Phosphorylation of 40-S ribosomal subunits by cAMP-dependent, cGMP-dependent and protease-activated protein kinases

Phosphorylation of proteoglycans from human articular cartilage by a cAMP-dependent protein kinase

Purified proteoglycan subunits from human articular, bovine articular and nasal cartilages, and a rat chondrosarcoma were phosphorylated in vitro by beef heart cAMP-dependent protein kinase in the presence of gamma 32P-ATP. In these experiments, a maximum of 1.7 moles of 32P were incorporated per mole of proteoglycan from human cartilage. Phosphorylation was dependent on the presence of cAMP. Analysis by autoradiography revealed that serine residues in the core protein of the proteoglycan were the sites of phosphorylation. Treatment of proteoglycan subunits with chondroitinase ABC and alkaline phosphatase prior to reaction with cAMP-dependent protein kinase increased the incorporation of 32P by 12-30% when compared with untreated proteoglycans. These data indicate that proteoglycans in cartilage can be phosphorylated by cAMP-dependent protein kinase.

Cyclic nucleotide- and calcium-independent phosphorylation of proteins in rat brain polyribosome: effects of ACTH, spermine, and hemin

The incorporation of [gamma-32P]ATP into proteins of rat brain polyribosomes was studied in vitro. The effects of cyclic nucleotides, calcium, hemin, ACTH, GTP, and spermine were examined. The incorporation of phosphate into proteins increased with time and phosphatase activity was very low; thus, the extent of phosphorylation was predominantly a reflection of protein kinase activity. Phosphorylation of proteins was not sensitive to Ca2+ in the presence or absence of either calmodulin or phosphatidylserine. Phosphorylation was also unaffected by cyclic nucleotides in the absence of exogenous enzymes. However, addition of a cAMP-dependent protein kinase together with cAMP resulted in a stimulation of the incorporation of phosphate into 4 phosphoproteins (pp70, pp58, pp43, and pp32); phosphorylation of pp32 was completely dependent on the addition of the kinase. ACTH (1-24), (11-24), and spermine inhibited the endogenous phosphorylation of one protein band (pp30). The phosphorylation of this 30 kD band was also selectively increased by hemin (5 microM). Higher concentrations of hemin exerted an inhibitory effect on the majority of the phosphoproteins. Protein phosphatase activity was not influenced by ACTH or spermine. The specific inhibition of pp30 phosphorylation by ACTH or spermine is most probably explained by an interaction with a cyclic nucleotide- and Ca2+ -independent protein kinase.

Cyclic AMP binding activity in liver supernatants during acute ethionine intoxication

There is a significant increase in hepatic cAMP binding activity 4 hr following ethionine intoxication. This activity is localized to the 100, 000 g supernatant. The binding of cAMP by the 100, 000 g supernatant of controls can be increased to experimental levels simply by dialysis. The increased binding activity seen during ethionine intoxication can be reversed by the administration of adenine. Neither S-adenosyl ethionine, S-adenosyl methionine, S-adenosyl homocysteine, nor AMP compete effectively with cAMP for the binding protein. Increasing ADP concentrations stimulates cAMP binding whereas increasing ATP concentration inhibits cAMP binding. At concentrations seen during ethionine intoxication the effects of ADP and ATP were equal but opposite in direction.

In organello and in vitro phosphorylation of chloroplast ribosomal proteins

Two chloroplast ribosomal proteins are phosphorylated in isolated chloroplast. One in the large subunit (L18) and one in the small subunit ( LS31 ). The phosphorylation is light dependent and occurs on a serine residue for both ribosomal proteins. These two proteins and other chloroplast ribosomal proteins are also phosphorylated in vitro using [gamma 32P]-ATP and a cAMP -dependent or a cAMP - independent protein kinase. The existence of a protein-kinase bound to chloroplast 70S ribosomes is also demonstrated, the enzyme is able to phosphorylate almost every chloroplast ribosomal protein.

