Identification of the protein kinase C phosphorylation site in neuromodulin

Neuromodulin (P-57, GAP-43, B-50, F-1) is a neurospecific calmodulin binding protein that is phosphorylated by protein kinase C. Phosphorylation by protein kinase C has been shown to abolish the affinity of neuromodulin for calmodulin [Alexander, K. A., Cimler, B. M., Meier, K. E., & Storm, D. R. (1987) J. Biol. Chem. 262, 6108-6113], and we have proposed that the concentration of free CaM in neurons may be regulated by phosphorylation and dephosphorylation of neuromodulin. The purpose of this study was to identify the protein kinase C phosphorylation site(s) in neuromodulin using recombinant neuromodulin as a substrate. Toward this end, it was demonstrated that recombinant neuromodulin purified from Escherichia coli and bovine neuromodulin were phosphorylated with similar Km values and stoichiometries and that protein kinase C mediated phosphorylation of both proteins abolished binding to calmodulin-Sepharose. Recombinant neuromodulin was phosphorylated by using protein kinase C and [gamma-32P]ATP and digested with trypsin, and the resulting peptides were separated by HPLC. Only one 32P-labeled tryptic peptide was generated from phosphorylated neuromodulin. The sequence of this peptide was IQASFR. The serine in this peptide corresponds to position 41 of the entire protein, which is adjacent to or contained within the calmodulin binding domain of neuromodulin. A synthetic peptide, QASFRGHITRKKLKGEK, corresponding to the calmodulin binding domain with a few flanking residues, including serine-41, was also phosphorylated by protein kinase C. We conclude that serine-41 is the protein kinase C phosphorylation site of neuromodulin and that phosphorylation of this amino acid residue blocks binding of calmodulin to neuromodulin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purification of a novel calmodulin binding protein from bovine cerebral cortex membranes

A new calmodulin (CaM) binding protein, designated P-57, has been purified to apparent homogeneity from bovine cerebral cortex membranes. In contrast to other calmodulin binding proteins, P-57 has higher affinity for calmodulin in the absence of bound Ca2+ than in its presence. The protein was purified by DEAE-Sephacel chromatography and two CaM-Sepharose affinity column steps. The first CaM-Sepharose column was run in the presence of Ca2+; the second was run in the presence of chelator in excess of Ca2+. P-57 was adsorbed by CaM-Sepharose only in the absence of bound Ca2+ and was eluted from the second column by buffers containing Ca2+. Sodium dodecyl sulfate (SDS)-polyacrylamide gels of the purified protein showed only one band at Mr 57 000. The major form of the protein on Bio-Gel A-1.5m and native polyacrylamide gradient gel electrophoresis ran with an apparent Stokes radius of 41 A. Photoaffinity labeling of P-57 with azido[125I]calmodulin yielded one cross-linked product on SDS gels with an Mr of 70 000. This interaction occurred only when excess ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid was present and was inhibited by the presence of Ca2+ in excess of chelator. It appears that P-57 has novel binding properties for calmodulin distinct from all other calmodulin binding proteins described thus far.     

