A 32-kDa protein associated with phospholipase A2-inhibitory activity from human placenta

Two monomeric 32-kDa proteins, termed 32K-I (pI 5.8) and 32K-II (pI 5.1), were isolated from human placenta, which was solubilized by a Ca2+-chelator. Only 32K-I was associated with PLA2-inhibitory activity. CNBr peptide mapping indicated that 32K-I was distinct from 32K-II and two 36-kDa proteins, called calpactin I and II or lipocortin II and I, which have been shown to possess PLA2-inhibitory activity. 32K-I bound to PS in a Ca2+-dependent manner. 32K-I was detected in many tissues except brain, cardiac and skeletal muscle.     

Characterizations of two distinct Ca2+-dependent phospholipid-binding proteins of 68-kDa isolated from human placenta

Two distinct 68-kDa proteins, named 68K-I (pI 6.4) and 68K-II (pI 5.6), were solubilized from human placenta by treatment with 5 mM EGTA. On DE52 cellulose column chromatography at pH 7.4, 68K-I in the EGTA eluate was recovered in the unadsorbed fractions, whereas 68K-II was retained on the column and eluted with 0.2 M NaCl. The 68K-I protein was obtained in more than 95% purity by further hydroxylapatite and cation exchange chromatographies, while the 68K-II protein was purified further by gel filtration and hydroxylapatite chromatographies. Partial amino acid sequence data showed that 68K-I protein was a novel protein which shared the same sequences as lipocortin I and that 68K-II was the same as human p68/67-kDa calelectrin (Crompton, M. R., Owens, R. J., Totty, N. F., Moss, S. E., Waterfield, M.D., and Crumpton, M. J. (1988) EMBO J. 7, 21-27; Südhof, T. C., Slaughter, C. A., Leznicki, I., Barjon, P., and Reynolds, G. A. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 664-668). The two proteins bound to acidic phospholipids, phosphatidylserine, and/or phosphatidylinositol, but not to phosphatidylcholine, in the presence of micromolar levels of Ca2+. 68K-I bound to phosphatidylinositol preferentially to phosphatidylserine, whereas 68K-II bound only to phosphatidylserine. Both 68K-I and 68K-II inhibited phospholipase A2 activity, and the inhibition by 68K-II was detectable only in the presence of 100 mM KCl. 68K-I, but not 68K-II, was found to bind to F-actin in a Ca2+-dependent (1 mM) manner. Moreover 68K-I, but not 68K-II, was phosphorylated in vitro at tyrosine residues by fps kinase and by epidermal growth factor receptor/kinase, the latter reaction being dependent on Ca2+ and epidermal growth factor. Western blot analysis with affinity purified anti-68K-I and anti-68K-II antibodies showed that 68K-I was located in only certain tissues, especially human placenta, whereas 68K-II was present in many human and rat tissues.     

Twenty-eight-kilodalton phosphorylatable calcium- and lipid-binding proteins purified from Physarum plasmodium.

Two 28-kDa calcium- and lipid-binding proteins were isolated from a detergent-insoluble fraction of the Physarum plasmodium. Both proteins have molecular masses of approximately 28 kDa by SDS-PAGE. The protein designated 28K-I has a slightly lower mobility than that designated 28K-II. The purified 28K-I has a dissociation constant of 1.0 microM for Ca2+ ions, while the 28K-II has two different dissociation constants: one of 0.32 microM and the other of 3.2 mM. The 28K-I binds to liposomes at Ca2+ concentrations higher than 1.0 microM and has a dissociation constant for lipid of 34 micrograms/ml at 10 microM Ca2+. The 28K-II binds to liposomes at concentrations of Ca2+ above the mM range and has a dissociation constant of 36 micrograms/ml for lipid at 2 mM Ca2+. There is no evidence of actin-binding activity by either of the 28-kDa (28K) proteins. The 28K proteins crossreacted with an antiserum against chicken brush border calpactin I. The two proteins have quite different phosphorylation levels between a fraction prepared from the cytosolic endoplasm and a fraction prepared from the whole cell. The 28K proteins may play some role in the membrane structure dynamics of the cortical gel layer.     