Heat shock causes diverse changes in the phosphorylation of the ribosomal proteins of mammalian cells

When HeLa cells or BHK cells were subjected to heat shock at 42 degrees C (for 2 h) or 45 degrees C (for 10 min) there was extensive dephosphorylation of ribosomal protein S6. Concomitantly ribosomal protein L14, which is not significantly phosphorylated in normal cells, became phosphorylated, as did a non-structural protein of Mr = 27000, associated with the ribosomes. The latter effects were not prevented by cycloheximide or actinomycin D. When cells shocked at 45 degrees C for 10 min were returned to 37 degrees C for 2 h there was rephosphorylation of ribosomal protein S6 and dephosphorylation of the 27 kDa protein, but not of ribosomal protein L14.

EGF, PGF2 alpha and insulin induce the phosphorylation of identical S6 peptides in swiss mouse 3T3 cells: effect of cAMP on early sites of phosphorylation

Epidermal growth factor (10(-9)M), prostaglandin (8.5 X 10(-7)M), F2 alpha, and insulin (10(-9)M), each of which only leads to a partial phosphorylation of 40S ribosomal protein S6, generate the same first eight phosphopeptides induced by 10% serum, suggesting all three activate a common regulatory pathway for the phosphorylation of S6. Added together, they induce almost maximal S6 phosphorylation and a phosphopeptide pattern nearly equivalent to that of serum. Unlike the agents above, 8-Br-cAMP or PGE1 has no significant effect on protein synthesis, but does induce a small increase in S6 phosphorylation. Surprisingly, the three peptides that become phosphorylated are identical with insulin-induced phosphopeptides 10b, 11, and 9, based on either comigration, limited acid hydrolysis, or V8 protease digestion. Incubation of 40S subunits with cAMP-dependent protein kinase induces the phosphorylation of these same three phosphopeptides. The in vitro and in vivo studies described here raise the possibility that cAMP could, in part, be responsible for mediating the phosphorylation of S6 during the mitogenic response.

Hepatic adenylate cyclase and phosphodiesterase activity during acute ethionine intoxication

Four hours after the administration of ethionine and in the face of an 80-90% depletion of hepatic ATP there is a twofold increase in cAMP. This increase in cAMP is shown not to be effected by a reduction in phosphodiesterase activity. Both the low Km and high Km forms of the enzyme retain their activity. The purification of liver plasma membranes is not affected by ethionine. There is a significant 20% increase in plasma membrane adenylate cyclase activity following acute ethionine intoxication. This was demonstrable using either ATP or 5'-adenylyl-imidodiphosphate as substrates. The plasma membranes, isolated in the presence of Ca2+, have a 20-fold higher basal adenylate cyclase activity then previously reported and are not further stimulated by GTP or NaF. A modified protocol for isolating cAMP from the adenylate cyclase reaction is described.

Purified rat brain calcium- and phospholipid-dependent protein kinase phosphorylates ribosomal protein S6

