mutation leads to a failure of paired homologous chromosomes to biorient, such that bivalents frequently orient toward only one pole in prometaphase and metaphase I. We show that incenp interacts genetically with ord, suggesting an important functional relationship between them in meiotic chromosome dynamics. The dcap-g and incenp mutations cause maternal effect lethality, with embryos from mutant mothers arrested in the initial mitotic divisions. O RGANISMS that undergo sexual reproduction utilize a specialized cell cycle, meiosis, to generate haploid gametes. Precise partitioning of the genome in meiosis is essential so that diploidy is reestablished upon fertilization, which is critical for embryonic development (Hassold and Hunt 2001). Meiosis employs distinct regulatory mechanisms such that the DNA is replicated exactly once and then divided twice without an additional intervening round of replication.In preparation for meiotic divisons, homologs pair and, in many systems, a proteinaceous structure, the synaptonemal complex (SC), forms an axis between homologs and regulates meiotic recombination (Page and Hawley 2003). Crossover events generate covalent linkages between homologs. These, in combination with sister-chromatid cohesion distal to the chiasmata (the physical structures resulting from crossing over), allow homologs to remain physically attached after SC disassembly and to thereby coordinate their movements.In meiosis I, homologs biorient on the spindle while sister chromatids coorient toward a single pole (reviewed in Petronczki et al. 2003). Release of cohesion distal to the chiasmata at the onset of anaphase I allows homologs to move apart; maintenance of centromere cohesion holds sister chromatids together as they travel toward a single spindle pole. The enduring attachment between sister chromatids is essential for them to biorient on the spindle in meiosis II. Centromere cohesion is severed at the onset of anaphase ...