Dephosphorylation of neuromodulin by calcineurin

Neuromodulin (p57, GAP-43, F1, B-50) is a major neural-specific, calmodulin binding protein found in brain, spinal cord, and retina that is associated with membranes. Phosphorylation of neuromodulin by protein kinase C causes a significant reduction in its affinity for calmodulin (Alexander, K. A., Cimler, B. M., Meirer, K. E., and Storm, D. R. (1987) J. Biol. Chem. 262, 6108-6113). It has been proposed that neuromodulin may function to bind and concentrate calmodulin at specific sites within neurons and that activation of protein kinase C causes the release of free calmodulin at high concentrations near its target proteins. It was the goal of this study to determine whether bovine brain contains a phosphoprotein phosphatase that will utilize phosphoneuromodulin as a substrate. Phosphatase activity for phosphoneuromodulin was partially purified from a bovine brain extract using DEAE-Sephacel and Sephacryl S-200 gel filtration chromatography. The neuromodulin phosphatase activity was resolved into two peaks by Affi-Gel Blue chromatography. One of these phosphatases, which represented approximately 60% of the total neuromodulin phosphatase activity, was tentatively identified as calcineurin by its requirement for Ca2+ and calmodulin (CaM) and inhibition of its activity by chlorpromazine. Therefore, bovine brain calcineurin was purified to homogeneity and examined for its phosphatase activity against bovine phosphoneuromodulin. Calcineurin rapidly dephosphorylated phosphoneuromodulin in the presence of micromolar Ca2+ and 3 microM CaM. The apparent Km and Vmax for the dephosphorylation of neuromodulin, measured in the presence of micromolar Ca2+ and 2 microM CaM, were 2.5 microM and 70 nmol Pi/mg/min, respectively, compared to a Km and Vmax of 4 microM and 55 nmol Pi/mg/min, respectively, for myosin light chain under the same conditions. Dephosphorylation of neuromodulin by calcineurin was stimulated 50-fold by calmodulin in the presence of micromolar free Ca2+. Half-maximal stimulation was observed at a calmodulin concentration of 0.5 microM. We propose that phosphoneuromodulin may be a physiologically important substrate for calcineurin and that calcineurin and protein kinase C may regulate the levels of free calmodulin available in neurons.     

Purification and characterization of a brain-specific protein kinase C substrate, neurogranin (p17). Identification of a consensus amino acid sequence between neurogranin and neuromodulin (GAP43) that corresponds to the protein kinase C phosphorylation site and the calmodulin-binding domain.

Neurogranin, formerly designated p17 (Baudier, J., Bronner, C., Kligman, D., and Cole, R. D.) (1989) J. Biol. Chem. 264, 1824-1828), a brain-specific in vitro substrate for protein kinase C (PKC), has been purified to homogeneity from bovine forebrain. The purified protein has a molecular mass of 7837.1 +/- 0.5 Da, determined by electrospray mass spectrometry. In the absence of reducing agent, dimers and higher oligomers accumulated. On sodium dodecyl sulfate-polyacrylamide gels the protein monomer migrated abnormally with an apparent molecular mass of 15,000-19,000 Da, depending on the percentage of polyacrylamide. The native protein is blocked at its amino terminus. The majority of the primary amino acid sequence was determined following proteolytic and chemical fragmentation. A comparison of the amino acid sequence of neurogranin with that of the brain-specific PKC substrate neuromodulin, revealed a strikingly conserved amino acid sequence AA(X)KIQA-SFRGH(X)(X)RKK(X)K. The two proteins are not related over the rest of their sequences. Neurogranin was shown to be phosphorylated in hippocampal slices incubated with 32Pi and phorbol esters stimulated neurogranin phosphorylation, suggesting that neurogranin is likely to be an in vivo substrate for PKC. In vitro phosphorylation of neurogranin by PKC produced a shift of the isoelectric point of the protein (pI 5.6) to a more acidic value (pI 5.4). Tryptic digestion of the phosphorylated protein yielded a single phosphopeptide having the sequence IQASFR, where the serine residue is the phosphorylated amino acid. This phosphopeptide is part of the conserved sequence shared with neuromodulin and also corresponds to the PKC phosphorylation site on neuromodulin (Apel, E. D., Byford, M. F., Au, D., Walsh, K. A., and Storm, D. R. (1990) Biochemistry 29, 2330-2335). Evidence was obtained suggesting that neurogranin binds to calmodulin in the absence of Ca2+, a feature that also characterizes neuromodulin. We propose that the amino acid sequence shared by neurogranin and neuromodulin reflects a functional relationship between these two proteins and that the consensus sequence represents a conserved PKC phosphorylation site and a calmodulin binding domain that characterizes a class of brain-specific PKC substrates.     