The Ca2+-phospholipid-regulated protein kinase has been purified to homogeneity from a 100,000 X g supernatant fluid of rat brain homogenate by a procedure that includes DEAE-cellulose chromatography and successive filtrations on Ultrogel AcA 34 in EGTA and in phosphatidylserine and Ca2+. A more rapid purification consisting of DEAE-cellulose chromatography, Ultrogel AcA 34 gel filtration chromatography, and DEAE-trisacryl chromatography, all in the presence of EGTA, was also developed. Although the enzyme obtained by the latter procedure is not homogeneous, it exhibits properties similar to those of the pure enzyme and is more stable. In addition, the DEAE-trisacryl step permitted resolution of a contaminating Ca2+-inhibitable protein kinase that can interfere with studies of the Ca2+-phospholipid-stimulated enzyme. The homogeneous enzyme, purified about 300-fold, was estimated to have a Mr of 84,000. Its activity was 20- to 30-fold higher in the presence of phospholipid and Ca2+ than in the presence of phospholipid and EGTA, EGTA, or Ca2+ alone. The specific activity of the activated kinase was 852 nmol of P incorporated into histone per min/mg at 20 degrees C. The pure enzyme underwent autophosphorylation in a Ca2+- and phospholipid-dependent manner. This reaction was inhibited in the presence of histones without affecting the kinetic properties of the enzyme. Under optimal assay conditions, the homogeneous enzyme was activated 10-20% by either 10 microM diolein or 100 nM phorbol 12-myristate 13-acetate. Activation of the purified enzyme by diolein or the phorbol ester was far greater (3- to 4-fold) when aggregated instead of freshly sonicated phospholipids were used, suggesting that these compounds affect the interaction of the enzyme with phospholipids and Ca2+. The purified enzyme catalyzed the phosphorylation of the 40S ribosomal subunit protein S6. The Km for S6 was approximately equal to 1 microM and it was estimated that 2 mol of phosphate were incorporated per mol of S6. The observation that protein S6 can be phosphorylated by the purified Ca2+-phospholipid-dependent protein kinase may link recent reports that phorbol ester tumor promoters activate the Ca2+-phospholipid-dependent protein kinase in vitro and stimulate phosphorylation of the ribosomal protein S6 in vivo.

Phosphorylation sites for ribosomal S6 protein kinases in mouse 3T3 fibroblasts stimulated with platelet-derived growth factor

Platelet release products and purified platelet-derived growth factor stimulated the phosphorylation of ribosomal protein S6 in cultured mouse Balb/c 3T3 fibroblasts. The post-nuclear fraction of the stimulated cells was enriched in S6 kinase activity specific for sites resembling those phosphorylated within intact cells in response to PDGF as determined by tryptic peptide mapping. 3T3-S6 sites closely resembled those phosphorylated in S6 of rat hepatocytes stimulated with insulin and included sites for both cAMP-dependent and independent kinases.

Phosphopeptide patterns of the ribosomal protein S6 following stimulation of guinea pig parotid glands by secretagogues involving either cAMP or calcium as second messenger

Stimulation of secretion in exocrine cells is associated with the incorporation of up to 3 to 4 phosphates into the ribosomal protein S6. This occurs with secretagogues involving either cAMP or free calcium as second messenger. An analysis of the phosphorylation pattern of S6 from stimulated guinea pig parotid glands reveals 3 phosphopeptides (termed A,B,C). The phosphopeptide pattern was identical for cAMP- or calcium-mediated stimulation, whereas phosphorylation of the S6 protein in vitro with catalytic subunit of cAMP-dependent protein kinase resulted only in the formation of phosphopeptides A and C. Therefore, secretagogue-mediated phosphorylation is not or not exclusively catalyzed by cAMP-dependent protein kinase even when cAMP is the second messenger.

Primary substrate specificity determinants for the H4-specific protease-activated protein phosphotransferase

The specificity of the histone-H4-specific, protease-activated protein kinase (H4-PK) was examined using two series of synthetic peptides corresponding to the phosphorylation sites in histone H4 and pyruvate kinase. Optimum kinetic constants for phosphorylation were observed using the peptide Val-Lys-Arg-Ile-Ser-Gly-Leu. Peptides in which the Lys was replaced by Arg or the Lys-Arg sequence was transposed were phosphorylated with less favorable kinetics. Peptides with either basic residue deleted did not serve as substrates. Only the H4 peptide, containing an Arg-Arg sequence, was phosphorylated by the cyclic-AMP-dependent protein kinase (CA-PK). Distinct specificity determinants for H4-PK and CA-PK were also observed using the pyruvate kinase peptide (Leu-Arg-Arg-Ala-Ser-Leu-Gly). Collectively the data indicated that the primary substrate specificity determinants for H4-PK are Lys-Arg-Xaa-Ser whereas the CA-PK selectively phosphorylates the sequence Arg-Arg-Xaa-Ser.