Cyclic GMP-dependent protein kinase stimulates the plasma membrane Ca2+ pump ATPase of vascular smooth muscle via phosphorylation of a 240-kDa protein.

The plasma membrane Ca2+ pump ATPase from porcine aorta was isolated by the calmodulin affinity chromatographic method of Kosk-Kosicka et al. (Kosk-Kosicka, D., Scaillet, S., and Inesi, G. (1986) J. Biol. Chem. 261, 3333-3338). Its activity was restored by adding either phosphatidylcholine or phosphatidylserine. Cyclic GMP-dependent protein kinase (G-kinase) stimulated the enzyme in a concentration-dependent manner. However, phosphatidylinositol kinase (PI-kinase) activity was not detected in the enzyme preparation, and the presence of phosphatidylinositol was not necessary for stimulation by G-kinase. Furthermore, adenosine, a potent PI-kinase inhibitor, did not affect the stimulation. The enzyme preparation contained three major proteins, with molecular masses of 240, 145, and 135 kDa, as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 240- and 135-kDa proteins were phosphorylated in association with the stimulation by G-kinase, but only the phosphorylation of the 240-kDa protein was dependent on the G-kinase concentration. A purified enzyme without the 240-kDa protein, prepared by our previous method (Imai, S., Yoshida, Y., and Sun, H.-T. (1990) J. Biochem. (Tokyo) 107, 755-761), was not activated by G-kinase. Immunoblotting with an antibody against the human erythrocyte Ca2+ pump revealed that the 135-kDa protein corresponded to one of the isoforms of the plasma membrane Ca2+ pump. These results suggest that the phosphorylation of the 240-kDa protein is responsible for stimulation of the plasma membrane Ca2+ pump ATPase by G-kinase.     

Vaccinia virus gene H5R encodes a protein that is phosphorylated by the multisubstrate vaccinia virus B1R protein kinase.

Vaccinia virus gene B1R encodes a protein kinase, the previously identified substrates of which include the proteins S2 and Sa of 40S ribosomal subunits. This work characterizes another substrate of the B1R kinase: a 36-kDa protein induced at the early stage of infection. Partially purified 36-kDa protein, eluted from a single-stranded DNA-cellulose column by 0.5 M NaCl, was separated by two-dimensional gel electrophoresis. Phosphorylation in vitro yielded multiple forms of the 36-kDa protein with approximate isoelectric points (pI) of 5.5, 5.7, 5.9, and 6.3, in addition to the apparently unphosphorylated form with a pI of approximately 6.8. The tryptic peptides derived from 36-kDa proteins with pI values of 5.7, 5.9, and 6.3 yielded almost identical high-pressure liquid chromatography profiles, strongly suggesting that the 36-kDa protein was modified by the phosphorylation of at least four sites, which were characterized as threonine residues. The amino acid sequence of several tryptic peptides derived from the 36-kDa protein showed that the 36-kDa protein was encoded by gene H5R of vaccinia virus. Consistent with this, the B1R kinase--either expressed in Escherichia coli or highly purified from HeLa cells--phosphorylated a recombinant trpE-H5R fusion protein in vitro. Fingerprints of the trpE-H5R and 36-kDa proteins phosphorylated by recombinant B1R kinase revealed common sites of phosphorylation, although some tryptic peptides were specific to either protein. Comparison was made of fingerprints of tryptic phosphopeptides derived from 36-kDa single-stranded DNA-binding protein labelled in vivo or in vitro. A common subset of peptides was observed, suggesting that some sites on H5R protein are phosphorylated by the B1R kinase in infected cells. These results suggest that some of the multiple threonine sites in the H5R protein are phosphorylated in vivo by the B1R protein kinase.     

