Published 13 February 2006. doi:10.1083/jcb1724iti5
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 172, Number 4, 480-480
Making 
-tubulin ring
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Spindles maintain some integrity despite the loss of components specific to  TuRC.
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-tubulin is required at the centrosome for the formation of bipolar spindles and arrives in the form of a large complex, TuRC. Vérollet et al. (page 517) and Haren et al. (page 505) find that a recently identified member of TuRC, Dgp71WD in Drosophila and its mammalian homologue NEDD1, is necessary for proper spindle formation. However, the protein functions are not fully conserved between the two species.
Two complexes in cells contain -tubulin. The smaller complex is TuSC, which contains -tubulin and two other proteins (GCP2 [Dgrip84] and GCP3 [Dgrip91]). TuRC is composed of TuSC, NEDD1/Dgp71WD, and three additional proteins (GCPs 4, 5, and 6).
Vérollet et al. found that depletion of any one of three TuRC-specific proteins (Drosophila homologues of GCP4, 5, or 6) disrupted the TuRC complex. Dgp71WD depletion did not affect complex formation, but frequently disrupted mitotic spindles. Most embryos lacking Dgp71WD were viable and appeared to survive by accumulating -tubulin at the centrosomes in the form of TuSC.
Meanwhile, Haren et al. showed that NEDD1 is absolutely required for centrosomal function and for accumulation of -tubulin at the centrosome in mammalian cells, but not for the assembly of TuRC. NEDD1-depleted cells resembled -tubulin mutants, with monopolar spindles forming in a large percentage of cells. These data agree with and extend recent work published by Lüders et al. (Nat. Cell Biol. 2005. doi:10.1038/NCB1349). Further analysis by Haren et al. demonstrated that NEDD1 is also required for duplication of the centrioles, which form the heart of the centrosome.
Why NEDD1 is essential in vertebrate cells, whereas flies can survive in the absence of Dgp71WD, may have to do with the abundance or functionality of the smaller TuSC in flies. Despite the apparent differences, the researchers expect that ultimately the main processes involving NEDD1/Dgp71WD and TuRC are likely to be similar between the organisms. The differences, they are finding now, may help them sort out possible models of centrosome assembly and function.
Rabiya S. Tuma
rabiya{at}nasw.org
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