Crystal structure of γ-tubulin complex protein GCP4 provides insight into microtubule nucleation

Microtubule nucleation in all eukaryotes involves gamma-tubulin small complexes (gamma TuSCs) that comprise two molecules of g-tubulin bound to gamma-tubulin complex proteins (GCPs) GCP2 and GCP3. In many eukaryotes, multiple gamma TuSCs associate with GCP4, GCP5 and GCP6 into large gamma-tubulin ring complexes (gamma TuRCs). Recent cryo-EM studies indicate that a scaffold similar to gamma TuRCs is formed by lateral association of gamma TuSCs, with the C-terminal regions of GCP2 and GCP3 binding gamma-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 gamma-tubulin. The human GCP4 structure is the prototype for all GCPs, as it can be precisely positioned within the gamma TuSC envelope, revealing the nature of protein-protein interactions and conformational changes regulating nucleation activity.