Ca2+-dependent dephosphorylation of kinesin heavy chain on beta-granules in pancreatic beta-cells. Implications for regulated beta-granule transport and insulin exocytosis

The specific biochemical steps required for glucose-regulated insulin exocytosis from beta-cells are not well defined. Elevation of glucose leads to increases in cytosolic [Ca2+]i and biphasic release of insulin from both a readily releasable and a storage pool of beta-granules. The effect of elevated [Ca2+]i on phosphorylation of isolated beta-granule membrane proteins was evaluated, and the phosphorylation of four proteins was found to be altered by [Ca2+]i. One (a 18/20-kDa doublet) was a Ca2+-dependent increase in phosphorylation, and, surprisingly, three others (138, 42, and 36 kDa) were Ca2+-dependent dephosphorylations. The 138-kDa beta-granule phosphoprotein was found to be kinesin heavy chain (KHC). At low levels of [Ca2+]i KHC was phosphorylated by casein kinase 2, but KHC was rapidly dephosphorylated by protein phosphatase 2B beta (PP2Bbeta) as [Ca2+]i increased. Inhibitors of PP2B specifically reduced the second, microtubule-dependent, phase of insulin secretion, suggesting that dephosphorylation of KHC was required for transport of beta-granules from the storage pool to replenish the readily releasable pool of beta-granules. This is distinct from synaptic vesicle exocytosis, because neurotransmitter release from synaptosomes did not require a Ca2+-dependent KHC dephosphorylation. These results suggest a novel mechanism for regulating KHC function and beta-granule transport in beta-cells that is mediated by casein kinase 2 and PP2B. They also implicate a novel regulatory role for PP2B/calcineurin in the control of insulin secretion downstream of a rise in [Ca2+]i.     

Protein kinase C isotypes and signaling in neutrophils. Differential substrate specificities of a translocatable calcium- and phospholipid-dependent beta-protein kinase C and a phospholipid-dependent protein kinase which is inhibited by long chain fatty acyl coenzyme A

Neutrophils possess a classical Ca2+, phosphatidyl serine (PS) and diglyceride (DG)-dependent protein kinase C (beta-PKC) which was translocatable from cytosol to membrane in response to elevated Ca2+ in the physiologic range or to pretreatment with phorbol myristate acetate (PMA). The translocatable beta-PKC was purified from neutrophil membranes prepared in the presence of Ca2+, eluted with EGTA and subjected to hydroxyapatite chromatography. An 80-kDa protein possessing Ca/DG/PS-dependent histone phosphorylating activity was recognized by a monoclonal antibody to beta-PKC but not to alpha-PKC or gamma-PKC. A cytosolic kinase activity remaining after Ca(2+)-induced translocation of beta-PKC was dependent on PS and DG but did not require Ca2+. This novel Ca(2+)-independent, PS/DG-dependent kinase, termed nPKC, eluted from hydroxyapatite between alpha-PKC and beta-PKC, ran as a 76-kDa band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and was reactive to a polyclonal consensus antibody but not to monoclonal antibodies to alpha-PKC, beta-PKC, or gamma-PKC. Long chain fatty acyl-CoA, but not the corresponding free fatty acids, inhibited nPKC in the 1-10 microM range. The chemotactic peptide fMet-Leu-Phe triggered prompt but transient increases in neutrophil long chain fatty acid acyl-CoA, suggesting that nPKC is regulated by fatty acyl-CoA as well as DG during neutrophil activation. Purified beta-PKC phosphorylated a number of cytosolic proteins in a Ca(2+)-dependent manner, including a major 47-kDa cytosolic protein, which may be implicated in superoxide anion generation. In contrast, nPKC did not phosphorylate the 47-kDa protein, but phosphorylated numerous cytosolic proteins in a Ca(2+)-independent manner, including a 66-kDa protein which was not phosphorylated by beta-PKC. Differences in location, substrate specificity, and cofactor dependence between nPKC and beta-PKC suggest these kinases may play selective roles in the activation sequence of the neutrophil.     

Phosphorylation by protein kinase C inactivates an inositol 1,4,5-trisphosphate 3-kinase purified from human platelets

An inositol 1,4,5-trisphosphate 3-kinase purified from human platelets contains two major components, 53 and 36 kDa polypeptides. Each polypeptide expresses Ca2+/calmodulin-dependent enzymatic activity and is phosphorylated by an unidentified protein kinase in the enzyme preparation. The 36-kDa polypeptide may be further phosphorylated on serine residues by protein kinase C to a stoichiometry of 0.8 mole phosphate per mole of protein. Phosphorylation of the 36-kDa component is correlated with inhibition of the kinase activity; the inhibitory effect is dependent upon Ca2+ and phosphatidylserine/diolein and may be blocked by a selective peptide inhibitor of protein kinase C. Phosphorylation by protein kinase C decreases the Vmax of the enzyme from 160 to 28 nmol/mg/min; the Km (0.76 microM) is not altered. These data suggest that protein kinase C may negatively regulate inositol 1,4,5-trisphosphate 3-kinase activity in the human platelet.     

