Centrioles are essential components of centrosome, the main microtubule-organizing center of animal cells required for robust spindle bipolarity [1,2]. They are duplicated once during the cell cycle [3], and the duplication involves assembly of a cartwheel on the pre-existing centriole followed by assembly of triplet microtubules around the cartwheel [4,5]. Although the molecular details of cartwheel formation are understood [6][7][8][9][10][11][12][13], the mechanisms initiating the formation of centriolar microtubules are not known. Here, we show that the central component of cartwheel, HsSAS-6 plays a crucial role in the formation of centriolar microtubules by interacting with the microtubule nucleation machinery, g-tubulin ring complex (g-TuRC) in human cells. The globular N terminus and the central coiled-coil domain of SAS-6 are required for formation of the cartwheel [7,14], whereas the function of its C-terminal outer cartwheel region in centriole duplication remains unclear. We find that deletion of HsSAS-6 C terminus disrupts microtubule formation in daughter centriole, and as a result, cells fail to form the new centriole. Consequently, this results in mitotic cells having only two centrioles localized at a single site. Detailed molecular analyses showed that HsSAS-6 interacts with the g-TuRC proteins and associates with the g-TuRC at the centrosome, and furthermore, the C terminus is essential for this association. High-resolution microscopy revealed localization of the g-TuRC protein, g-tubulin as multiple lobes surrounding the HsSAS-6-containing central hub in the centriole. Together, the results indicate that HsSAS-6 regulates centriolar microtubule assembly by anchoring g-TuRCs to the pro-centriole at the onset of daughter centriole formation.
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