Transcription factor phosphorylation by pp90(rsk2) - Identification of Foskinase and NGFI-B kinase I as pp90(rsk2)

The in vitro phosphorylation of transcription factors by growth factor-acti vated protein kinases has resulted in the discovery of a number of activiti es whose identities and relationships to one another are unclear. Fos kinas e is a growth factor-stimulated serine/threonine protein kinase that phosph orylates c-Fos at serine 362 within the carboxyl-terminal regulatory domain . Fos kinase activation is dependent on p21(ras) and mitogen-activated prot ein kinase/ERK kinase kinase (MEK) activity and is independent of phosphati dylinositol 3-kinase activity. We have purified Fos kinase by affinity chro matography using the Sepharose-linked protein kinase inhibitor, bisindolylm aleimide (BIM), Fos kinase has an apparent molecular mass of 88 kDa, and ma ss spectrophotometric analysis of the isolated protein showed that it produ ced tryptic fragments identical to those predicted for pp90(rsk2), Fos kina se isolated from nerve growth factor-stimulated PC12 cells is indistinguish able from NGFI-B kinase I, based on their chromatographic behavior, substra te specificities, and relative sensitivity to BIM, Furthermore, we have dis tinguished Fos kinase from calcium/cAMP response element-binding protein (C REB) kinase, Therefore, Fos kinase and NGFI-B kinase I and pp90(rsk2) repre sent the same protein kinase species, Moreover, we report that pp90(rsk2) e xists within nerve growth factor-stimulated PC12 cells as two chromatograph ically and immunologically distinct species, Finally, we demonstrate that C REB kinase is distinct from pp90(rsk2).