The centrosome is the major microtubule organizing structure in somatic cells. Centrosomal microtubule nucleation depends on the protein γ-tubulin. In mammals, γ-tubulin associates with additional proteins into a large complex, the γ-tubulin ring complex (γTuRC). We characterize NEDD1, a centrosomal protein that associates with γTuRCs. We show that the majority of γTuRCs assemble even after NEDD1 depletion but require NEDD1 for centrosomal targeting. In contrast, NEDD1 can target to the centrosome in the absence of γ-tubulin. NEDD1-depleted cells show defects in centrosomal microtubule nucleation and form aberrant mitotic spindles with poorly separated poles. Similar spindle defects are obtained by overexpression of a fusion protein of GFP tagged to the carboxy-terminal half of NEDD1, which mediates binding to γTuRCs. Further, we show that depletion of NEDD1 inhibits centriole duplication, as does depletion of γ-tubulin. Our data suggest that centriole duplication requires NEDD1-dependent recruitment of γ-tubulin to the centrosome.

Microtubule nucleation in all eukaryotes involves γ-tubulin small complexes (γTuSCs) that comprise two molecules of γ-tubulin bound to γ-tubulin complex proteins (GCPs) GCP2 and GCP3. In many eukaryotes, multiple γTuSCs associate with GCP4, GCP5 and GCP6 into large γ-tubulin ring complexes (γTuRCs). Recent cryo-EM studies indicate that a scaffold similar to γTuRCs is formed by lateral association of γTuSCs, with the C-terminal regions of GCP2 and GCP3 binding γ-tubulin molecules. However, the exact role of GCPs in microtubule nucleation remains unknown. Here we report the crystal structure of human GCP4 and show that its C-terminal domain binds directly to γ-tubulin. The human GCP4 structure is the prototype for all GCPs, as it can be precisely positioned within the γTuSC envelope, revealing the nature of protein-protein interactions and conformational changes regulating nucleation activity.

Hinge prolines can be considered as 'quaternary structure helpers'. The presence of a proline should be considered when searching for a determinant of oligomerization with arm exchange and could be used to engineer synthetic oligomers or to displace a monomers to oligomers equilibrium by mutation of this proline residue.

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Edited by Velia FowlerMicrotubules are nucleated from multiprotein complexes containing ␥-tubulin and associated ␥-tubulin complex proteins (GCPs). Small complexes (␥TuSCs) comprise two molecules of ␥-tubulin bound to the C-terminal domains of GCP2 and GCP3. ␥TuSCs associate laterally into helical structures, providing a structural template for microtubule nucleation. In most eukaryotes ␥TuSCs associate with additional GCPs (4, 5, and 6) to form the core of the so-called ␥-tubulin ring complex (␥TuRC). GCPs 2-6 constitute a family of homologous proteins. Previous structural analysis and modeling of GCPs suggest that all family members can potentially integrate into the helical structure. Here we provide experimental evidence for this model. Using chimeric proteins in which the N-and C-terminal domains of different GCPs are swapped, we show that the N-terminal domains define the functional identity of GCPs, whereas the C-terminal domains are exchangeable. FLIM-FRET experiments indicate that GCP4 and GCP5 associate laterally within the complex, and their interaction is mediated by their N-terminal domains as previously shown for ␥TuSCs. Our results suggest that all GCPs are incorporated into the helix via lateral interactions between their N-terminal domains, whereas the C-terminal domains mediate longitudinal interactions with ␥-tubulin. Moreover, we show that binding to ␥-tubulin is not essential for integrating into the helical complex.In all eukaryotes, microtubules are nucleated from specialized multiprotein complexes containing ␥-tubulin and associated proteins (1-3). These complexes resemble small rings by electron microscopy and are thus called ␥-tubulin ring complexes (␥TuRCs) 4 (4 -7). Closer inspection revealed that these ␥TuRCs are helices of one turn, with the two ends overlapping. They are ubiquitous and essential for viability: the growth of new microtubules is crucial to drive mitotic spindle formation and cell division. ␥TuRCs are mainly composed of ␥-tubulin and of proteins of the GCP (␥-tubulin complex protein) family. GCPs are characterized by sequence homology in two specific regions, also referred to as the grip1 and grip2 motifs (8). Five members of this family are known: GCPs 2, 3, 4, 5, and 6. GCPs 2 and 3 associate with ␥-tubulin to form a V-shaped subcomplex, called ␥-tubulin small complex (␥TuSC). GCPs 2 and 3 constitute the arms of the V, interacting laterally via their N-terminal domains (Fig. 1, A and B). The C-terminal domains are located at the two tips of the V, each binding one molecule of ␥-tubulin (9). In the budding yeast Saccharomyces cerevisiae, ␥TuSCs are directly recruited to the spindle pole body (yeast centrosome equivalent) by the protein Spc110. Oligomers of Spc110 interact with the basis of the V-shaped ␥TuSCs and stabilize their lateral association (10 -13). Likely, seven ␥TuSCs assemble stepwise into a helix of one turn plus a small overlap (14). In this helical array, the ␥-tubulin molecules are exposed to form a platform from which ␣/␤-tubulin dimers assemble into...

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Mitotic spindle formation in animal cells involves microtubule nucleation from two centrosomes that are positioned at opposite sides of the nucleus. Microtubules are captured by the kinetochores and stabilized. In addition, microtubules can be nucleated independently of the centrosome and stabilized by a gradient of Ran—GTP, surrounding the mitotic chromatin. Complex regulation ensures the formation of a bipolar apparatus, involving motor proteins and controlled polymerization and depolymerization of microtubule ends. The bipolar apparatus is, in turn, responsible for faithful chromosome segregation. During recent years, a variety of experiments has indicated that defects in specific motor proteins, centrosome proteins, kinases and other proteins can induce the assembly of aberrant spindles with a monopolar morphology or with poorly separated poles. Induction of monopolar spindles may be a useful strategy for cancer therapy, since ensuing aberrant mitotic exit will usually lead to cell death. In this review, we will discuss the various underlying molecular mechanisms that may be responsible for monopolar spindle formation.

We report here that PBP1a can dimerize but does not interact with PBP1b to form PBP1a/PBP1b heterodimers in Escherichia coli. These findings support the idea of a relevant involvement of dimerization of both PBP1a and PBP1b during murein synthesis and suggest the existence of different peptidoglycan synthesis complexes.