A ribosomal protein S6 kinase copurifies with phosphatase-1 activating factor

Phosphorylation of elongation factor 1 and ribosomal protein S6 by multipotential S6 kinase and insulin stimulation of translational elongation

Stimulation of protein synthesis in response to insulin is concomitant with increased phosphorylation of initiation factors 4B and 4G and ribosomal protein S6 (Morley, S. J., and Traugh, J. A. (1993) Biochimie 75, 985-989) and is due at least in part to multipotential S6 kinase. When elongation factor 1 (EF-1) from rabbit reticulocytes was examined as substrate for multipotential S6 kinase, up to 1 mol/mol of phosphate was incorporated into the alpha, beta, and delta subunits. Phosphorylation of EF-1 resulted in a 2-2. 6-fold stimulation of EF-1 activity, as measured by poly(U)-directed polyphenylalanine synthesis. The rate of elongation was also stimulated by approximately 2-fold with 80 S ribosomes phosphorylated on S6 by multipotential S6 kinase. When the rates of elongation in extracts from serum-fed 3T3-L1 cells and cells serum-deprived for 1.5 h were compared, a 40% decrease was observed upon serum deprivation. The addition of insulin to serum-deprived cells for 15 min stimulated elongation to a rate equivalent to that of serum-fed cells. Similar results were obtained with partially purified EF-1, with both EF-1 and ribosomes contributing to stimulation of elongation. These data are consistent with a ribosomal transit time of 3.2 min for serum-deprived cells and 1.6 min following the addition of insulin for 15 min. Taken together, the data suggest that insulin stimulation involves coordinate regulation of EF-1 and ribosomes through phosphorylation by multipotential S6 kinase.

A membrane-bound protein kinase from rabbit reticulocytes is an active form of multipotential S6 kinase

An active ribosomal protein S6 kinase has been highly purified from the membranes of rabbit reticulocytes by chromatography of the Triton X-100 extract on DEAE-cellulose, SP-Sepharose Fast Flow, and by FPLC on Mono Q and Superose-12. The S6 kinase elutes around 40 000 daltons upon gel filtration on Superose-12 or Sephacryl S-200. It has a subunit molecular weight of 40-43 kDa as determined by protein kinase activity following denaturation/renaturation in SDS-polyacrylamide gels containing S6 peptide. It also phosphorylates translational initiation factors eIF-2 and eIF-4F, glycogen synthase, histone 1, histone 2B, myelin basic protein, but not prolactin, skeletal myosin light chain, histone 4, tubulin, and casein. Apparent Km values have been determined to be 15 microM for ATP, 1.2 microM for S6 and 10 microM for S6 peptide. Two-dimensional tryptic phosphopeptide mapping shows the same sites on S6 are phosphorylated as those identified previously with proteolytically activated multipotential S6 kinase from rabbit reticulocytes, previously denoted as protease activated kinase II. Examination of relative rates of phosphorylation and kinetic constants of synthetic peptides based on previously identified phosphorylation sites, indicates a minimum substrate recognition sequence to be arginine at the n - 3 position. Based on these characteristics, including molecular weight and an expanded substrate specificity, the membrane S6 kinase can be distinguished from the p90 (Type I) and p70 (Type II) S6 kinases, and from protein kinase C and the catalytic subunit of cAMP-dependent protein kinase.

Identification and purification of a novel serine/threonine messenger-independent growth-related protein kinase from lactating goat mammary gland

A second messenger-independent serine/threonine protein kinase from lactating goat mammary gland is purified and characterized. The purification steps include: homogenization, ultracentrifugation, ammonium sulphate precipitation, DEAE-Sepharose, phosphocellulose, hydrophobic and Mono Q columns. On the final step of purification the enzyme is revealed as a single band of mol wt 45,000 on silver-stained SDS-PAGE. Mg2+ and K+ are necessary for its optimum activity. Phosvitin and casein are substrates for the enzyme but kemptide, RRREEETEEE, protamine and histone mixture are all poorly phosphorylated. The kinase is inhibited by quercetin, heparin, random tyrosine- and glutamic acid-containing polymers, Ca2+, NaF, 2,3-bis-phosphoglycerate. 1 mM Mn2+ affects positively the basal level of the kinase activity but 5 mM Mn2+ completely suppress the effect of 10 mM Mg2+. Km of this enzyme for ATP is 1.57 microM and pH optimum is from 6 to 7. Isolation of this kinase is facilitated by its unusually high affinity for phosphocellulose.