Model for MT nucleation of γ-TuSCs at the SPB. (A) An extended ring of 17 γ-tubulin molecules is assembled as a nucleation platform. This template is stably associated with the SPB in a cell cycle–dependent manner and further stabilized by the nucleation of MTs. Tubulin dimers interacting more strongly with γ-tubulin than with the MT plus end are the basis for MT nucleation. (B) Model of how the excess of Tub4 and Spc98 could stabilize the γ-TuSC ring.