γ‐Tubulin Ring Complexes and EB1 play antagonistic roles in microtubule dynamics and spindle positioning

γ‐Tubulin is critical for microtubule ( MT ) assembly and organization. In metazoa, this protein acts in multiprotein complexes called γ‐ T ubulin R ing C omplexes (γ‐ T u RC s). While the subunits that constitute γ‐ T ubulin S mall C omplexes (γ‐ T u SC s), the core of the MT nucleation machinery, are essential, mutation of γ‐ T u RC ‐specific proteins in Drosophila causes sterility and morphological abnormalities via hitherto unidentified mechanisms. Here, we demonstrate a role of γ‐ T u RC s in controlling spindle orientation independent of MT nucleation activity, both in cultured cells and in vivo , and examine a potential function for γ‐ T u RC s on astral MT s. γ‐ T u RC s locate along the length of astral MT s, and depletion of γ‐ T u RC ‐specific proteins increases MT dynamics and causes the plus‐end tracking protein EB 1 to redistribute along MT s. Moreover, suppression of MT dynamics through drug treatment or EB 1 down‐regulation rescues spindle orientation defects induced by γ‐ T u RC depletion. Therefore, we propose a role for γ‐ T u RC s in regulating spindle positioning by controlling the stability of astral MT s.