Biochemical Characterization of Annexins I and II Isolated from Pig Nervous Tissue

Five proteins having molecular masses of 90, 67, 37, 36, and 32 kDa (p90, p67, p37, p36, and p32, respectively) were identified in the particulate fractions of pig brain cortex and pig spinal cord prepared in the presence of 0.2 mM Ca2+ and further purified using a protocol previously described for the purification of calpactins. Proteins p90, p37, and p36 are related to annexins I and II. Annexin II, represented by p90, is found as an heterotetramer, composed of two heavy chains of 36 kDa and two light chains of 11 kDa, and as a monomer of 36 kDa. Protein p37, which differs immunologically from p36, is a monomer and could be related to annexin I. All three proteins are Ca(2+)-dependent phospholipid- and F-actin-binding proteins; they are phosphorylated on a serine and on a tyrosine residue by protein kinases associated with synaptic plasma membranes. Purified p36 monomer and p36 heterotetramer proteins bind to actin at millimolar Ca2+ concentrations. The stoichiometry of p36 binding to F-actin at saturation is 1:2, corresponding to one tetramer or monomer of calpactin for two actin monomers (KD, 3 x 10(-6) M). Synaptic plasma membranes supplemented with the monomeric or tetrameric forms of p36 phosphorylate the proteins on a serine residue. The monomer is phosphorylated on a serine residue by a Ca(2+)-independent protein kinase, whereas the heterotetramer is phosphorylated on a serine residue and a tyrosine residue by Ca(2+)-dependent protein kinases. Antibodies to brain p37 and p36 together with antibodies to lymphocytes lipocortins 1 and 2 were used to follow the distribution of these proteins in nervous tissues. Polypeptides of 37, 34, and 36 kDa cross-react with these antibodies. Anti-p37 and antilipocortin 1 cross-react on the same 37- and 34-kDa polypeptides; anti-p36 and antilipocortin 2 cross-react only on the 36-kDa polypeptides.     

Lipocortins are major substrates for protein kinase C in extracts of human neutrophils.

We have identified two major proteins in human neutrophils that are phosphorylated in vitro by protein kinase C (PKC) as lipocortins III and a fragment of a lipocortin-like 68-kDa protein. In electroporated cells, the 68-kDa protein was phosphorylated during stimulation of the cells with either FMLP or PMA. Lipocortins are of interest because of their Ca2(+)- and phospholipid-dependent actin binding properties and ability to inhibit phospholipase A2. Two crude fractions of enzymes and proteins exposed to [gamma-32]PATP in the presence of Ca2+, Mg2+, phosphatidylserine and 1,2-oleoyl-acetyl-rac-glycerol were analyzed by gel electrophoresis and autoradiography. A number of proteins in a detergent-free fraction, including proteins at 36 and 32 kDa, were phosphorylated in the presence of these cofactors. In contrast, only two major proteins (35 and 32 kDa) were phosphorylated in a detergent-extracted fraction. Phosphorylation of the 36, 35, and 32 kDa proteins required the presence of Ca2+, Mg2+, and phosphatidylserine in our soluble fraction and detergent extract, indicating PKC-dependent phosphorylation. The 32-kDa protein phosphorylated in both the soluble fraction and detergent extract was identified as lipocortin III by immunoprecipitation with a cross-reactive antibody that recognized lipocortin I and comparison of cyanogen bromide (CNBr) cleavage patterns of this protein with a lipocortin III standard. The 68-kDa protein was identified as a lipocortin VI-like protein by immunoprecipitation with anti-calelectrin. Additionally, the CNBr cleavage pattern of the 68-kDa protein was similar to that of the 36-kDa protein phosphorylated in our soluble fraction. Autoradiograms of the 68- and 36-kDa fragments immunoprecipitated from our soluble fraction with anticalelectrin and cleaved with CNBr showed that both of these proteins were phosphorylated in this sample. Because phosphorylation is known to change the functional characteristics of the lipocortins, the potential exists to link PKC and lipocortins in neutrophils to regulation of granulemembrane interactions or mediation of inflammation.     