Calmodulin and Ca2+-dependent phosphorylation and dephosphorylation of 63-kDa subunit-containing bovine brain calmodulin-stimulated cyclic nucleotide phosphodiesterase isozyme

Bovine brain contains calmodulin-dependent cyclic nucleotide phosphodiesterase isozymes which are composed of two distinct subunits: Mr 60,000 and 63,000. The 60-kDa but not the 63-kDa subunit-containing isozyme can be phosphorylated by cAMP-dependent protein kinase resulting in decreased affinity of this subunit toward calmodulin (Sharma, R. K., and Wang, J. H. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 2603-2607). In contrast, purified 63-kDa subunit-containing isozyme has been found to be phosphorylated by a preparation of bovine brain calmodulin-binding proteins in the presence of Ca2+ and calmodulin. The phosphorylation resulted in the maximal incorporation of 2 mol of phosphate/mol of the phosphodiesterase subunit with a 50% decrease in the enzyme affinity toward calmodulin. At a constant calmodulin concentration of 6 nM, the phosphorylated isozyme required a higher concentration of Ca2+ for activation than the nonphosphorylated phosphodiesterase. The Ca2+ concentrations at 50% activation by calmodulin of the nonphosphorylated and phosphorylated isozymes were 1.1 and 1.9 microM, respectively. Phosphorylation can be reversed by the calmodulin-dependent phosphatase, calcineurin, but not by phosphoprotein phosphatase 1. The results suggest that the Ca2+ sensitivities of brain calmodulin-dependent cyclic nucleotide phosphodiesterase isozymes can be modulated by protein phosphorylation and dephosphorylation mechanisms in response to different second messengers.     

Phosphorylation of neuromodulin (GAP-43) by casein kinase II. Identification of phosphorylation sites and regulation by calmodulin.

Neuromodulin (P-57, GAP-43, B-50, F-1) is a neurospecific calmodulin-binding protein believed to play a role in regulation of neurite outgrowth and neuroplasticity. Neuromodulin is phosphorylated by protein kinase C, and this phosphorylation prevents calmodulin from binding to neuromodulin (Alexander, K. A., Cimler, B. M., Meier, K. E. & Storm, D. R. (1987) J. Biol. Chem. 262, 6108-6113). The only other protein kinase known to phosphorylate neuromodulin is casein kinase II (Pisano, M. R., Hegazy, M. G., Reimann, E. M. & Dokas, L. A. (1988) Biochem. Biophys. Res. Commun. 155, 1207-1212). Phosphoamino acid analyses revealed that casein kinase II modified serine and threonine residues in both native bovine and recombinant mouse neuromodulin. Two serines located in the C-terminal end of neuromodulin, Ser-192 and Ser-193, were identified as the major casein kinase II phosphorylation sites. Thr-88, Thr-89, or Thr-95 were identified as minor casein kinase II phosphorylation sites. Phosphorylation by casein kinase II did not affect the ability of neuromodulin to bind to calmodulin-Sepharose. However, calmodulin did inhibit the phosphorylation of neuromodulin by casein kinase II with a Ki of 1-2 microM. Calmodulin inhibition of casein kinase II phosphorylation was due to calmodulin binding to neuromodulin rather than to the protein kinase. These data suggest that the minimal secondary and tertiary structure exhibited by neuromodulin may be sufficient to juxtapose its calmodulin-binding domain, located at the N-terminal end, with the neuromodulin casein kinase II phosphorylation sites at the C-terminal end of the protein. We propose that calmodulin regulates casein kinase II phosphorylation of neuromodulin by binding to neuromodulin and sterically hindering the interaction of casein kinase II with its phosphorylation sites on neuromodulin.     

Cyclic GMP regulation of the plasma membrane (Ca2+-Mg2+)ATPase in vascular smooth muscle

Plasma membrane (Ca2+-Mg2+)ATPase purified from bovine aortic microsomes by calmodulin affinity chromatography was incorporated into soybean phospholipid liposomes. In the reconstituted proteoliposomes, a protein corresponding to the ATPase was phosphorylated by [gamma-32P]ATP in the presence of cGMP and cGMP-dependent protein kinase. Both the affinity for Ca2+ and the maximum Ca2+ uptake activity by the proteoliposomes were increased by the cGMP-dependent phosphorylation, and there was good parallelism between the Ca2+-uptake rate and the extent of phosphorylation. These results strongly suggest that the Ca2+-transport ATPase of the vascular smooth muscle plasma membrane is regulated through its cGMP-dependent phosphorylation.     