Insulin-mediated phosphorylation of ribosomal protein S6 in mouse melanoma cells and melanoma x fibroblast hybrid cells in relation to cell proliferation

The possible role of insulin-mediated phosphorylation of ribosomal protein S6 in the control of cell proliferation was examined in insulin-unresponsive mouse melanoma calls (PG19) and insulin-responsive melanoma x fibroblast clone 100A. In the hybrid cells, under conditions of growth arrest in medium with low serum, ribosomal protein S6 was rapidly phosphorylated in response to insulin or serum. The phosphorylation of the S6 protein increased over a wide range of insulin concentrations, suggesting that insulin stimulated the phosphorylation by interacting with both high- and low-affinity receptors. In contrast, in growth-arrested melanoma cells, an intermediate level of S6 phosphorylation was observed. Insulin caused only a marginal increase and serum caused a small but consistent increase in the level of S6 phosphorylation in the melanoma cells. Cell cycle analysis revealed that both cell lines arrested growth to a similar degree in the G1 phase of the cell cycle; thus, the higher baseline level of S6 phosphorylation observed in the melanoma cells was not attributable to less complete growth arrest of these cells in medium with low serum. The S6 phosphorylation results correlate well with previous results suggesting that the hybrid cells, but not the parental melanoma cells, can become growth-limited for processes regulated by insulin.

Proteolysis of the epidermal growth factor receptor by endogenous calcium-activated neutral protease from rat liver

In A-431 membranes but not in rat liver membranes, the epidermal growth factor (EGF) receptor was converted from a Mr=180,000 to a Mr=160,000 form by a protease activated when cells were broken in the presence of calcium. Calcium-activated neutral protease (CANP) activity in rat liver cytosol was separated from its protein inhibitor by DEAE-cellulose chromatography. When fractions containing this protease activity were incubated with rat liver membranes in the presence of calcium, the Mr=180,000 form of the receptor was converted to the Mr=160,000 form. This conversion was blocked both by the separated endogenous inhibitor and by leupeptin. Apparently CANP is a highly regulated endogenous protease which could degrade the EGF receptor-kinase in most tissues.

Ordered phosphorylation of 40S ribosomal protein S6 after serum stimulation of quiescent 3T3 cells

The amino acids and tryptic peptides that become phosphorylated in 40S ribosomal protein S6 after serum stimulation of quiescent 3T3 cells were examined by two-dimensional thin-layer electrophoresis. In the maximally phosphorylated form of the protein, most of the phosphate was incorporated into serine and a small amount, into threonine. Digestion of this form of the protein with trypsin revealed 10 major phosphopeptides. All 10 contained phosphoserine and 2 of the 10 also contained phosphothreonine. Next, the five forms of increasingly phosphorylated S6 were individually separated on two-dimensional polyacrylamide gels or total S6 was isolated from cells that were stimulated for only a short time and their phosphotryptic maps were analyzed. The results showed that, as larger amounts of phosphate were added to S6, the phosphopeptides appeared in a specific order.

The phosphorylation of ribosomal protein S6 from progesterone-stimulated Xenopus laevis oocytes. Kinetic studies and phosphopeptide analysis

Xenopus laevis oocytes were prelabeled with [32P]orthophosphate overnight before maturation was induced by progesterone stimulation. The phosphorylation status of ribosomal protein S6 from control oocytes and the temporal changes in S6 phosphorylation after progesterone treatment were analyzed by two-dimensional gel electrophoresis. S6 protein was separated in up to five distinct S6 species, which differed in their degree of phosphorylation. 32P labeling of S6, as judged from the shift of radioactivity into more highly phosphorylated S6 derivatives, continuously increased in progesterone-stimulated oocytes even at later times when germinal vesicle breakdown was completed. S6 protein of unstimulated oocytes was labeled to a lower degree. Trypsin cleavage of total S6 protein, isolated from control and maturing oocytes, gave rise to different complex phosphopeptide patterns reflecting the existence of various multiply phosphorylated S6 derivatives in both samples. Two of the more highly phosphorylated S6 derivatives showed considerable differences between the phosphopeptide elution profiles of control and stimulated oocytes indicating that dissimilar sites had been modified under both physiological conditions. Only phosphoserine was detected in the phosphoamino acid analysis of individual S6 derivatives.

