Centrosomes are the cellular organelles that nucleate microtubules (MTs) via the activity of gamma-tubulin ring complex(s) (γTuRC) bound to the pericentriolar material of the centrosomes. BRCA1, the breast and ovarian cancer specific tumor suppressor, inhibits centrosomal MT nucleation via its ubiquitin ligase activity, and one of the known BRCA1 substrates is the key γTuRC component, γ-tubulin. We analyzed the mechanism by which BRCA1 regulates centrosome function using an in vitro reconstitution assay, which includes separately staged steps. Our results are most consistent with a model by which the BRCA1 ubiquitin ligase modifies both γ-tubulin plus a second centrosomal protein that controls localization of γTuRC to the centrosome. We suggest that this second protein is an adapter protein or protein complex that docks γ-TuRC to the centrosome. By controlling γ-TuRC localization, BRCA1 appropriately inhibits centrosome function, and loss of BRCA1 would result in centrosome hyperactivity, supernumerary centrosomes and, possibly, aneuploidy. ©2007 Landes Bioscience.