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Gamma Tubulin Ring Complexes As Minus End Capping Proteins During Centrosome Mediated Nucelation

1238 words - 5 pages

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Microtubule nucleation from centrosomes involves a lockwasher-shaped protein complex containing gamma-tubulin, named the gamma-tubulin ring complex (gammaTuRC). Here we investigate the mechanism by which the gammaTuRC nucleates microtubules, using a direct labelling method to visualize the behaviour of individual gammaTuRCs. A fluorescently-labelled version of the gammaTuRC binds to the minus ends of microtubules nucleated in vitro. Both gammaTuRC-mediated nucleation and binding of the gammaTuRC to preformed microtubules block further minus-end growth and prevent microtubule depolymerization. The gammaTuRC therefore acts as a minus-end-capping protein, as confirmed by ...view middle of the document...

, 1995; Knop and Schiebel, 1997; Barbosa et al., 2000; Colombié et al., 2006). In contrast, γ-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 γ-TuRC to the centrosome and along spindle MTs (Haren et al., 2006; Lüders et al., 2006).

The γ-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 γ-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 how γ-TuRC proteins could influence MT dynamics, we determined dynamic parameters on individual MTs in Drosophila S2 cells during interphase. For the first time in metazoan cells, we show that γ-TuRCs contribute to the regulation of MT dynamics, independently of their nucleation activity. The γ-TuRCs localize along MTs, where they limit catastrophe events, thus enhancing MT stability.

Tubulin is a small globular protein found in all eukaryotic cells. The tubulin family represents about 3-4% of the total protein content in a cell and its members include α-, β-, γ-, δ-, ε-, and ζ-tubulin [1]. Although the different forms of tubulin are similar, they can have different cellular locations and functions. It should be noted that although both actin and tubulin are basic components of the cytoskeleton and possess the ability to hydrolyze NTP, they are evolutionarily distinct with actin being related in structure to hexokinase and tubulins being distantly related to heterotrimeric G-proteins and other GTPases such as Ras [2].

[pic]Figure 1. y-TURC

Repeating units of ~110kDa α/β-tubulin heterodimers are the primary component of microtubules. Microtubule assembly starts with the assembly of a tubulin complex. Here, γ-tubulin and other protein components form the γ-tubulin ring complex (γ-TuRC) [2]. The specific cellular roles for additional tubulins such as δ-, ε- and ζ- tubulins are less well known, but they can be found in centrioles and may be involved during mitotic spindle assembly and cell division [3]. Post-translational modification of...

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