Purification and reconstitution of Drosophila gamma-tubulin complexes

Regulation of microtubule assembly and organization in mitosis by the AAA+ ATPase Pontin

To identify novel proteins important for microtubule assembly in mitosis, we have used a centrosome-based complementation assay to enrich for proteins with mitotic functions. An RNA interference (RNAi)-based screen of these proteins allowed us to uncover 13 novel mitotic regulators. We carried out in-depth analyses of one of these proteins, Pontin, which is known to have several functions in interphase, including chromatin remodeling, DNA repair, and transcription. We show that reduction of Pontin by RNAi resulted in defects in spindle assembly in Drosophila S2 cells and in several mammalian tissue culture cell lines. Further characterization of Pontin in Xenopus egg extracts demonstrates that Pontin interacts with the gamma tubulin ring complex (gamma-TuRC). Because depletion of Pontin leads to defects in the assembly and organization of microtubule arrays in egg extracts, our studies suggest that Pontin has a mitosis-specific function in regulating microtubule assembly.

Phosphoregulation and depolymerization-driven movement of the Dam1 complex do not require ring formation

During mitosis, kinetochores form persistent attachments to microtubule tips and undergo corrective detachment in response to phosphorylation by Ipl1 (Aurora B) kinase. The Dam1 complex is required to establish and maintain bi-oriented attachment to microtubule tips in vivo, and it contains multiple sites phosphorylated by Ipl1 (Refs 2, 3, 4, 5, 6, 7, 8, 9, 10). Moreover, a number of kinetochore-like functions can be reconstituted in vitro with pure Dam1 complex. These functions are believed to derive from the ability of the complex to self-assemble into rings. Here we show that rings are not necessary for dynamic microtubule attachment, Ipl1-dependent modulation of microtubule affinity or the ability of Dam1 to move processively with disassembling microtubule tips. Using two fluorescence-based assays, we found that the complex exhibited a high affinity for microtubules (Kd of approximately 6 nM) that was reduced by phosphorylation at Ser 20, a single Ipl1 target residue in Dam1. Moreover, individual complexes underwent one-dimensional diffusion along microtubules and detached 2.5-fold more frequently after phosphorylation by Ipl1. Particles consisting of one to four Dam1 complexes - too few to surround a microtubule - were captured and carried by disassembling tips. Thus, even a small number of binding elements could provide a dynamic, phosphoregulated microtubule attachment and thereby facilitate accurate chromosome segregation.

Efficient protein trafficking requires trailer hitch, a component of a ribonucleoprotein complex localized to the ER in Drosophila

Translational control of localized messenger mRNAs (mRNAs) is critical for cell polarity, synaptic plasticity, and embryonic patterning. While progress has been made in identifying localization factors and translational regulators, it is unclear how broad a role they play in regulating basic cellular processes. We have identified Drosophila trailer hitch (tral) as a gene that is required for the proper secretion of the dorsal-ventral patterning factor Gurken, as well as the vitellogenin receptor Yolkless. Surprisingly, biochemical purification of Tral revealed that it is part of a large RNA-protein complex that includes the translation/localization factors Me31B and Cup as well as the mRNAs for endoplasmic reticulum (ER) exit site components. This complex is localized to subdomains of the ER that border ER exit sites. Furthermore, tral is required for normal ER exit site formation. These findings raise exciting new possibilities for how the mRNA localization machinery could interface with the classical secretory pathway to promote efficient protein trafficking in the cell.

Characterization of a Drosophila centrosome protein CP309 that shares homology with Kendrin and CG-NAP

The centrosome in animal cells provides a major microtubule-nucleating site that regulates the microtubule cytoskeleton temporally and spatially throughout the cell cycle. We report the identification in Drosophila melanogaster of a large coiled-coil centrosome protein that can bind to calmodulin. Biochemical studies reveal that this novel Drosophila centrosome protein, centrosome protein of 309 kDa (CP309), cofractionates with the gamma-tubulin ring complex and the centrosome-complementing activity. We show that CP309 is required for microtubule nucleation mediated by centrosomes and that it interacts with the gamma-tubulin small complex. These findings suggest that the microtubule-nucleating activity of the centrosome requires the function of CP309.

Characterization of a new gammaTuRC subunit with WD repeats

The gamma-tubulin ring complex (gammaTuRC), consisting of multiple protein subunits, can nucleate microtubule assembly. Although many subunits of the gammaTuRC have been identified, a complete set remains to be defined in any organism. In addition, how the subunits interact with each other to assemble into gammaTuRC remains largely unknown. Here, we report the characterization of a novel gammaTuRC subunit, Drosophila gamma ring protein with WD repeats (Dgp71WD). With the exception of gamma-tubulin, Dgp71WD is the only gammaTuRC component identified to date that does not contain the grip motifs, which are signature sequences conserved in gammaTuRC components. By performing immunoprecipitations after pair-wise coexpression in Sf9 cells, we show that Dgp71WD directly interacts with the grip motif-containing gammaTuRC subunits, Dgrips84, 91, 128, and 163, suggesting that Dgp71WD may play a scaffolding role in gammaTuRC organization. We also show that Dgrips128 and 163, like Dgrips84 and 91, can interact directly with gamma-tubulin. Coexpression of any of these grip motif-containing proteins with gamma-tubulin promotes gamma-tubulin binding to guanine nucleotide. In contrast, in the same assay Dgp71WD interacts with gamma-tubulin but does not facilitate nucleotide binding.