Synthetic peptides derived from the nonmuscle myosin light chains are highly specific substrates for protein kinase C

Phorbol esters increase MLC phosphorylation and actin remodeling in bovine lung endothelium without increased contraction

Direct protein kinase C (PKC) activation with phorbol myristate acetate (PMA) results in the loss of endothelial monolayer integrity in bovine lung endothelial cells (EC) but produces barrier enhancement in human lung endothelium. To extend these findings, we studied EC contractile events and observed a 40% increase in myosin light chain (MLC) phosphorylation in bovine endothelium following PMA challenge. The increase in PMA-mediated MLC phosphorylation occurred at sites distinct from Ser19/Thr18, sites catalyzed by MLC kinase (MLCK), and immunoblotting with antibodies specific to phosphorylated Ser19/Thr18 demonstrated profound time-dependent Ser19/Thr18 dephosphorylation. These events occurred in conjunction with rearrangement of stress fibers into a grid-like network, but without an increase in cellular contraction as measured by silicone membrane wrinkling assay. The PMA-induced MLC dephosphorylation was not due to kinase inhibition but, rather, correlated with rapid increases in myosin-associated phosphatase 1 (PPase 1) activity. These data suggest that PMA-mediated EC barrier regulation may involve dual mechanisms that alter MLC phosphorylation. The increase in bovine MLC phosphorylation likely occurs via direct PKC-dependent MLC phosphorylation in conjunction with decreases in Ser19/Thr18 phosphorylation catalyzed by MLCK due to PMA-induced increases in PPase 1 activity. Together, these events result in stress fiber destabilization and profound actin rearrangement in bovine endothelium, which may result in the physiological alterations observed in these models.

Phosphorylation of myosin light chain kinase by p21-activated kinase PAK2

Phosphorylation of myosin II regulatory light chains (RLC) by Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) is a critical step in the initiation of smooth muscle and non-muscle cell contraction. Post-translational modifications to MLCK down-regulate enzyme activity, suppressing RLC phosphorylation, myosin II activation, and tension development. Here we report that PAK2, a member of the Rho family of GTPase-dependent kinases, regulates isometric tension development and myosin II RLC phosphorylation in saponin permeabilized endothelial monolayers. PAK2 blunts tension development by 75% while inhibiting diphosphorylation of myosin II RLC. Cdc42-activated placenta and recombinant, constitutively active PAK2 phosphorylate MLCK in vitro with a stoichiometry of 1.71 +/- 0. 21 mol of PO(4)/mol of MLCK. This phosphorylation inhibits MLCK phosphorylation of myosin II RLC. PAK2 catalyzes MLCK phosphorylation on serine residues 439 and 991. Binding calmodulin to MLCK blocks phosphorylation of Ser-991 by PAK2. These results demonstrate that PAK2 can directly phosphorylate MLCK, inhibiting its activity and limiting the development of isometric tension.

Phosphorylation of non-muscle myosin II regulatory light chain by p21-activated kinase (gamma-PAK)

Myosin regulatory light chain (RLC) phosphorylation has been implicated in Rho-mediated stress fibre formation. The recent observation that Rho kinase phosphorylates RLC in vitro suggests that serine/threonine kinases other than those in the myosin light chain kinase (MLCK) family have the potential to activate myosin II. In this study we report that gamma-PAK, which is activated by the GTP-binding proteins Cdc42 and Rac, catalyses phosphorylation of intact non-muscle myosin II and isolated recombinant RLC. gamma-PAK phosphorylated endothelial cell myosin II to 0.85 +/- 0.02 mol PO4 per mol RLC. Phosphorylation is Ca2+/calmodulin-independent and the enzyme has a K(m) and Vmax for myosin II regulatory light chain of 12 microM and 180 nmol/min/mg respectively. No myosin II heavy chain phosphorylation was detected. Phosphopeptide maps and phosphoamino acid analysis revealed that gamma-PAK phosphorylates Ser-19 but does not phosphorylate Thr-18. A panel of recombinant RLC mutants was used to confirm that Ser-19 is the only phosphorylation site modified by gamma-PAK. On substitution of both Ser-19 and Thr-18 with Ala or Glu, no phosphorylation of other Ser/Thr residues in the RLC was detected. Similar to MLCK, Arg-16 is required for interaction of gamma-PAK with the substrate, since converting Arg-16 to Ala significantly reduced RLC phosphorylation. Endothelial cell monolayers permeabilized with saponin retract upon exposure to either Cdc42 or trypsin-activated gamma-PAK and ATP. Activation of gamma-PAK is required to initiate Ca2+/calmodulin-independent cell retraction and actin rearrangement. Taken together, these data suggest that myosin II activation by the p21-activated family of kinases may be physiologically important in regulating cytoskeletal organization.

