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Centrioles function as the major components of centrosomes, which organize microtubule (MT) arrays in proliferating cells, and as basal bodies for primary cilia formation in quiescent cells. Centrioles and basal bodies are structurally similar, barrel-shaped organelles composed of MTs. In proliferating cells, two new centrioles, termed procentrioles, form during the S phase of the cell cycle in close proximity to the proximal ends of the two preexisting parental centrioles, often at a near-orthogonal angle [1]. Considerable progress has been made toward understanding the biogenesis of centrioles, but the mechanisms that determine their lengths remain unknown. Here we show that overexpression of the centriolar protein CPAP in human cells enhances the accumulation of centriolar tubulin, leading to centrioles of strikingly increased length. Consistent with earlier work [2], we also find that elongated MT structures can be induced by depletion of the distal end-capping protein CP110 from centrioles. Importantly, though, these structures differ from genuine primary cilia. We thus propose that CPAP and CP110 play antagonistic roles in determining the extent of tubulin addition during centriole elongation, thereby controlling the length of newly formed centrioles.
