Ca2+ differentially regulates conventional protein kinase Cs' membrane interaction and activation.

The regulation of conventional protein kinase Cs by Ca2+ was examined by determining how this cation affects the enzyme's 1) membrane binding and catalytic function and 2) conformation. In the first part, we show that significantly lower concentrations of Ca2+ are required to effect half-maximal membrane binding than to half-maximally activate the enzyme. The disparity between binding and activation kinetics is most striking for protein kinase C betaII, where the concentration of Ca2+ promoting half-maximal membrane binding is approximately 40-fold higher than the apparent Km for Ca2+ for activation. In addition, the Ca2+ requirement for activation of protein kinase C betaII is an order of magnitude greater than that for the alternatively spliced protein kinase C betaI; these isozymes differ only in 50 amino acids at the carboxyl terminus, revealing that residues in the carboxyl terminus influence the enzyme's Ca2+ regulation. In the second part, we use proteases as conformational probes to show that Ca2+dependent membrane binding and Ca2+-dependent activation involve two distinct sets of structural changes in protein kinase C betaII. Three separate domains spanning the entire protein participate in these conformational changes, suggesting significant interdomain interactions. A highly localized hinge motion between the regulatory and catalytic halves of the protein accompanies membrane binding; release of the carboxyl terminus accompanies the low affinity membrane binding mediated by concentrations of Ca2+ too low to promote catalysis; and exposure of the amino-terminal pseudosubstrate and masking of the carboxyl terminus accompany catalysis. In summary, these data reveal that structural determinants unique to each isozyme of protein kinase C dictate the enzyme's Ca2+-dependent affinity for acidic membranes and show that, surprisingly, some of these determinants are in the carboxyl terminus of the enzyme, distal from the Ca2+-binding site in the amino-terminal regulatory domain.

Protein kinase C is regulated in vivo by three functionally distinct phosphorylations.

BACKGROUND: Protein kinase Cs are a family of enzymes that transduce the plethora of signals promoting lipid hydrolysis. Here, we show that protein kinase C must first be processed by three distinct phosphorylations before it is competent to respond to second messengers. RESULTS: We have identified the positions and functions of the in vivo phosphorylation sites of protein kinase C by mass spectrometry and peptide sequencing of native and phosphatase-treated kinase from the detergent-soluble fraction of cells. Specifically, the threonine at position 500 (T500) on the activation loop, and T641 and S660 on the carboxyl terminus of protein kinase C beta II are phosphorylated in vivo. T500 and S660 are selectively dephosphorylated in vitro by protein phosphatase 2A to yield an enzyme that is still capable of lipid-dependent activation, whereas all three residues are dephosphorylated by protein phosphatase 1 to yield an inactive enzyme. Biochemical analysis reveals that protein kinase C autophosphorylates on S660, that autophosphorylation on S660 follows T641 autophosphorylation, that autophosphorylation on S660 is accompanied by the release of protein kinase C into the cytosol, and that T500 is not an autophosphorylation site. CONCLUSIONS: Structural and biochemical analyses of native and phosphatase-treated protein kinase C indicate that protein kinase C is processed by three phosphorylations. Firstly, trans-phosphorylation on the activation loop (T500) renders it catalytically competent to autophosphorylate. Secondly, a subsequent autophosphorylation on the carboxyl terminus (T641) maintains catalytic competence. Thirdly, a second autophosphorylation on the carboxyl terminus (S660) regulates the enzyme's subcellular localization. The conservation of each of these residues (or an acidic residue) in conventional, novel and atypical protein kinase Cs underscores the essential role for each in regulating the protein kinase C family.

Isozyme-specific inhibition of protein kinase C by RNA aptamers.

In vitro selection technology has been used to purify RNA aptamers from a random sequence pool that can bind to, and specifically inhibit, protein kinase C beta II. Two of the selected RNA aptamers bind to this isozyme of protein kinase C with nanomolar affinities and inhibit activation with unprecedented selectivity; the highly related, alternatively spliced beta I isozyme, which differs by 23 residues, is inhibited with 1 order of magnitude lower potency; the next most similar isozyme, alpha, shows no detectable inhibition. The production of isozyme-specific inhibitors of protein kinase C opens the possibilities for dissecting the roles of specific protein kinase Cs in the myriad of intracellular signalling pathways.

In vivo regulation of protein kinase C by trans-phosphorylation followed by autophosphorylation.

