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The centrosome nucleates microtubule polymerization, affecting mkrotubule number, polarity, and structure. We use an in vitro system based on extmcts of Xenopus eggs to examine the role of y-tubulin in centrosome assembly and function. y-Tubuiin is present in the cytoplasm of frog eggs and vertebrate somatic cells in a large-255 complex. The egg extracts assemble centroaomes around sperm centrloies. Formation of a centrosome in the extract requires both the y-tubuiln complex and ATP and can take place in the absence of microtubulea. y-Tubulin is not present on the sperm prior to incubation in extract, but is recruited from the cytoplasm during centroaome assembly. The+ Mlin complex also binds to mkrotubuies, likely the minus end, indepemknt of the centrosome. These reeults suggest that y-tubulin is an essential component of the link between the centroeome and the microtubule, probably playing a direct …