Regulation of cAMP concentration by calmodulin-dependent cyclic nucleotide phosphodiesterase

Bovine brain contains two major calmodulin (CaM) dependent phosphodiesterase isozymes which are homodimeric proteins with subunit molecular masses of 60 and 63 kilodaltons (kDa), respectively. The 60-kDa subunit isozyme can be phosphorylated by cAMP-dependent protein kinase, resulting in a decrease in the enzyme affinity towards CaM. The phosphorylation is blocked by Ca2+ and CaM and reversed by the CaM-stimulated phosphatase (calcineurin). The 63-kDa subunit isozymes can also be phosphorylated, but in this case by a CaM-dependent protein kinase(s). This phosphorylation is also accompanied by a decrease in the isozyme affinity towards CaM and can be reversed by the CaM-dependent phosphatase. Analysis of the complex regulatory properties of the phosphodiesterase isozymes has led to the suggestion that fluxes of cAMP and Ca2+ during cell activations are closely coupled and that the CaM-dependent phosphodiesterase isozymes play key roles in this signal coupling phenomenon.     

Dihydropyridine-sensitive calcium channels from skeletal muscle. II. Functional effects of differential phosphorylation of channel subunits.

Dihydropyridine-sensitive Ca2+ channels from skeletal muscle are multisubunit proteins and are regulated by protein phosphorylation. The purpose of this study was to determine: 1) which subunits are the preferential targets of various protein kinases when the channels are phosphorylated in vitro in their native membrane-bound state and 2) the consequences of these phosphorylations in functional assays. Using as substrates channels present in purified transverse (T) tubule membranes, cAMP-dependent protein kinase (PKA), protein kinase C (PKC), and a multifunctional Ca2+/calmodulin-dependent protein kinase (CaM protein kinase) preferentially phosphorylated the 165-kDa alpha 1 subunit to an extent that was 2-5-fold greater than the 52-kDa beta subunit. A protein kinase endogenous to the skeletal muscle membranes preferentially phosphorylated the beta peptide and showed little activity toward the alpha 1 subunit; however, the extent of phosphorylation was low. Reconstitution of partially purified channels into liposomes was used to determine the functional consequences of phosphorylation by these kinases. Phosphorylation of channels by PKA or PKC resulted in an activation of the channels that was observed as increases in both the rate and extent of Ca2+ influx. However, phosphorylation of channels by either the CaM protein kinase or the endogenous kinase in T-tubule membranes was without effect. Phosphorylation did not affect the sensitivities of the channels toward the dihydropyridines. Taken together, the results demonstrate that the alpha 1 subunit is the preferred substrate of PKA, PKC, and CaM protein kinase when the channels are phosphorylated in the membrane-bound state and that phosphorylation of the channels by PKA and PKC, but not by CaM protein kinase or an endogenous T-tubule membrane protein kinase, results in activation of the dihydropyridine-sensitive Ca2+ channels from skeletal muscle.     

Identification of an endogenous protein kinase C activity and its intrinsic 15-kilodalton substrate in purified canine cardiac sarcolemmal vesicles

