Identification of insulin-stimulated protein kinase-1 as the rabbit equivalent of rskmo-2: identification of two threonines phosphorylated during activation by mitogen-activated protein kinase

An improved procedure has been developed for the isolation of insulin-stimulated protein kinase-1 (ISPK-1), an S6 kinase-II homologue, by which 0.5 mg highly purified enzyme can be obtained within four days. The sequences of tryptic peptides from ISPK-1 (100 residues) revealed 100% identity with the predicted protein product of rsk(mo)-2, a cDNA clone isolated from a mouse F2 cell line library [Alcorta, D. A., Crews, C. M., Sweet, L. J., Bankston, L., Jones, S. W. and Erikson, R. L. (1989) Mol. Cell. Biol. 9, 3850-3859], demonstrating that rsk(mo)-2 encodes an S6 kinase-II. Two isoforms of mitogen-activated protein (MAP) kinase (p42(mapk) and p44(mapk)) were the only ISPK-1-reactivating enzymes detected after Mono Q chromatography of extracts prepared from rabbit skeletal muscle or phaeochromocytoma 12 cells stimulated by nerve or epidermal growth factors. One of the residues on ISPK-1 phosphorylated by p42(mapk) was a threonine located nine residues N-terminal to the conserved Ala-Pro-Glu motif in the C-terminal protein kinase domain, an analogous location to phosphorylation sites essential for the activity of cAMP-dependent protein kinase, MAP kinase and p34cdc2. A further threonine located five residues N-terminal to the same Ala-Pro-Glu motif was also phosphorylated, probably via autophosphorylation catalysed by ISPK-1 itself.