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Query: UMLS:C0694563 (eds)
 1,062 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Many cell lines respond to mitogenic stimuli (serum, growth factors) with rapid phosphorylation of the ribosomal protein S6 at several serine sites. We have tried to identify the protein kinase(s) mediating this effect of growth stimuli. Examining post-DEAE chromatography fractions of S49 kin- cell extracts, we could detect a highly active effector-independent S6 kinase with specificity for serine residues. The study was extended to the presumably homologous human enzyme, using HeLa S3 cells as model system. Activity yields increased up to sevenfold when exhausted HeLa cells were supplied with fresh medium plus serum. The enzyme uses ATP, not GTP, as cosubstrate, 40-S or 80-S (reassociated from subunits) ribosomal particles being substrate. The optimal K+ concentration, measured at 3 mM Mg2+, is 35 mM. Under optimized assay conditions S6 phosphorylation proceeded faster in vitro than it appeared to do in vivo. The apparent Mr of the enzyme, as estimated by gel filtration on Sephadex G-100, is 56,000 (determination in the presence of 200 mM KCl in 25 mM phosphate buffer). Tighter binding to DEAE-Sephacel and higher specificity for S6 distinguishes this enzyme from the following S6-phosphorylating protein kinases: protein kinase C, protease-activated kinase II, histone-4 phosphotransferase and an enzyme with the properties of casein kinase I. In published summaries of observations shown here and in a follow-up study with chick embryo fibroblasts, the enzyme(s) has been referred to as mitogen-responsive S6 kinase(s) [Martini, O. H. W. and Lawen, A. (1985) in Hormones and cell regulation (Dumont, J. E., Hamprecht, B. and Nunez, J., eds) vol. 9, pp. 411-412, Elsevier Company, North-Holland, Amsterdam; Lawen, A. and Martini, O. H. W. (1985) FEBS Lett. 185, 272-276].
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PMID:Mitogen-responsive S6 kinase. 254 4

The effector binding domain and the switch II region of c-Ha-Ras are necessary for p120GAP-stimulated GTP hydrolysis. We report a third region of c-Ha-Ras located within the alpha 3 helix (amino acids 101-103) which is also required for efficient p120GAP, but not neurofibromin-mediated hydrolysis. This highly conserved region of the Ras protein was investigated using an insertion-deletion mutant (Ras-100LIR104) originally characterized by Willumsen et al. (Willumsen, B. M., Adari, H., Zhang, K., Papageorge, A. G., Stone, J. C., McCormick, F., and Lowy, D. R (1989) in The Guanine Nucleotide Binding Proteins; Common Structural and Functional Properties (Bosch, L., Kraal, B., and Parmeggiani, A., eds) pp. 165-178, Plenum Press, New York). The 100LIR104 substitution did not alter the intrinsic hydrolytic rate of the protein. The p120GAP-stimulated hydrolysis of Ras-100LIR104, however, was decreased by 2-3-fold compared to wild type Ras. This decrease in p120GAP-stimulated hydrolysis was not due to its inability to physically associate with Ras-100LIR104. GTP (as determined by competitive binding assays). Surprisingly, neurofibromin-stimulated GTP hydrolysis was unaltered by the mutation. Finally, no differences were observed in the ability of either the p120GAP catalytic domain or the neurofibromin GRD to accelerate Ras-100LIR104 GTPase activity, indicating that the amino-terminal noncatalytic GAP region is critical for p120GAP-stimulated GTP hydrolysis. This is the first report of a Ras mutation which differentiates between p120GAP and neurofibromin activity.
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PMID:A conserved region of c-Ha-Ras is required for efficient GTPase stimulation by GTPase activating protein but not neurofibromin. 749 25