The cardiac sarcolemmal 15-kDa protein, previously shown to be the principal sarcolemmal substrate phosphorylated in intact heart in response to beta-adrenergic stimulation (Presti, C. F., Jones, L. R., and Lindemann J. P. (1985) J. Biol. Chem. 260, 3860-3867), was demonstrated to be the major substrate phosphorylated in purified canine cardiac sarcolemmal vesicles by an intrinsic protein kinase C activity. The intrinsic protein kinase C, detected by its ability to phosphorylate H1 histones, was most concentrated in cardiac sarcolemmal vesicles and absent from sarcoplasmic reticulum membranes. Unmasking techniques localized the intrinsic protein kinase activity and its principal endogenous substrate, the 15-kDa protein, to the cytoplasmic surfaces of sarcolemmal vesicles; phospholamban contaminating the sarcolemmal preparation was not significantly phosphorylated. The intrinsic protein kinase C required micromolar Ca2+ for activity, but not calmodulin. Half-maximal phosphorylation of the 15-kDa protein occurred at 10 microM Ca2+; optimal phosphorylation of the 15-kDa protein by protein kinase C and Ca2+ was additive to that produced by cAMP-dependent protein kinase. Exogenous phospholipids were not required to activate endogenous protein kinase C. However, heat-treated sarcolemmal vesicles, in which intrinsic protein kinase activities were inactivated, were sufficient to maximally activate soluble protein kinase C prepared from rat brain, suggesting that all the necessary phospholipid cofactors were already present in sarcolemmal vesicles. Of the many proteins present in sarcolemmal vesicles, only the 15-kDa protein was phosphorylated significantly in heat-inactivated sarcolemmal vesicles by soluble protein kinase C, confirming that the 15-kDa protein was a preferential substrate for this enzyme. Consistent with a protein kinase C activity in sarcolemmal vesicles, the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate stimulated 15-kDa protein phosphorylation severalfold, producing approximately 70% of the maximal phosphorylation even in the absence of significant ionized Ca2+. The results are compatible with an intrinsic protein kinase C activity in sarcolemmal vesicles whose major substrate is the 15-kDa protein.     

Phosphorylation of the 165-kDa dihydropyridine/phenylalkylamine receptor from skeletal muscle by protein kinase C

Dihydropyridine-sensitive Ca2+ channels exist in many different types of cells and are believed to be regulated by various protein phosphorylation and dephosphorylation reactions. The present study concerns the phosphorylation of a putative component of dihydropyridine-sensitive Ca2+ channels by the calcium and phospholipid-dependent protein kinase, protein kinase C. A skeletal muscle peptide of 165 kDa, which is known to contain receptors for dihydropyridines, phenylalkylamines, and other Ca2+ channel effectors, was found to be an efficient substrate for protein kinase C when the peptide was phosphorylated in its membrane-bound state. Protein kinase C incorporated 1.5-2.0 mol of phosphate/mol of peptide within 2 min into the 165-kDa peptide in incubations carried out at 37 degrees C. In contrast to the membrane-bound peptide, the purified 165-kDa peptide in detergent solution was phosphorylated to a markedly less extent than its membrane-bound counterpart; less than 0.1 mol of phosphate/mol of peptide was incorporated. Preincubation of the membranes with several types of drugs known to be Ca2+ channel activators or inhibitors had no specific effects on the rate and/or extent of phosphorylation of the 165-kDa peptide by protein kinase C. The phosphorylation of the membrane-bound 165-kDa peptide by protein kinase C was compared to that catalyzed by cAMP-dependent protein kinase and was found to be not additive. Prior phosphorylation of the 165-kDa peptide by cAMP-dependent protein kinase prevented subsequent phosphorylation of the peptide by protein kinase C. Phosphoamino acid analysis indicated that protein kinase C phosphorylated the 165-kDa peptide at both serine and threonine residues. Phosphopeptide mapping experiments showed that protein kinase C phosphorylated one unique site in the 165-kDa peptide, and, in addition, other sites that were phosphorylated by either cAMP-dependent protein kinase or a multifunctional Ca2+/calmodulin-dependent protein kinase. The results suggest that the 165-kDa dihydropyridine/phenylalkylamine receptor could serve as a physiological substrate of protein kinase C in intact cells. It is therefore possible that the regulation of dihydropyridine-sensitive Ca2+ channels by activators of protein kinase C may occur at the level of this peptide.     

Plasma membrane Ca(2+)-pump ATPase is not a substrate for cGMP-dependent protein kinase.

A plasma membrane Ca(2+)-pump ATPase preparation purified from porcine aorta was incubated with cGMP-dependent protein kinase (G-kinase) under the conditions under which dose-dependent stimulation of the enzyme by G-kinase was observed. Several proteins were phosphorylated, but two isoforms of plasma membrane Ca(2+)-pump ATPase with molecular masses of 135- and 145-kDa were not phosphorylated. The protein that was phosphorylated by G-kinase and identified in our previous study as the 135-kDa isoform of Ca(2+)-pump ATPase, on the basis of its almost identical mobility on SDS-PAGE, was found to be another protein with a molecular mass of 138 kDa. Fractionation of the enzyme preparation after incubation with G-kinase by a newly developed calmodulin affinity chromatographic method resulted in the separation of all the G-kinase substrates from the two isoforms of plasma membrane Ca(2+)-pump ATPase. These results suggest that the direct phosphorylation of the Ca(2+)-pump ATPase does not occur in association with the stimulation of the plasma membrane Ca(2+)-pump ATPase by G-kinase.     