Protein kinase C substrates from bovine brain. Purification and characterization of neuromodulin, a neuron-specific calmodulin-binding protein

Although such solubility is uncommon among proteins generally, several bovine brain proteins were found to be soluble in 2.5% perchloric acid, and many of them were in vitro substrates for protein kinase C (Ca2+/phospholipid-dependent enzyme). Two of the perchloric acid-soluble brain proteins were purified, p43 and p17. P43 and p17 could be phosphorylated by protein kinase C only in the presence of Ca2+ and phospholipids and neither was a substrate for protein kinase II. P43 was subsequently identified as the neurospecific, calmodulin-binding protein, neuromodulin (also designated P-57, GAP43, B50, or F1) (Alexander, K. H., Wakim, B. T., Doyle, G. S., Walsh, K. A., and Storm, D. R. (1988) J. Biol. Chem. 263, 7544-7549). A rapid purification method for neuromodulin was developed taking advantage of its newly discovered property, solubility in 2.5% perchloric acid, and of its previously recognized calmodulin-binding property. Evidence was obtained that neuromodulin isolated from cytosolic extract exists as a mixture of molecular forms and that the Ca2+-binding S100 protein-beta discriminates among the different neuromodulin isoforms in forming covalent complexes via disulfide bridges; this discrimination may be explained by analogous differences observed between the NH2-terminal amino acid sequences of p57 and F1. Solubility in 2.5% perchloric acid was demonstrated for another rat brain protein kinase C substrate, p87. We suggest that perchloric acid solubility might be a common property of protein kinase C substrates.     

Phosphorylation and characterization of bovine heart calmodulin-dependent phosphodiesterase.

Calmodulin-dependent phosphodiesterase was purified to apparent homogeneity from the total calmodulin-binding fraction of bovine heart in a single step by immunoaffinity chromatography. The isolated enzyme had significantly higher affinity for calmodulin than the bovine brain 60-kDa phosphodiesterase isozyme. The cAMP-dependent protein kinase was found to catalyze the phosphorylation of the purified cardiac calmodulin-dependent phosphodiesterase with the incorporation of 1 mol of phosphate/mol of subunit. The phosphodiesterase phosphorylation rate was increased severalfold by histidine without affecting phosphate incorporation into the enzyme. Phosphorylation of phosphodiesterase lowered its affinity for calmodulin and Ca2+. At constant saturating concentrations of calmodulin (650 nM), the phosphorylated calmodulin-dependent phosphodiesterase required a higher concentration of Ca2+ (20 microM) than the nonphosphorylated phosphodiesterase (0.8 microM) for 50% activity. Phosphorylation could be reversed by the calmodulin-dependent phosphatase (calcineurin), and dephosphorylation was accompanied by an increase in the affinity of phosphodiesterase for calmodulin.     

Phosphorylation of Rab proteins from the brain of Bombyx mori

Rab proteins play fundamental roles in the regulation of membrane traffic. Previously, from the brain of Bombyx mori we isolated two cDNA clones (BRab1 and BRab14), each of which encoded a different member of Rab-protein family and was expressed in Escherichia coli and purified using an affinity chromatography. In this study, one cDNA clone (BRab8) was isolated from a cDNA library from the brain of B. mori. The recombinant protein was expressed in E. coli and purified. Next, the phosphorylations of these three purified BRab proteins were examined, using mammalian protein kinases in vitro. Protein kinase C (PKC) phosphorylated BRab8 and BRab14 proteins. Protein kinase A faintly phosphorylated BRab8 and BRab14 proteins. Calcium/calmodulin-dependent protein kinase faintly phosphorylated BRab8 protein. Next, brains of B. mori were dissected and homogenized. The homogenate showed a calcium-dependent protein kinase activity of BRab8 and BRab14 proteins. So PKC from the brain of B. mori was partially purified by a sequence of chromatographies on DEAE-Cellulofine and affinity chromatography. This PKC phosphorylated BRab8 and BRab14 proteins. These results suggest that the function of Rab proteins in the brain of B. mori is regulated by calcium-dependent protein kinases.     

Purification of recombinant proteins based on the interaction between a phenothiazine-derivatized column and a calmodulin fusion tail.

