Stimulation of phosphorylation of lipocortin at threonine residues by epidermal growth factor (EGF) and the EGF receptor: addition of protein kinase P with polylysine inhibits this effect

Inhibitory role of Src family tyrosine kinases on Ca2+-dependent insulin release

Both neurotransmitter release and insulin secretion occur via regulated exocytosis and share a variety of similar regulatory mechanisms. It has been suggested that Src family tyrosine kinases inhibit neurotransmitter release from neuronal cells (H. Ohnishi, S. Yamamori, K. Ono, K. Aoyagi, S. Kondo, and M. Takahashi. Proc Natl Acad Sci USA 98: 10930-10935, 2001). Thus the potential role of Src family kinases in the regulation of insulin secretion was investigated in this study. Two structurally different inhibitors of Src family kinases, SU-6656 and PP2, but not the inactive compound, PP3, enhanced Ca2+-induced insulin secretion in both rat pancreatic islets and INS-1 cells in a concentration-dependent and time-dependent manner. Furthermore, Src family kinase-mediated insulin secretion appears to be dependent on elevated intracellular Ca2+ and independent of glucose metabolism, the ATP-dependent K+ channel, adenylyl cyclase, classical PKC isoforms, extracellular signal-regulated kinase 1/2, and insulin synthesis. The sites of action for Src family kinases seem to be distal to the elevation of intracellular Ca2+ level. These results indicate that one or more Src family tyrosine kinases exert a tonic inhibitory role on Ca2+-dependent insulin secretion.

A rice membrane-bound calcium-dependent protein kinase is activated in response to low temperature

Calcium-dependent protein kinases (CDPKs) are found in various subcellular localizations, which suggests that this family of serine/threonine kinases may be involved in multiple signal transduction pathways. CDPKs are believed to be involved in the response of plants to low temperatures, but the precise role in the signal transduction pathway is largely unknown. Previous reports described changes in CDPKs' mRNA levels in response to cold treatment, but whether these changes are accompanied by increases in protein level and/or kinase activities is unknown. In the present study, we identify in rice (Oryza sativa L. cv Don Juan) plants a 56-kD membrane-bound CDPK that is activated in response to cold treatment. Immunoblot analysis of the enzyme preparations from control and cold-treated plants showed that the kinase level was similar in both preparations. However, both kinase and autophosphorylating activities of the enzyme prepared from cold-treated plants were significantly higher than that obtained from control plants. The activation of the CDPK is detected after 12 to 18 h of cold treatment, which indicates that the kinase does not participate in the initial response to low temperature but in the adaptative process to adverse conditions. To our knowledge, this is the first demonstration of a CDPK that is posttranscriptionally activated in response to low temperature.

A rice membrane calcium-dependent protein kinase is induced by gibberellin

A rice (Oryza sativa) seed plasma-membrane calcium-dependent serine/threonine protein kinase (CDPK) has been partially purified. Comparing results in seeds that were treated with and without the plant hormone gibberellin (GA) for 10 min showed that rice CDPK was highly induced by GA. After separating solubilized membrane proteins by sodium dodecyl sulfate-gel electrophoresis, followed by renaturation, a radiolabeled phosphoprotein band of approximately 58 kD was detected, and it was apparently produced by autophosphorylation. There are five aspects of the rice CDPK that show similarity to mammalian protein kinase C (PKC) and to other plant CDPKs: (a) Histone IIIS and PKC peptide-ser25 (19-31) are phosphorylated by rice CDPK. (b) The phosphorylation reaction is strictly dependent on calcium. (c) The activity of the rice CDPK is inhibited by either staurosporine or the PKC inhibitory peptide (19-36). (d) Addition of calmodulin has no effect on the activity of the enzyme; however, the CDPK is inhibited by the calmodulin antagonists trifluoperazine and W-7. (e) The rice CDPK reacts with a mammalian anti-PKC antibody in immunoblotting analysis. However, there is one major difference between the rice CDPK and other CDPKs: the rice CDPK is induced by GA, whereas no mammalian PKC or other plant CDPKs are known to be induced by any hormone.

