The signaling pathways that mediate the transforming activity of the Rac1 GTPase remain to be determined. In the present study, we used effector domain mutants of the constitutively activated Rac(61L) mutant that display differential transforming activities and differential activation of downstream effector pathways to investigate the contribution of p70 S6 kinase (p70S6K) to Rac1 transformation and to decipher the signaling pathways leading from Rac1 to p70S6K. First, we found that Rac1 transforming activity could be dissociated from Rac1 activation of p70S6K. A weakly transforming Rac1 mutant retained the ability to activate p70S6K, whereas some potently transforming effector mutants were impaired in their ability to activate p70S6K. These data suggest that p70S6K is not necessary to promote full Rac1 transforming activity. We also found a strong correlation between the ability of the Rac(61L) effector mutants to activate p70S6K and their ability to activate the JNK mitogen-activated protein kinase. We found that the MLK3 serine/threonine kinase activated JNK and p70S6K, whereas activation of p70S6K by Rac(61L) was significantly inhibited by dominant-negative MLK3. Additionally, the ability of the Rac(61L) effector mutants to activate MLK3 correlated well with their ability to activate p70S6Kand JNK. Taken together, these results provide evidence that Rac1 coordinately activates p70S6K and JNK via MLK3 activation. Finally, we found that co-expression of wild type, but not kinase-dead, MLK3 significantly inhibited Rac1 transforming activity. These results suggest that MLK3 may be a negative regulator of the growth-promoting and transforming properties of Rac1.
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Published, JBC Papers in Press, November 16, 2001, DOI 10.1074/jbc.M109379200
This work was supported by National Institutes of Health Grants CA42978, CA55008, and CA63071 (all to C. J. D.) and Grant GM59767 (to L. M. G.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.