TRANSFORMATION OF MAMMALIAN-CELLS BY CONSTITUTIVELY ACTIVE MAP KINASEKINASE

Mitogen-activated protein (MAP) kinase kinase (MAPKK) activates MAP ki nase in a signal transduction pathway that mediates cellular responses to growth and differentiation factors. Oncogenes such as ras, src, ra f, and mos have been proposed to transform cells by prolonging the act ivated state of MAPKK and of components downstream in the signaling pa thway. To test this hypothesis, constitutively active MAPKK mutants we re designed that had basal activities up to 400 times greater than tha t of the unphosphorylated wild-type kinase. Expression of these mutant s in mammalian cells activated AP-1-regulated transcription. The cells formed transformed foci, grew efficiently in soft agar, and were high ly tumorigenic in nude mice. These findings indicate that constitutive activation of MAPKK is sufficient to promote cell transformation.