- Tubulin ring complexes regulate microtubule plus end dynamics

The organization and dynamics of microtubules (MTs) are essential for different cellular processes such as migration or division. In animal cells, MT nucleation usually occurs at the centrosome, where -tubulin plays a key role. This protein is organized in multiprotein complexes (Moritz et al., 1995; Zheng et al., 1995; Raynaud-Messina and Merdes, 2007). In Drosophila melanogaster, two main complexes have been characterized (Oegema et al., 1999). The -tubulin small complex (-TuSC) is composed of -tubulin and two other proteins, Dgrip84 and -91. The -tubulin ring complex (-TuRC) is formed of -TuSCs associated with additional grip motif polypeptides, Dgrip75, -128, and -163, and a WD motif protein, Dgp71WD. Components of the -TuSC are highly conserved in eukaryotes. Deletion of any -TuSC subunit is lethal both in fungi and in Drosophila. This loss of function results in the accumulation of cells in mitosis, which is associated with defects such as monopolar spindles, impairment in centrosome maturation, and aneuploidy (Oakley and Oakley, 1989; Sunkel et al., 1995; Knop and Schiebel, 1997; Barbosa et al., 2000; Colombié et al., 2006). In contrast, -TuRC–specific grip motif proteins are nonessential for viability in yeast and in Drosophila (Fujita et al., 2002; Schnorrer et al., 2002; Anders et al., 2006; Vérollet et al., 2006; Vogt et al., 2006). Nevertheless, these grip proteins are necessary for the assembly of the large complex, for efficient mitotic progression (Vérollet et al., 2006; Izumi et al., 2008), and for specialized functions such as the organization of MT subarrays during oogenesis (Schnorrer et al., 2002; Vogt et al., 2006). Analysis of the nongrip component Dgp71WD reveals that this protein regulates the function and targeting of the -TuRC to the centrosome and along spindle MTs (Haren et al., 2006; Lüders et al., 2006). The -tubulin complexes are involved in the nucleation of MTs from centrosomes but also from chromatin and spindle MTs (Joshi et al., 1992; Sunkel et al., 1995; Knop and Schiebel, 1997; Oegema et al., 1999; Wilde and Zheng, 1999; Wiese and Zheng, 2000; Goshima et al., 2008; Zhu et al., 2008). Additional observations in fungi suggest that -tubulin and its partners also affect the organization or dynamics of MTs (Oakley and Oakley, 1989; Paluh et al., 2000; Vardy and Toda, 2000; Fujita et al., 2002; Venkatram et al., 2004; Zimmerman and Chang, 2005; Masuda et al., 2006). To determine whether and  -Tubulin is critical for the initiation and regulation of microtubule (MT) assembly. In Drosophila melanogaster, it acts within two main complexes: the -tubulin small complex (-TuSC) and the -tubulin ring complex (-TuRC). Proteins specific of the -TuRC, although nonessential for viability, are required for efficient mitotic progression. Until now, their role during interphase remained poorly understood. Using RNA interference in Drosophila S2 cells, we show that the -TuRC is not critical for overall MT organization. However, depletion of any component of this complex results in an increase of MT dynamics. Combined immunofluorescence and live imaging analysis allows us to reveal that the -TuRC localizes along interphase MTs and that distal -tubulin spots match with sites of pause or rescue events. We propose that, in addition to its role in nucleation, the -TuRC associated to MTs may regulate their dynamics by limiting catastrophes. -Tubulin ring complexes regulate microtubule plus end dynamics