A potent and highly selective peptide substrate for protein kinase C assay

Protein kinases exhibit substrate specificities that are often primarily determined by the amino acids around the phosphorylation sites. Peptides corresponding to protein kinase C phosphorylation sites in several different proteins were synthesized on SPOTs membrane which has recently been found to be applicable for studies of protein kinase specificity. After phosphorylation with protein kinase C, we chose the best phosphorylated peptides for the investigation of the importance of amino acids immediately adjacent to the phosphorylation site. The selectivity of the best protein kinase C substrates from this study was analysed with protein kinases A, CK1 and CK2. According to these tests, the most favourable characteristics of SPOTs-membrane-associated peptides were demonstrated by peptide KRAKRKTAKKR. Kinetic analysis of peptide phosphorylation with protein kinase C revealed an apparent Km of 0.49 +/- 0.13 microM and Vmax of 10.0 +/- 0.5 nmol/min per mg with soluble peptide KRAKRKTAKKR. In addition, we assayed several other soluble peptides commonly used as protein kinase C substrates. Peptide KRAKRKTAKKR showed the lowest Km and the highest Vmax/Km value in comparison with peptides FKKSFKL, pEKRPSQRSKYL and KRAKRKTTKKR. Furthermore, of the peptides tested, KRAKRKTAKKR was the most selective substrate for protein kinase C. The favourable kinetic parameters combined with the selectivity should make the KRAKRKTAKKR peptide useful as a substrate for protein kinase C in the assays of both purified enzyme and in crude cell extracts.

Protein phosphatase assay using a modification of the P81 paper protein kinase assay procedure

Synthetic peptides have been used to define specificity determinants and to distinguish reactivities of numerous protein kinases and phosphoprotein phosphatases. Direct analysis of peptide phosphorylation is most often determined using P81 phosphocellulose paper to separate modified peptide and unreacted [gamma-32P]ATP; however phosphopeptide dephosphorylation is usually determined by extraction and quantitation of phosphomolybdate complexes or ion exchange chromatography. We describe here the adaptation of the rapid, direct P81 paper protein kinase assay for the determination of phosphopeptide dephosphorylation. The S6-21 peptide (AKRRRLSSLRASTSKSESSQK), which is derived from the multiphosphorylated carboxyl terminal domain of the S6 ribosomal protein, was phosphorylated by a human placenta S6 kinase and dephosphorylation by purified phosphoprotein phosphatase type 1 in the presence of a variety of buffers, and inhibitors/activators was determined using the new assay. Results comparable to those obtained with the ion-exchange chromatography were obtained, and the assay was significantly less expensive, more rapid, and more accurate than methods previously used to quantitate phosphopeptide dephosphorylation.

Amblyomma americanum (L.): protein kinase C-independent fluid secretion by isolated salivary glands

Protein kinase C activity was partially purified from tick salivary glands by fast protein liquid chromatography anion-exchange chromatography. Enzyme activity was stimulated by Ca2+, phosphatidylserine, and diacylglycerol with the highest activity observed in the presence of all three modulators. Enzyme activity was inhibited by a synthetic pseudosubstrate peptide with an amino acid sequence resembling the protein kinase C substrate phosphorylation site. The protein kinase C activator, 1-oleoyl-2-acetyl-sn-glycerol (OAG), when added to whole in vitro salivary glands previously prelabeled with 32P, stimulated the phosphorylation of salivary gland proteins. Activators of protein kinase C (phorbol ester or OAG) did not stimulate fluid secretion by isolated tick salivary glands. OAG and phorbol ester had only minimal affects on the ability of dopamine to stimulate secretion by isolated salivary glands and dopamine's ability to increase salivary gland cyclic AMP.

