Functional Analysis of γ-Tubulin Complex Proteins Indicates Specific Lateral Association via Their N-terminal Domains

Microtubules are nucleated from multiprotein complexes containing gamma-tubulin and associated gamma-tubulin complex proteins (GCPs). Small complexes (gamma TuSCs) comprise two molecules of gamma-tubulin bound to the C-terminal domains of GCP2 and GCP3. gamma TuSCs associate laterally into helical structures, providing a structural template for microtubule nucleation. In most eukaryotes gamma TuSCs associate with additional GCPs (4, 5, and 6) to form the core of the so-called gamma-tubulin ring complex (gamma TuRC). GCPs2-6 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 gamma 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 gamma-tubulin. Moreover, we show that binding to gamma-tubulin is not essential for integrating into the helical complex.