Phosphorylation of c-jun mediated by MAP kinases

Nuclear localization and regulation of erk- and rsk-encoded protein kinases

We demonstrate that members of the erk-encoded family of mitogen-activated protein (MAP) kinases (pp44/42mapk/erk) and members of the rsk-encoded protein kinases (RSKs or pp90rsk) are present in the cytoplasm and nucleus of HeLa cells. Addition of growth factors to serum-deprived cells results in increased tyrosine and threonine phosphorylation and in the activation of cytosolic and nuclear MAP kinases. Activated MAP kinases then phosphorylate (serine/threonine) and activate RSKs. Concurrently, a fraction of the activated MAP kinases and RSKs enter the nucleus. In addition, a distinct growth-regulated RSK-kinase activity (an enzyme[s] that phosphorylates recombinant RSK in vitro and that may be another member of the erk-encoded family of MAP kinases) was found associated with a postnuclear membrane fraction. Regulation of nuclear MAP kinase and RSK activities by growth factors and phorbol ester is coordinate with immediate-early gene expression. Indeed, in vitro, MAP kinase and/or RSK phosphorylates histone H3 and the recombinant c-Fos and c-Jun polypeptides, transcription factors phosphorylated in a variety of cells in response to growth stimuli. These in vitro studies raise the possibility that the MAP kinase/RSK signal transduction pathway represents a protein-Tyr/Ser/Thr phosphorylation cascade with the spatial distribution and temporal regulation that can account for the rapid transmission of growth-regulating information from the membrane, through the cytoplasm, and to the nucleus.

Activation of extracellular signal-regulated kinase, ERK2, by p21ras oncoprotein

To examine signal transduction events activated by oncogenic p21ras, we have studied kinases that are activated following the scrape loading of p21ras into quiescent cells. We observe rapid activation of 42 kDa and 46 kDa protein kinases. The 42 kDa kinase is the mitogen and extracellular-signal regulated kinase ERK2, (MAP2 kinase), which is activated by phosphorylation on tyrosine and threonine in response to oncogenic p21ras, while the 46 kDa kinase is likely to be another member of the ERK family. Stimulation of these kinases by oncogenic p21ras does not require the presence of growth factors, showing that oncogenic p21ras uncouples kinase activation from external signals. In ras transformed cell lines, these kinases are constitutively activated. We propose that the kinases are important components of the signal transduction pathway activated by p21ras oncoprotein.

Crossed signals: oncogenic transcription factors

Transcription factors operate at key regulatory junctions in the cell's responses to diverse extracellular stimuli. We discuss how the transforming activity of mutated transcription factors encoded by several oncogenes (v-erbA, v-fos, v-jun and v-rel) may result in part from a loss of the integrated response to the signalling network.