Regulation of tubulin synthesis and cell cycle progression in mammalian cells by γ-tubulin-mediated microtubule nucleation

We have previously shown that gamma-tubulin, the third member of the tubulin family that functions in microtubule nucleation, when overexpressed, accumulates throughout the cytoplasm and forms numerous ectopic microtubule nucleation sites in mammalian cells (Shu and Joshi [1995] J. Cell. Biol. 130:1137-1147). We now show that overexpression of gamma-tubulin differentially upregulates the synthesis of alpha- and beta-tubulins in mammalian cells. Surprisingly, despite a dramatic increase in the level of gamma-tubulin protein in transfected cells, there is no obvious alteration in the level of endogenous gamma-tubulin mRNA, suggesting that synthesis of gamma-tubulin might employ a regulatory mechanism other than the autoregulatory pathway shared by alpha- and beta-tubulins. Interestingly, a significant number of mammalian cells transfected with gamma-tubulin fail to form normal bipolar mitotic spindle during mitosis; instead, numerous microtubules occur in the cytoplasm intermingled with the condensed chromosomes. In addition, they reduplicate their DNA after an abnormal mitotic exit. These results thus suggest that the number of microtubule nucleation sites, or even gamma-tubulin itself, might play an important role in the regulation of tubulin synthesis as well as cell cycle progression. (C) 2001 Wiley-Liss, Inc.