STABLE EXPRESSION OF HETEROLOGOUS MICROTUBULE-ASSOCIATED PROTEINS (MAPS) IN CHINESE-HAMSTER OVARY CELLS - EVIDENCE FOR DIFFERING ROLES OF MAPS IN MICROTUBULE ORGANIZATION

To study the effects of microtubule-associated proteins (MAPs) on in v ivo microtubule assembly, cDNAs containing the complete coding sequenc es of a Drosophila 205-kD heat stable MAP, human MAP 4, and human tau were stably transfected into CHO cells. Constitutive expression of the transfected genes was low in most cases and had no obvious effects on the viability of the transfected cell lines. High levels of expressio n, as judged by Western blots, immunofluorescence, and Northern blots, could be induced by treating cells with sodium butyrate. High levels of MAPs were maintained for at least 24-48 h after removal of the sodi um butyrate. Immunofluorescence analysis indicated that all three MAPs bound to cellular microtubules, but only the transfected tau caused a rearrangement of microtubules into bundles. Despite high levels of ex pression of these exogenous MAPs and the bundling of microtubules in c ells expressing tau, transfected cells had normal levels of assembled and unassembled tubulin. With the exception of the tau-induced bundles , microtubules in transfected cells showed the same sensitivity as con trol cells to microtubule depolymerization by Colcemid. Further, all t hree MAPs were ineffective in reversing the taxol-dependent phenotype of a CHO mutant cell line. The absence of a quantitative effect of any of these heterologous proteins on the assembly of tubulin suggests th at these MAPs may have different roles in vivo from those inferred pre viously from in vitro experiments.