Trends in Cell Biology
ReviewTargeting of γ-tubulin complexes to microtubule organizing centers: conservation and divergence
Section snippets
The γ-TuC as an MT nucleation template: self-oligomerization promoted by receptors at MT organizing centers (MTOCs) or assembly with γ-TuRC-specific proteins (GCPs)
MTs are polarized, hollow cylinders that are assembled by the polymerization of heterodimeric α/β-tubulin. In cells, MT nucleation primarily depends on γ-tubulin, a member of the tubulin superfamily. γ-Tubulin associates with defined partner proteins to form γ-TuCs, which then act as scaffolds for α/β-tubulin dimers to initiate polymerization. γ-TuCs are regulated by various factors that recruit γ-TuCs to different cellular locations, spatially controlling MT nucleation activity. Such sites
Budding yeast represents the simplest MT nucleation system: self-oligomerization of γ-TuSCs promoted by γ-TuSC receptors
In S. cerevisiae, Spc110 and Spc72 are the only γ-TuSC recruiting factors targeting γ-TuSCs to the nuclear and cytoplasmic side of the SPB, respectively (Figure 1) 19, 20, 21, 22, 23. The N-terminal region of Spc110 directly interacts with Spc98/GCP3 to recruit γ-TuSCs to the nuclear side of SPBs 20, 24. Overexpression of yeast γ-TuSC components does not induce ectopic MT nucleation [25], supporting the view that γ-TuSC activating factors reside at SPBs. Spc110-N in vitro data raise the
Increased complexity of MT nucleation sites: fission yeast and A. nidulans
By homology search and immunological approaches, proteins homologous to Spc110 have been identified from fungi to vertebrates 32, 33, 34, 35, 36, 37, 38. While the overall sequence similarity is low, these proteins all share a conserved pericentrin and AKAP450 centrosomal targeting (PACT) domain that is exclusively involved in the recruitment of the proteins to centrosomes or SPBs 36, 39, 40, 41 (except CG-NAP, which also localizes at the Golgi apparatus 42, 43, 44). Consistently, the lack of
γ-TuRC recruiting factors are employed at multiple MT nucleation sites during mitotic spindle formation: a burst of complexity in Metazoa
In S. pombe and most likely other fungi that employ closed mitosis, intranuclear spindle MTs are nucleated solely at SPBs during a normal cell cycle. Non-SPB MT nucleators are involved in cytoplasmic MT arrays but not mitotic spindle formation. By contrast, in most metazoan cells that undergo open mitosis, multiple MT nucleation pathways cooperatively contribute to the integrity of spindle formation and the efficiency of chromosome biorientation establishment. Accordingly, since the physical
Emergence of paralogs in vertebrates
Additional members of the same type of γ-TuC recruiting factor are likely to have evolved through gene duplication as paralogs. These putative paralogs not only share main structural and functional features but have evolved specialized functions for interphase MT arrays and/or mitotic spindles. For example, the MOZART protein has evolved into MOZART1 and MOZART2A/B. While MOZART1 is required for targeting γ-TuRCs at mitotic centrosomes and spindle formation [63], MOZART2A/B contributes to
Emergence of γ-TuC targeting factors differentially regulating multiple MTOCs in different cell types: NEDD1/GCP-WD and the augmin complex
Most, if not all, species of the two largest subphyla of metazoans, the Chordata (including vertebrates) and the Arthropoda, have evolved to contain a complete set of GCP proteins (GCP2–6) and additional γ-TuC recruiting factors that display no homology to γ-TuSC receptors: NEDD1 and the augmin complex. Depending on the cell type, NEDD1 and the augmin complex can be employed for different MT nucleation sites. Such complexity in function may reflect the adaptation of spindle assembly to more
Concluding remarks
Studies of the γ-TuC and its recruiting factors in various model organisms are revealing formerly unappreciated levels of diversity in MT nucleation. The structural features of γ-TuSC receptors as well as the mechanisms they adopt to promote γ-TuSC oligomerization and γ-TuRC assembly are identifying the common regulation strategy behind the diversity. Understanding the function of γ-TuC recruiting factors in spindle assembly will help us reveal the various degrees of dependency on multiple MT
Acknowledgments
The authors were supported by a grant from the Deutsche Forschungsgemeinschaft (Schi 295/4-2). They thank I. Hagan for improvement of the writing style and critical comments on the manuscript. They apologize to colleagues whose work could not be discussed or cited owing to space limitations.
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