δ-Tubulin and ɛ-tubulin: two new human centrosomal tubulins reveal new aspects of centrosome structure and function

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Abstract
The centrosome organizes microtubules, which are made up of α-tubulin and β-tubulin, and contains centrosome-bound γ-tubulin, which is involved in microtubule nucleation. Here we identify two new human tubulins and show that they are associated with the centrosome. One is a homologue of the Chlamydomonas δ-tubulin Uni3, and the other is a new tubulin, which we have named ɛ-tubulin. Localization of δ-tubulin and ɛ-tubulin to the centrosome is independent of microtubules, and the patterns of localization are distinct from each other and from that of γ-tubulin. δ-Tubulin is found in association with the centrioles, whereas ɛ-tubulin localizes to the pericentriolar material. ɛ-Tubulin exhibits a cell-cycle-specific pattern of localization, first associating with only the older of the centrosomes in a newly duplicated pair and later associating with both centrosomes. ɛ-Tubulin thus distinguishes the old centrosome from the new at the level of the pericentriolar material, indicating that there may be a centrosomal maturation event that is marked by the recruitment of ɛ-tubulin.