Phosphorylation of plasma membrane aquaporin regulates temperature-dependent opening of tulip petals

The opening and closing of tulip petals was reproduced in the dark by changing the temperature from 5 degrees C to 20 degrees C for opening and 20 degrees C to 5 degrees C for closing. The opening process was accompanied by (3)H(2)O transport through the stem from the incubation medium to the petals. A Ca(2+)-channel blocker and a Ca(2+)-chelator inhibited petal opening and (3)H(2)O transport. Several proteins in the isolated plasma membrane fraction were phosphorylated in the presence of 25 micro M Ca(2+) at 20 degrees C. The 31-kDa protein that was phosphorylated, was suggested immunologically as the putative plasma membrane aquaporin (PM-AQP). This phosphorylated PM-AQP clearly reacted with the anti-phospho-Ser. In-gel assay revealed the presence of a 45-kDa Ca(2+)-dependent protein kinase in the isolated plasma membrane. Phosphorylation of the putative PM-AQP was thought to activate the water channel composed of PM-AQP. Dephosphorylation of the phosphorylated PM-AQP was also observed during petal closing at 5 degrees C, suggesting the inactivation of the water channel.     

Activation of myelin basic protein kinases during echinoderm oocyte maturation and egg fertilization

At least five activated protein kinases were detectable in soluble extracts from maturing as compared to immature sea star oocytes. These kinases could be distinguished on the basis of the time courses of their activation following exposure of the oocytes to 1-methyladenine, their substrate specificities, and their chromatographic properties on DEAE-Sephacel and Sephacryl S-200. A histone H1 kinase (HH1K) (Mr 110,000) underwent maximal activation near the time of 1-methyladenine-induced germinal vesicle breakdown (GVBD). When myelin basic protein (MBP) was used as a substrate, HH1K and two additional kinases (MBPK-I and MBPK-II) were detectable. MBPK-II (Mr 110,000) was fully activated at the time of GVBD, whereas peak activation of MBPK-I (Mr 45,000) occurred after this event. Two "ribosomal protein S6 kinases" (S6K-I and S6K-II) could be detected with a synthetic peptide (RRLSSLRA), which was patterned after a major phosphorylation site in S6. The two S6 kinases (Mr 110,000 for both) underwent activation post-GVBD. HH1K and S6K-I coeluted from DEAE-Sephacel at a conductivity of 5.5-6.0 mmho, whereas MBPK-I, MBPK-II, and S6K-II coeluted from this resin in a second peak at a conductivity = 10-11 mmho. The HH1K and MBPK-II activities both declined prior to the emission of the first polar body (i.e., meiotic cell division), but the MBPK-I, S6K-I, and S6K-II activities remained elevated during this time. The activities of these kinases were also examined during the early cell divisions in sea urchin embryos. Within 5 min after fertilization, the high level of MBPK-I activity in sea urchin eggs rapidly declined. However, along with the HH1K and MBPK-II activities, the MBPK-I activity was transiently increased prior to each cell division. No appreciable postfertilization changes in the S6K-I and S6K-II activities were apparent during the first three cycles of cell division.     

Epidermal growth factor dependent phosphorylation of a 35-kilodalton protein in placental membranes