Phosphorylation of myosin light chain by a protease-activated kinase from rabbit skeletal muscle

A protease-activated protein kinase that phosphorylates the P light chain of myosin in the absence of Ca2+ and calmodulin has been isolated from rabbit skeletal muscle. The enzyme has properties similar to protease-activated kinase I from rabbit reticulocytes [S. M. Tahara and J. A. Traugh (1981) J. Biol. Chem. 256, 11588-11564], which has been shown to phosphorylate the P light chain of myosin [P. T. Tuazon, J. T. Stull, and J. A. Traugh (1982) Biochem. Biophys. Res. Commun. 108, 910-917]. The protease-activated kinase from skeletal muscle has been partially purified by chromatography on DEAE-cellulose, phosphocellulose and hydroxyapatite. The enzyme phosphorylates histone as well as the P light chain of myosin following activation by proteolysis. Stoichiometric phosphorylation of myosin light chain was observed with the protease-activated kinase and myosin light chain kinase. The sites phosphorylated by the protease-activated kinase and myosin light chain kinase were examined by two-dimensional peptide mapping following chymotryptic digestion. The phosphopeptides observed with the protease-activated kinase were different from those obtained with the Ca2+-dependent myosin light chain kinase, indicating that the two enzymes phosphorylated different sites on the P light chain of skeletal muscle myosin. When actomyosin from skeletal muscle was examined as substrate, the P light chain was phosphorylated following activation of the protease-activated kinase by limited proteolysis.

Differential phosphorylation of ribosomal protein S6 in isolated rat hepatocytes after incubation with insulin and glucagon

Glucagon and insulin both stimulated the 32P-labelling of ribosomal protein S6 in rat hepatocytes that had been incubated with 32Pi. Glucagon selectively enhanced the labelling of the tryptic peptide phosphorylated by cyclic AMP-dependent protein kinase, demonstrating that 6 S is a physiological substrate for this enzyme. Insulin stimulated the phosphorylation of distinct tryptic peptides, at least one of which appears to be very close in the primary structure to the sites phosphorylated by cyclic AMP-dependent protein kinase.

Differential activation of two protease-activated protein kinases from reticulocytes by a Ca2+-stimulated protease and identification of phosphorylated translational components

Two protein kinases have been partially purified from rabbit reticulocytes and shown to be activated by limited proteolysis with trypsin [S.M. Tahara and J.A. Traugh (1981) J. Biol. Chem. 256, 11558-11564; P.T. Tuazon, W.C. Merrick, and J.A. Traugh (1980) J. Biol. Chem. 255, 10954-10958]. Reticulocyte lysate was examined for protease activities which might be involved in activation of the protein kinases in vivo. Two neutral proteases, differentially activated by Fe2+ and Ca2+, were identified and partially purified. The Ca2+-stimulated protease specifically activated protease-activated kinase II; no effect was observed on protease-activated kinase I. The Fe2+-stimulated protease was not active on either protein kinase. The protease-activated kinases were examined using initiation factors (eIF) and 40-S ribosomal subunits as substrate. Protease-activated kinase I phosphorylated one subunit of eIF-3 (Mr 130000), eIF-4B and 40-S ribosomal protein S10. Protease-activated kinase II modified the beta subunit of eIF-2 (Mr 53000) and 40-S ribosomal protein S6. The substrate specificities are unique when compared with other cAMP-dependent and cAMP-independent protein kinases from reticulocytes.