Neuronal cells mature faster on polyethyleneimine coated plates than on polylysine coated plates

Cell morphology, protein/DNA ratio, ganglioside analysis, taurine uptake, and the activity of neurone specific enolase showed that neuronal cells mature faster when grown on polyethyleneimine coated plates compared to cells grown on polylysine coated plates. Our results also show higher protein/DNA ratio and total and neuron specific enolase activities in cells grown in serum supplemented medium, when compared to their counterparts grown in synthetic medium. Moreover, our results show that only some specific markers can be used to determine the early and late events of cell maturation, whereas other markers continuously vary with time.

Alteration of the kinetic properties of the epidermal growth factor receptor tyrosine kinase by basic proteins

Previous studies have shown that lysine- and arginine-rich proteins can enhance the activity of tyrosine and serine/threonine protein kinases. However, the kinetics and mechanism of this activation are not fully understood. Therefore we investigated the ability of poly(amino acids) and the arginine-rich protein, protamine, to alter the kinetic properties of epidermal growth factor (EGF) receptor protein-tyrosine kinase activity using immunoaffinity-purified receptor isolated from human epidermoid carcinoma (A431) cells. Poly(L-lysine), poly(L-arginine) and protamine stimulated EGF receptor kinase activity by 3-5-fold at non-saturating doses of ATP and peptide substrate, while poly(L-glutamate) had no effect. Initial kinetic studies demonstrated an increase in the maximum velocity and a decrease in the apparent Km for the peptide substrate angiotensin II in the presence of the basic effectors. Further analysis of the kinetic mechanism by product inhibition revealed that protamine altered the pattern of ADP inhibition towards the peptide substrate but not towards ATP. The change was indicative of the receptor's ability to form an enzyme-angiotensin II-ADP ternary complex in the presence of protamine but not in its absence. In addition, the basic effectors had a substantially decreased influence on the kinase activity of a C-terminally truncated form of the EGF receptor. Thus the changes in kinase activity may be partially mediated by the C-terminal region of the receptor, which contains the sites of receptor self-phosphorylation. These results suggest that the basic domains of proteins can interact with the EGF receptor to induce changes in its kinetic properties, especially with regard to reactant recognition and binding.

Heparin stimulates epidermal growth factor receptor-mediated phosphorylation of tyrosine and threonine residues

We have described previously that in extracts of A431 cells epidermal growth factor (EGF) stimulates the phosphorylation of tyrosine as well as of threonine residues in the EGF receptor and in lipocortin 1. We now report that heparin at low concentrations also stimulates the autophosphorylation of the EGF receptor and of the recombinant 56-kDa domain of the EGF receptor that lacks the EGF binding site. To study the stimulations of phosphorylation of threonine residues, a fusion protein was prepared with glutathione S-transferase (GST) and an EGF receptor fragment, TK8 (residues 647-688), that contains the threonine phosphorylation site but no tyrosine. We show that the phosphorylation of threonine residues in GST-TK8 by extracts of A431 cells is stimulated by heparin but not by EGF. These and other results suggest that heparin acts as a chaperone, a substrate modulator, that enhances the susceptibility of the substrate to phosphorylation by protein kinases.

Control of src kinase activity by activators, inhibitors, and substrate chaperones

The activities of src tyrosine kinases are greatly influenced by substrate modulators (chaperones). In the presence of bovine serum albumin, the phosphorylation of a random polymer of glutamic acid, alanine, and tyrosine (1:1:1) by src kinases is stimulated 20- to 100-fold, but there is little stimulation with a polymer of glutamic acid and tyrosine (4:1) as substrate. This suggests that serum albumin interacts with the substrates rather than with the enzyme. groEL and several other heat shock proteins also stimulate the phosphorylation of a random polymer of glutamic acid, alanine, and tyrosine (1:1:1). In the absence of substrate modulators, the phosphorylation of calmodulin and of several ras proteins by src kinase is barely detectable. In the presence of polylysine or protamine, marked phosphorylation is observed. Another type of control of src kinase activities appears to be directed toward the enzyme rather than the substrate. Triton X-100 extracts of plasma membranes of bovine brain contain a heat-stable factor that stimulates c-src kinase activity with any of the polymers as substrate. The same extract contains a heat-labile factor that preferentially inhibits c-src kinase activity. The two factors are separated by DEAE-Sephacel and phosphocellulose chromatography. The presence of the activator enhances the potency of the inhibitor.