Dephosphorylation by the catalytic subunits of protein phosphatases 1 (CS1) and 2A (CS2) reveals that mature protein kinase C is phosphorylated at two distinct sites. Treatment of protein kinase C beta II with CS1 causes a significant increase in the protein's electrophoretic mobility (approximately 4 kDa) and a coincident loss in catalytic activity. The CS1-dephosphorylated enzyme cannot autophosphorylate or be phosphorylated by mature protein kinase C, indicating that a different kinase catalyzes the phosphorylation at this site. The loss of activity is consistent with dephosphorylation on protein kinase C's activation loop (Orr, J. W., and Newton, A. C., (1994) J. Biol. Chem. 269, 27715-27718). Treatment with CS2 results in a smaller shift in electrophoretic mobility (approximately 2 kDa) and no loss in catalytic activity. Furthermore, the CS2-dephosphorylated form can autophosphorylate and thus regain the electrophoretic mobility of mature enzyme, consistent with dephosphorylation at protein kinase C's carboxyl-terminal autophosphorylation site, which is modified in vivo (Flint, A. J., Paladini, R. D., and Koshland, D. E., Jr. (1990) Science 249, 408-411). In summary, two phosphorylations process protein kinase C to generate the mature form: a transphosphorylation that renders the kinase catalytically competent and an autophosphorylation that may be important for the subcellular localization of the enzyme.

Phosphatidyl-L-serine is necessary for protein kinase C's high-affinity interaction with diacylglycerol-containing membranes.

The contributions of phospholipid headgroup structure, diacylglycerol, and Ca2+ in regulating the interaction of protein kinase C beta II with membranes or detergent/lipid mixed micelles were examined. Binding measurements revealed that, in the absence of diacylglycerol, protein kinase C displays no significant selectivity for headgroup structure other than change: the enzyme binds with equal affinity to phosphatidyl-L-serine, phosphatidyl-D-serine, and other monoanionic lipids such as phosphatidylglycerol. In contrast, selectivity for headgroup occurs in the presence of diacylglycerol. This second messenger increases the affinity of protein kinase C for phosphatidyl-L-serine-containing membranes or micelles by 2 orders of magnitude, but has only moderate effects on the affinity of protein kinase C for surfaces containing other anionic lipids. Ca2+ does not affect the diacylglycerol-mediated increase in protein kinase C's affinity for phosphatidylserine, but does increase the enzyme's affinity for acidic phospholipids. Lastly, ionic strength studies reveal that electrostatic interactions are the primary driving force in the interaction of protein kinase C with membranes. In the absence of either diacylglycerol or phosphatidylserine, these interactions are sufficiently weak that little binding occurs at physiological ionic strength; thus, protein kinase C is unlikely to translocate to the plasma membranes in the absence of diacylglycerol, even if intracellular Ca2+ levels are high. Our data reveal that, although there is no specificity for binding acidic lipids in the absence of diacylglycerol, specific structural elements of the L-serine headgroup are required for the high-affinity binding of protein kinase C to diacylglycerol-containing membranes.

Reversible exposure of the pseudosubstrate domain of protein kinase C by phosphatidylserine and diacylglycerol.

The lipid activators of protein kinase C, phosphatidylserine and diacylglycerol, induce a reversible conformational change that exposes the auto-inhibitory pseudosubstrate domain of the enzyme. The pseudosubstrate domain of beta-II protein kinase C is cleaved after the first residue, arginine 19, by the endoproteinase Arg-C only when the kinase is bound to the activating lipid phosphatidylserine. Exposure of this residue is markedly enhanced by diacylglycerol. In contrast, the pseudosubstrate domain is not cleaved in the absence of lipids, when protein kinase C is bound to non-activating acidic lipids, when the kinase has autophosphorylated on the amino terminus, or after dilution of the activating lipids. This work reveals specificity in the interaction of protein kinase C with phosphatidylserine since only this phospholipid causes the specific conformational change detected in the regulatory domain of the enzyme, and demonstrates that allosteric regulators expose the intramolecular auto-inhibitory domain of a kinase.

Evidence of two isomers of phosphatidylinositol in plant tissue.

The structure of phosphatidylinositol in barley (Hordeum vulgare) aleurone layers was investigated by chemical degradation. In vivo myo-[2-(3)H]inositol-labeled phosphatidylinositol was first converted to glycerophosphoinositol and, subsequently, after removal of the glycerol moiety, to inositol monophosphate. Here, we present data that show that, in addition to the commonly occurring 1,2-diacylglycero-3-(d-myo-inositol-1-phosphate), barley aleurone cells contain a novel second isomer of phosphatidylinositol that differs in structure of the head group.

Phosphoinositides in barley aleurone layers and gibberellic Acid-induced changes in metabolism.

Phospholipids of barley (Hordeum vulgare L. cv Himalaya) aleurone layers were labeled with myo-[2-(3)H]inositol or [(32)Pi], extracted, and analyzed by physical (chromatography) and chemical (deacylation) techniques. Three phospholipids were found to incorporate both myo-[2-(3)H]inositol and [(32)Pi]-phosphatidylinositol, phosphatidylinositol-monophosphate, and phosphatidylinositol-bisphosphate. Stimulation of [(3)H]inositol prelabeled aleurone layers with GA(3) showed enhanced incorporation of label into phosphatidylinositol within 30 seconds and subsequent rapid breakdown. Stimulation of phosphatidylinositol labeling observed in these studies is the earliest response of aleurone cells to gibberellic acid reported.