Background: Growth factors activate an array of cell survival signaling pathways. Mitogen-activated protein (MAP) kinases transduce signals emanating from their upstream activators MAP kinase kinases (MEKs). The MEK–MAP kinase signaling cassette is a key regulatory pathway promoting cell survival. The downstream effectors of the mammalian MEK–MAP kinase cell survival signal have not been previously described.

Results: We identify here a pro-survival role for the serine/threonine kinase Rsk1, a downstream target of the MEK–MAP kinase signaling pathway. In cells that are dependent on interleukin-3 (IL-3) for survival, pharmacological inhibition of MEKs antagonized the IL-3 survival signal. In the absence of IL-3, a kinase-dead Rsk1 mutant eliminated the survival effect afforded by activated MEK. Conversely, a novel constitutively active Rsk1 allele restored the MEK–MAP kinase survival signal. Experiments in vitro and in vivo demonstrated that Rsk1 directly phosphorylated the pro-apoptotic protein Bad at the serine residues that, when phosphorylated, abrogate Bad's pro-apoptotic function. Constitutively active Rsk1 caused constitutive Bad phosphorylation and protection from Bad-modulated cell death. Kinase-inactive Rsk1 mutants antagonize Bad phosphorylation. Bad mutations that prevented phosphorylation by Rsk1 also inhibited Rsk1-mediated cell survival.

Conclusions: These data support a model in which Rsk1 transduces the mammalian MEK–MAP kinase signal in part by phosphorylating Bad.