Site-specific phosphorylation of a synthetic peptide derived from ribosomal protein S6 by human placenta protein kinases

A synthetic peptide S6-21 (AKRRRLSSLRASTSKSESSQK) which contains the phosphorylated residues in the ribosomal protein S6 has been used as a substrate for two partially purified human placenta protein kinases. Two distinct classes of protein kinases which catalyze either amino terminal (AKRRRLSS) or carboxyl terminal (LRASTSKSESSQK) peptide phosphorylation were identified. Multiple sites were phosphorylated in each domain. A single protein kinase which catalyzed phosphorylation of sites in both domains was identified. Although growth factors are known to promote phosphorylation of S6 at five serine sites, no enzyme which could modify S6-21 to that extent was observed.

Protein phosphorylation and kinase activities in tumour cells after hyperthermia

Phosphorylation of various proteins and the activities of specific kinases were studied in tumour cells after hyperthermia. P388 lymphoid tumour cells were treated at 40-45 degrees C for 1 h in vitro. Immediately after heat treatment, particulate and cytosol cell fractions were isolated, phosphorylated proteins separated and various kinase activities were measured. Hyperthermic treatment of the cells caused a significant decrease in protein kinase C activity while the activity of calcium-ion and phospholipid-independent protein kinases increased. Phosphorylation of cytosol proteins of 120, 80, 33, 25 and 14 kDa increased significantly after hyperthermia, and protein kinase C selectively phosphorylated the last three of these proteins. The phosphorylation of three heat shock proteins (44, 70 and 85 kDa) was not changed after hyperthermic treatment. Four tyrosine kinase activities were separated. The protein tyrosine kinase activity decreased to one-tenth of the control value after 45 degrees C for 1 h hyperthermia. The changes in kinase activities and protein phosphorylation induced by hyperthermia proved to be temperature- and time-dependent.

Substrate recognition determinants for rhodopsin kinase: studies with synthetic peptides, polyanions, and polycations

Rhodopsin kinase phosphorylates serine- and threonine-containing peptides from bovine rhodopsin's carboxyl-terminal sequence. Km's for the peptides decrease as the length of the peptide is increased over the range 12-31 amino acids, reaching 1.7 mM for peptide 318-348 from the rhodopsin sequence. The Km for phosphorylation of rhodopsin is about 10(3) lower than that for the peptides, which suggests that binding of rhodopsin kinase to its substrate, photolyzed rhodopsin, involves more than just binding to the carboxyl-terminal peptide region that is to be phosphorylated. A synthetic peptide from the rhodopsin sequence that contains both serines and threonines is improved as a substrate by substitution of serines for the threonines, suggesting that serine residues are preferred as substrates. Analogous 25 amino acid peptides from the human red or green cone visual pigment, a beta-adrenergic receptor, or M1 muscarinic acetylcholine receptors are better substrates for bovine rhodopsin kinase than is the peptide from bovine rhodopsin. An acidic serine-containing peptide from a non-receptor protein, alpha s1B-casein, is also a good substrate for rhodopsin kinase. However, many basic peptides that are substrates for other protein kinases--histone IIA, histone IIS, clupeine, salmine, and a neurofilament peptide--are not phosphorylated by rhodopsin kinase. Polycations such as spermine or spermidine are nonessential activators of phosphorylation of rhodopsin or its synthetic peptide 324-348. Polyanions such as poly(aspartic acid), dextran sulfate, or poly(adenylic acid) inhibit the kinase. Poly(L-aspartic acid) is a competitive inhibitor with respect to rhodopsin (KI = 300 microM) and shows mixed type inhibition with respect to ATP.