Monoclonal antibody studies of creatine kinase. Antibody-binding sites in the N-terminal region of creatine kinase and effects of antibody on enzyme refolding

Chaperone-like effect of the linker on the isolated C-terminal domain of rabbit muscle creatine kinase

Intramolecular chaperones (IMCs), which are specific domains/segments encoded in the primary structure of proteins, exhibit chaperone-like activity against the aggregation of the other domains in the same molecule. In this research, we found that the truncation of the linker greatly promoted the thermal aggregation of the isolated C-terminal domain (CTD) of rabbit muscle creatine kinase (RMCK). Either the existence of the linker covalently linked to CTD or the supply of the synthetic linker peptide additionally could successfully protect the CTD of RMCK against aggregation in a concentration-dependent manner. Truncated fragments of the linker also behaved as a chaperone-like effect with lower efficiency, revealing the importance of its C-terminal half in the IMC function of the linker. The aggregation sites in the CTD of RMCK were identified by molecular dynamics simulations. Mutational analysis of the three key hydrophobic residues resulted in opposing effects on the thermal aggregation between the CTD with intact or partial linker, confirming the role of linker as a lid to protect the hydrophobic residues against exposure to solvent. These observations suggested that the linkers in multidomain proteins could act as IMCs to facilitate the correct folding of the aggregation-prone domains. Furthermore, the intactness of the IMC linker after proteolysis modulates the production of off-pathway aggregates, which may be important to the onset of some diseases caused by the toxic effects of aggregated proteolytic fragments.

Dissimilarity in the folding of human cytosolic creatine kinase isoenzymes

Creatine kinase (CK, EC 2.7.3.2) plays a key role in the energy homeostasis of excitable cells. The cytosolic human CK isoenzymes exist as homodimers (HMCK and HBCK) or a heterodimer (MBCK) formed by the muscle CK subunit (M) and/or brain CK subunit (B) with highly conserved three-dimensional structures composed of a small N-terminal domain (NTD) and a large C-terminal domain (CTD). The isoforms of CK provide a novel system to investigate the sequence/structural determinants of multimeric/multidomain protein folding. In this research, the role of NTD and CTD as well as the domain interactions in CK folding was investigated by comparing the equilibrium and kinetic folding parameters of HMCK, HBCK, MBCK and two domain-swapped chimeric forms (BnMc and MnBc). Spectroscopic results indicated that the five proteins had distinct structural features depending on the domain organizations. MBCK BnMc had the smallest CD signals and the lowest stability against guanidine chloride-induced denaturation. During the biphasic kinetic refolding, three proteins (HMCK, BnMc and MnBc), which contained either the NTD or CTD of the M subunit and similar microenvironments of the Trp fluorophores, refolded about 10-fold faster than HBCK for both the fast and slow phase. The fast folding of these three proteins led to an accumulation of the aggregation-prone intermediate and slowed down the reactivation rate thereby during the kinetic refolding. Our results suggested that the intra- and inter-subunit domain interactions modified the behavior of kinetic refolding. The alternation of domain interactions based on isoenzymes also provides a valuable strategy to improve the properties of multidomain enzymes in biotechnology.

Dissecting the key residues crucial for the species-specific thermostability of muscle-type creatine kinase

Species-specific protein thermal stability is closely correlated to the living conditions of the organism, especially to its body temperature. In this research, human and zebrafish muscle-type creatine kinases (MMCKs) were taken as model proteins to investigate the molecular adaptation of proteins in poikilothermal and homoiothermal animals. Both the optimal temperature for catalysis and the thermal stability of human MMCK was much higher than those of zebrafish MMCK. Sequence alignment identified 9 amino acid variations conserved in either the teleost MMCKs or the mammal and electric ray MMCKs. Bidirectional mutations were performed to find the residues with beneficial mutations. The results showed that two residues close to the dimer interface of MMCK, the 46th and 146th residue, were crucial for species-specific thermal stability.

Sequential events in the irreversible thermal denaturation of human brain-type creatine kinase by spectroscopic methods

The non-cooperative or sequential events which occur during protein thermal denaturation are closely correlated with protein folding, stability, and physiological functions. In this research, the sequential events of human brain-type creatine kinase (hBBCK) thermal denaturation were studied by differential scanning calorimetry (DSC), CD, and intrinsic fluorescence spectroscopy. DSC experiments revealed that the thermal denaturation of hBBCK was calorimetrically irreversible. The existence of several endothermic peaks suggested that the denaturation involved stepwise conformational changes, which were further verified by the discrepancy in the transition curves obtained from various spectroscopic probes. During heating, the disruption of the active site structure occurred prior to the secondary and tertiary structural changes. The thermal unfolding and aggregation of hBBCK was found to occur through sequential events. This is quite different from that of muscle-type CK (MMCK). The results herein suggest that BBCK and MMCK undergo quite dissimilar thermal unfolding pathways, although they are highly conserved in the primary and tertiary structures. A minor difference in structure might endow the isoenzymes dissimilar local stabilities in structure, which further contribute to isoenzyme-specific thermal stabilities.

