Amitosis requires γ-tubulin-mediated microtubule assembly in Tetrahymena thermophila

To reveal the molecular systems involved in the division of a cell and its contents during cell proliferation is one of the major subjects in cell biology. Although cytoskeletal organization during mitosis has been well studied, consensus on the molecular basis of amitosis has not been achieved. Here we adapted an immunofluorescence method and investigated the cellular localization of γ-tubulin and microtubules (MTs) in dividing Tetrahymena. Although the macronucleus (Mac) lacks a bipolar spindle, γ-tubulin and MTs are specifically detected in the dividing Mac and show a marked change in the pattern of localization. First, γ-tubulin and MTs appear in whole Mac, then, γ-tubulin gathers at the center of the Mac where the aster-like structure of MTs forms. On Mac expansion, MTs associated with numerous dots of γ-tubulin are reorganized into longitudinally arranged bundles, suggesting that the mutual sliding of each filament and polymerization of MTs may induce Mac expansion. Moreover, normal Mac expansion and equal segregation of the Mac are severely disturbed when γ-tubulin is shut off. We propose that γ-tubulin-mediated MT assembly is required in Mac amitosis of Tetrahymena.