In human placental membranes isolated in the presence of ethylenediaminetetraacetic acid (EDTA), epidermal growth factor (EGF) stimulated the [gamma-32P]ATP-dependent phosphorylation of tyrosine residues on the 170-kilodalton (kDa) EGF receptor and on a 35-kDa protein. The initial rate of phosphorylation of these proteins in the presence of EGF was 5.2 and 3.5 nmol of phosphate min-1 (mg of receptor protein)-1, and this was approximately 10- and 6-fold higher than the basal rate, respectively. Half-maximal phosphorylation of both proteins occurred at about 2.5 nM EGF. In the presence of p-nitrophenyl phosphate, EGF stimulated the phosphorylation of the 35-kDa protein but not the EGF receptor, suggesting that hormone-stimulated autophosphorylation of the receptor/kinase was not required for kinase activation. The 35-kDa protein exists in two forms: (1) 35Keluate, which was associated with the membrane in the presence of Ca2+ but was eluted with EDTA, and (2) 35Kmemb, which was not eluted from membranes with EDTA. Both forms were immunologically related to a 35-kDa protein previously isolated from A431 cells. Antiserum against the 35-kDa protein also reacted with a protein with an apparent size of 66 kDa that was phosphorylated in an EGF-dependent manner. In phosphorylation reactions performed in the presence of Mg2+, Ca2+ was required for phosphorylation of the 35Keluate form, but Ca2+ was not required for phosphorylation of the 35Kmemb form. Phosphorylation appears to change the membrane-binding properties of the 35Kmemb form because 32P-labeled 35Kmemb could be eluted from the membrane by EDTA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Detection and Characterization of Some New Basic Proteins in Chicken Postsynaptic Densities

Chicken brain postsynaptic density (PSD) polypeptides, obtained by treating synaptosomes with 0.5% Triton X-100 and then further purified on a sucrose gradient, are demonstrated to contain four basic proteins of 76K (pI greater than 9.2), 58K (pI 8.1-8.8, heterogeneous), 40K (pI 9.0), and 24K (pI 8.9). Nonequilibrium pH gradient-sodium dodecyl sulfate two-dimensional gels further reveal six more basic proteins with pI values higher than 9.2: 76K, 52K, 47K, 45K, 36K, and 34K. These basic proteins are a major part of the total chicken PSD polypeptides appearing on the gels. Some of these basic proteins (58K, 52K, 47K, 36K, 24K, and two at 76K) are distinguishable from those of brain mitochondria, the major contaminant. The 40K and 34K proteins may be common mitochondrial polypeptides. The 45K protein is probably a mitochondrial contaminant. A number of proteins including 76K (synapsin I-like protein) and 58K, along with some other minor ones, can be phosphorylated by endogenous protein kinase(s) in the presence of Ca2+, Mg2+, and [gamma-32P]ATP. No PSD basic proteins bind Ca2+.     

Phosphorylation of high-mobility-group proteins by the calcium-phospholipid-dependent protein kinase and the cyclic AMP-dependent protein kinase

Purified lamb thymus high-mobility-group (HMG) proteins 1, 2, and 17 have been investigated as potential substrates for the Ca2+-phospholipid-dependent protein kinase and the cAMP-dependent protein kinase. HMG proteins 1, 2, and 17 are phosphorylated by the Ca2+-phospholipid-dependent protein kinase; the reactions are totally Ca2+ and lipid dependent and are not inhibited by the inhibitor protein of the cAMP-dependent protein kinase. HMG 17 is phosphorylated predominantly in a single seryl residue, Ser 24 in the sequence Gln-Arg-Arg-Ser 24-Ala-Arg-Leu-Ser 28-Ala-Lys, with the second seryl moiety, Ser 28, modified to a markedly lesser degree. HMGs 1 and 2 are also phosphorylated in only seryl residues but with each there are multiple phosphorylation sites. HMG 17, but not HMG 1 or 2, is also phosphorylated by the cAMP-dependent protein kinase with the site phosphorylated being the minor of the two phosphorylated by the Ca2+-phospholipid-dependent protein kinase; the Km for phosphorylation by the cAMP-dependent enzyme is 50-fold higher than that by the Ca2+-phospholipid-dependent enzyme. HMG 17 is an equally effective substrate for the Ca2+-phospholipid-dependent protein kinase either as the pure protein or bound to nucleosomes. Preliminary evidence has indicated that lamb thymus HMG 14 is also a substrate for the Ca2+-phospholipid-dependent enzyme. It is phosphorylated with a Km similar to that of HMG 17 (4-6 microM), and a comparison of tryptic peptides suggests that it is phosphorylated in a site that is homologous with Ser 24 of HMG 17 and distinct from the sites phosphorylated by the cAMP-dependent protein kinase.