Overexpression of an epitope-tagged beta-tubulin in Chinese hamster ovary cells causes an increase in endogenous alpha-tubulin synthesis

Abstract

A Chinese hamster beta-tubulin cDNA, engineered to express a 9 amino acid epitope from the influenza hemagglutinin antigen (HA), was transfected into Chinese hamster ovary (CHO) cells. The recombinant protein (HA beta 1-tubulin) appeared to behave normally by the following criteria: immunofluorescence indicated that HA beta 1-tubulin incorporated into all classes of interphase and spindle microtubules as well as microtubule organizing centers. The sensitivity of the cells expressing HA beta 1-tubulin to Colcemid and taxol was unchanged. A 210 kD microtubule associated protein (MAP) remained associated with microtubules that incorporate HA beta 1-tubulin. The synthesis of both endogenous beta-tubulin and HA beta 1-tubulin was repressed by colchicine. The HA beta 1-tubulin incorporated into microtubules to the same extent as the endogenous beta-tubulin, and the overall extent of microtubule assembly in transfected cells was unchanged. Finally, transfected cells had normal growth rates and morphologies. When effects on endogenous tubulin production were measured, it was found that expression of the HA beta 1-tubulin reduced the synthesis of endogenous wild-type beta-tubulin but increased the synthesis of alpha-tubulin. At steady state, a small increase in total tubulin consistent with the increased synthesis of alpha-tubulin was found. The results indicate that expression of excess exogenous beta-tubulin perturbs the synthesis of endogenous alpha-tubulin in a manner that is not easily explained by current models of tubulin regulation. The changes in tubulin synthesis along with degradation of excess tubulin subunits may reflect mechanisms that exist to ensure coordinate levels of alpha- and beta-tubulin for assembly.