A method to purify proteins by fusing them to the Ca2+-dependent protein calmodulin is described by using glutathione-S-transferase (GST) from Schistosoma japonicum as a model. Glutathione-S-transferase was genetically fused to calmodulin (CaM). The designed GST-CaM fusion protein has a selective factor Xa cleavage site located between the C-terminus of GST and the N-terminus of CaM. The recombinant fusion protein was expressed in Escherichia coli, and the crude cell extract was loaded onto a phenothiazine affinity column in the presence of Ca2+. Calmodulin was used as an affinity tail to enable binding of the fusion protein to the phenothiazine column. Removal of Ca2+ with a calcium-complexing solution causes elution of the fusion protein. The GST-CaM fusion protein was then digested with factor Xa, and the target protein GST was isolated. The purity of the isolated GST was verified by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).     

Characterization of the human calmodulin-like protein expressed in Escherichia coli.

The protein-coding region of an intronless human calmodulin-like gene [Koller, M., & Strehler, E. E. (1988) FEBS Lett. 239, 121-128] has been inserted into a pKK233-2 expression vector, and the 148-residue, M(r) = 16,800 human protein was purified to apparent homogeneity by phenyl-Sepharose affinity chromatography from cultures of Escherichia coli JM105 transformed with the recombinant vector. Several milligrams of the purified protein were obtained from 1 L of bacterial culture. A number of properties of human CLP were compared to those of bacterially expressed human calmodulin (CaM) and of bovine brain CaM. CLP showed a characteristic Ca(2+)-dependent electrophoretic mobility shift on SDS-polyacrylamide gels, although the magnitude of this shift was smaller than that observed with CaM. CLP was able to activate the 3',5'-cyclic nucleotide phosphodiesterase to the same Vmax as normal CaM, albeit with a 7-fold higher Kact. In contrast, the erythrocyte plasma membrane Ca(2+)-ATPase could only be stimulated to 62% of its maximal CaM-dependent activity by CLP. CLP was found to contain four Ca(2+)-binding sites with a mean affinity constant of 10(5) M-1, a value about 10-fold lower than that for CaM under comparable conditions. The highly tissue-specifically-expressed CLP represents a novel human Ca(2+)-binding protein showing characteristics of a CaM isoform.     

KDR酪氨酸激酶的克隆表达及纯化

VEGF作为一种血管内皮细胞有丝分裂原,通过与内皮细胞表面特定受体结合,从而导致新生血管的形成,其中VEGFR-2(KDR/FIk-1)在肿瘤血管形成中起重要作用,因此其抑制剂有可能发展成为治疗肿瘤的新药.采用大肠杆菌成功表达了具有酪氨酸激酶活性的KDR酪氨酸激酶催化域(KDR-CD).采用RT-PCR从人脐静脉内皮细胞中提取总RNA,获得KDR-CD的编码基因,将其克隆至pET-30a载体,在E. coli BL21(DE3)中进行了成功表达,采用Ni-NTA亲和层析对其进行了纯化,Western blotting结果显示表迭的KDR-CD蛋白自身磷酸化,重组KDR-CD蛋白具有利用ATP催化底物发生磷酸化反应的激酶活性.     

Characterization of the (Ca2+-Mg2+)ATPase purified by calmodulin-affinity chromatography from bovine aortic smooth muscle

A calmodulin inhibitor, trifluoperazine, suppresses ATP-dependent Ca2+ uptake into microsomes prepared from bovine aortic smooth muscle. From this microsomal preparation which we expected to contain calmodulin-dependent Ca2+-transport ATPase [EC 3.6.1.3], we purified (Ca2+-Mg2+)ATPase by calmodulin affinity chromatography. The protein peak eluted by EDTA had calmodulin-dependent (Ca2+-Mg2+)ATPase activity. The major band (135,000 daltons) obtained after sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) accounted for about 80% of the total protein eluted. This major band was phosphorylated by [gamma-32P]ATP in a Ca2+-dependent manner. All the 32P incorporated into the major band was released by hydroxylaminolysis. The ATPase reconstituted in soybean phospholipid liposomes showed ATP, calmodulin-dependent Ca2+ uptake. The affinity of the ATPase for Ca2+, Km, was 7 microM and the maximum ATPase activity was 1.4 mumol/mg/min. These values were changed to 0.17 microM and 3.5 mumol/mg/min, respectively by the addition of calmodulin. The activity of the purified (Ca2+-Mg2+)ATPase was inhibited by orthovanadate, and the concentration required for half-maximal inhibition was about 1.8 microM which is close to that of plasma membrane ATPases. Judging from the effect of orthovanadate and the molecular weight, the purified (Ca2+-Mg2+)ATPase was considered to have originated from the plasma membrane not from the sarcoplasmic reticulum.     

