Models of nucleation complex attachment and activation. (a) In the absence of c-tubulin ring complex (c-TuRC)-specific components, as in Saccharomyces, Spc110 or its equivalent directly attaches c-tubulin small complex (c-TuSC) to microtubule-organizing centers (MTOCs), promoting ring assembly. (b) In organisms with complete c-TuRCs, active complexes attach to organizing centers directly through c-TuSCs or potentially through unique sites in the c-TuRC-specific components. Localization of c-TuRCs at non-MTOC locations, such as within the mitotic spindle, is mediated through the c-TuRC-specific proteins. In both models, c-TuSC interactions define the geometry of the nucleating template. Reprinted with permission .