Microtubule nucleation from an asymmetric γ-TuRC. (A) γ-TuSC and γ-TuRC viewed from the top. A “closed” conformation at positions 1–8 enables contacts between neighboring γ-tubulin molecules. GCPs in positions 9–14 are spaced farther apart (“open”). Actin, MOZART1 (Mzt1), and an N-terminal extension of GCP6 are part of the luminal bridge. (B) Side views of γ-TuRCs in partly open and closed conformations. A minimal nucleus of tubulin dimers (α/β) can form at positions 1–8. Lateral association of additional dimers completes the cylinder, thus matching a perfect template for nucleation. GCPs in positions 9–14 undergo a conformational switch that might occur stochastically or driven by allosteric effects of α/β-tubulin or regulatory proteins.