牛呼吸道合胞体病毒G蛋白的截短表达与鉴定

经生物学软件DNA Star分析,将牛呼吸道合胞体病毒G基因截短成2个片段G1和G2。然后用人工合成的牛呼吸道合胞体病毒G基因为模板,用PCR分别扩增G1和G2基因片段,其大小分别为570 bp和308 bp。将目的片段定向克隆到pET30a表达载体中,酶切及测序鉴定均正确后,转化BL21表达菌,经IPTG诱导后G1和G2基因片段都获得了表达,且都为可溶性表达。用Ni柱亲和层析法在非变性条件下纯化重组蛋白,经免疫印迹试验鉴定证明纯化的重组蛋白G1具有良好的抗原性和特异性,而重组蛋白G2无反应性。应用纯化的重组蛋白G1进行的间接ELISA与免疫印迹试验在国内牛血清中检测到了BRSV血清抗体。本研究所表达的重组蛋白G1为基于牛呼吸道合胞体病毒G蛋白的血清学诊断方法的建立与牛呼吸道合胞体病毒G蛋白生物学功能的研究奠定了基础。     

牛γ-干扰素在大肠杆菌中的表达及抗原性鉴定

目的在大肠杆菌中高效表达牛γ-干扰素（bovine interferon-γ,BovIFN-γ）,并对其生物活性进行初步鉴定。方法依据GenBank上基因序列人工合成BovIFN-γ基因,PCR方法扩增该基因,将其插入PET-28a载体构建原核表达质粒,转化大肠杆菌BL21中,在IPTG诱导下表达BovIFN-γ,并进行Western blot鉴定。Ni-NTA亲和层析法和电洗脱方法纯化表达的重组蛋白,用Western blot和商品化的BovIFN-γ检测试剂盒进行重组蛋白的抗原性检测。结果成功构建了BovIFN-γ原核表达载体PET-28a-BIFN-γ,并在大肠杆菌中高效表达,表达蛋白约占菌体总蛋白的32%,表达产物主要以可溶性形式存在于菌体裂解液上清中;重组蛋白可与BovIFN-γ单克隆抗体反应,Ni-NTA亲和层析法纯化的重组蛋白抗原活性比电洗脱方法纯化的抗原活性高。结论在大肠杆菌中成功表达了可溶性的BovIFN-γ蛋白,可与BovIFN-γ单抗发生反应,纯化的重组蛋白具有良好的反应原性。     

Purification and characterization of a calcium-calmodulin-dependent phospholamban kinase from canine myocardium

A Ca2+-calmodulin-dependent protein kinase was purified to apparent homogeneity from the cytosolic fraction of canine myocardium, with phospholamban as substrate. Purification involved sequential chromatography on DEAE-cellulose, calmodulin-agarose, DEAE-Bio-Gel A, and phosphocellulose. This procedure resulted in a 987-fold purification with a 5.4% yield. The purified enzyme migrated as a single band on native polyacrylamide gels, and it exhibited an apparent molecular weight of 550,000 upon gel filtration. Gel electrophoresis under denaturing conditions revealed a single protein band with Mr 55,000. The purified kinase could be autophosphorylated in a Ca2+-calmodulin-dependent manner, and under optimal conditions, 6 mol of Pi was incorporated per mole of 55,000-dalton subunit. The activity of the enzyme was dependent on Ca2+, calmodulin, and ATP.Mg2+. Other ions which could partially substitute for Ca2+ in the presence of Mg2+ and saturating calmodulin concentrations were Sr2+ greater than Mn2+ greater than Zn2+ greater than Fe2+. The substrate specificity of the purified Ca2+-calmodulin-dependent protein kinase for cardiac proteins was determined by using phospholamban, troponin I, sarcoplasmic reticulum membranes, myofibrils, highly enriched sarcolemma, and mitochondria. The protein kinase could only phosphorylate phospholamban and troponin I either in their purified forms or in sarcoplasmic reticulum membranes and myofibrils, respectively. Exogenous proteins which could also be phosphorylated by the purified protein kinase were skeletal muscle glycogen synthase greater than gizzard myosin light chain greater than brain myelin basic protein greater than casein. However, phospholamban appeared to be phosphorylated with a higher rate as well as affinity than glycogen synthase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification and characterization of the calmodulin-binding domain of neuromodulin, a neurospecific calmodulin-binding protein

