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The p90[rsk] (Rsk) and p70[s6k] kinases were first identified based on their ability to phosphorylate the 40S ribosomal protein S6 in vitro. Both of these enzymes are differentially regulated by serine/threonine phosphorylation in response to mitogenic stimulation. ERK1 and ERK2 have been shown to regulate Rsk activity. Once activated by this phosphorylation, a significant amount of Rsk can be found in the nucleus, suggesting that it has a role in nuclear signaling events. The regulation of nuclear Rsk and ERK activities by growth factors is coordinated with the induction of several early response genes. Rsk has also been shown to be activated by ionizing radiation, presumably through an activated MAP kinase. Studies in Xenopus oocytes and mouse NIH/3T3 cells indicate that inactive Rsk and ERK2 exist in a complex of approximately 110kDa. Upon phosphorylation of Rsk and ERK2, the heterodimer dissociates and at least a portion of these activated kinases translocate to the nucleus.