Neuromodulin (formerly designated P-57) is an abundant, neural specific, calmodulin-binding protein which exhibits higher affinity for calmodulin in the absence of free Ca2+ than in the presence of free Ca2+. In this study a series of proteolytic fragments of neuromodulin were systematically screened for calmodulin-Sepharose binding activity. A 9-amino acid fragment, designated M1-C1 and having the sequence RGHITRKKL, was identified as the putative CaM-binding domain of neuromodulin. Two heptadecapeptides, designated FP57-Phe and FP57-Trp, were synthesized, each containing the M1-C1 sequence and the four flanking amino acids from each site. The FP57-Trp peptide contained a tryptophan residue in place of the native phenylalanine. Anti-FP57-Phe antibody binding to neuromodulin was inhibited by preincubation of antibodies with excess FP57-Phe. 125I-CaM gel overlay of neuromodulin was inhibited by anti-FP57-Phe antibodies. Addition of CaM to FP57-Trp increased peptide tryptophanyl fluorescence. In the presence of Ca2+, the stoichiometry of the FP57-Trp.CaM complex was 1:1, FP57-Trp binding to CaM was competitive with neuromodulin. The Ca2+-independent dissociation constant of the FP57-Phe.CaM complex was 0.41 microM. The Ca2+-dependent affinity of the complex could not be measured directly but appeared to be significantly greater than the Ca2+-independent affinity.     

异戊二烯合成酶(IspS)在大肠杆菌中的表达及其产异戊二烯的研究

将来源于银白杨的异戊二烯合成酶基因按照大肠杆菌密码子偏爱性进行优化,克隆到表达载体pACYCDu-et-1上,在大肠杆菌BL21(DE3)中异源表达,采用镍柱亲和层析纯化重组蛋白并测定其异戊二烯合成酶活性,通过摇瓶发酵实验对重组菌产异戊二烯进行进一步研究。结果显示:银白杨异戊二烯合成酶在大肠杆菌中能够高效表达,经过镍柱纯化后,电泳检测到特异性表达条带;该重组异戊二烯合成酶能够催化异戊二烯的合成,重组菌的异戊二烯产量可达到60μg/L。     

Expression of calreticulin in Escherichia coli and identification of its Ca2+ binding domains

Recombinant calreticulin and discrete domains of calreticulin were expressed in Escherichia coli, using the glutathione S-transferase fusion protein system, and their Ca2+ binding properties were determined. Native calreticulin bound 1 mol of Ca2+/mol of protein with high affinity, and also bound approximately 20 mol of Ca2+/mol of protein with low affinity. Both Ca2+ binding sites were present in the recombinant calreticulin indicating that proper folding of the protein was achieved using this system. Calreticulin is structurally divided into three distinct domains: the N-domain encompassing the first 200 residues; the P-domain which is enriched in proline residues (residue 187-317); and the C-domain which covers the carboxyl-terminal quarter of the protein (residues 310-401), and contains a high concentration of acidic residues. These domains were expressed in E. coli, isolated, and purified, and their Ca2+ binding properties were analyzed. The C-domain bound approximately 18 mol of Ca2+/mol of protein with a dissociation constant of approximately 2 mM. The P-domain bound approximately 0.6-1 mol of Ca2+/mol of protein with a dissociation constant of approximately 10 microM. The P-domain and the C-domain, when expressed together as the P+C-domain, bound Ca2+ with both high affinity and low affinity, reminiscent of both full length recombinant calreticulin and native calreticulin. In contrast the N-domain, did not bind any detectable amount of 45Ca2+. We conclude that calreticulin has two quite distinct types of Ca2+ binding sites, and that these sites are in different structural regions of the molecule. The P-domain binds Ca2+ with high affinity and low capacity, whereas the C-domain binds Ca2+ with low affinity and high capacity.