Mutation of the conserved Asp122 in the linker impedes creatine kinase reactivation and refolding

Creatine kinase (CK), a key enzyme in maintaining the intracellular energetic homeostasis, contains two domains connected by a long linker. In this research,we found that the mutations of the conserved Asp122 in the linker slightly affected CK activity, structure and stability. The hydrogen bonding and the ion pair contributed 2-5 kJ/mol to the conformational stability of CK. Interestingly, the ability of CK reactivation from the denatured state was completely removed by the mutations. These results suggested that the electrostatic interactions were crucial to the action of the linker in CK reactivation.

CK-MM autoantibodies: Prevalence, immune complexes, and effect on CK clearance

Although the blood level of creatine kinase (CK) is the most commonly used marker of muscle injury, there is marked interindividual variability in this measure. Part of this variability may be attributed to variability in the rate of CK clearance from the circulation. In this study, we assessed the possibility that CK-MM autoantibodies form immune complexes with CK following muscle injury and subsequently affect the CK clearance rate. Using an enzyme-linked immunosorbent assay, CK-MM autoantibodies were detected in all 25 human subjects studied but the levels varied greatly. Using protein A-sepharose, the percentage of the plasma CK activity found in immune complexes was determined to be correlated with the CK-MM autoantibody level at lower CK levels (<1,022 U/L). When CK-MM antibodies were administered to mice, plasma CK activity following a bolus CK injection was reduced by 11%-32%. We conclude that CK-MM autoantibodies can modulate the rate of CK clearance from the circulation. Thus, the relatively low blood CK levels seen in some individuals following injury may be attributed partly or entirely to an autoantibody-enhanced clearance of CK.

Antibodies as specific chaperones

Protein folding is often accompanied by formation of non-native conformations leading to protein aggregation. A number of reports indicate that antibodies can facilitate folding and prevent aggregation of protein antigens. The influence of antibodies on folding is strictly antigen specific. Chaperone-like antibody activity may be due to the stabilization of native antigen conformations or folding transition states, or screening of aggregating hydrophobic surfaces. Taking advantage of chaperone-like activity of antibodies for immunotherapy may prove to be a promising approach to the treatment of Alzheimer's and prion-related diseases. Antibody-assisted folding may enhance renaturation of recombinant proteins from inclusion bodies.

Mutation of conserved active-site threonine residues in creatine kinase affects autophosphorylation and enzyme kinetics

Muscle-type creatine kinase (MM-CK) is a member of an isoenzyme family with key functions in cellular energetics. It has become a matter of debate whether the enzyme is autophosphorylated, as reported earlier [Hemmer, Furter-Graves, Frank, Wallimann and Furter (1995) Biochim. Biophys. Acta 1251, 81-90], or exclusively nucleotidylated. In the present paper, we demonstrate unambiguously that CK is indeed autophosphorylated. However, this autophosphorylation is not solely responsible for the observed microheterogeneity of MM-CK on two-dimensional isoelectric focusing gels. Using phosphoamino-acid analysis of (32)P-labelled CK isoforms, phosphothreonine (P-Thr) residues were identified as the only product of autophosphorylation for all CK isoenzymes. The phosphorylated residues in chicken MM-CK were allocated to a region in the vicinity of the active site, where five putative phosphorylation sites were identified. Site-directed threonine-valine-replacement mutants reveal that autophosphorylation is not specific for one particular residue but occurs at all examined threonine residues. The enzyme kinetic parameters indicate that the autophosphorylation of CK exerts a modulatory effect on substrate binding and the equilibrium constant, rather than on the catalytic mechanism itself.

Effect of Mg2+ during reactivation and refolding of guanidine hydrochloride-denatured creatine kinase

Creatine kinase (ATP: creatine N-phosphotransferase, EC 2.7.3.2) was completely denatured using 3 M guanidine hydrochloride for 2 h as in previous studies [Yao et al. (1982), Sci. Sin. 25B, 1296-1302; Yao et al. (1984), Biochemistry 23, 2740-2744; Yao et al. (1982), Sci. Sin. 25B, 1186-1193]. Under suitable conditions, about 60-70% of the activity can be recovered in the presence of different Mg2+ concentrations. Both the reactivation and the refolding processes follow two-phase courses after dilution in the proper solutions. A comparison of the rate constants for the refolding of unfolded creatine kinase with those for the recovery of its catalytic activity at various Mg2+ concentrations shows that these are not synchronized. The reactivity of guanidine hydrochloride-denatured creatine kinase can be inhibited by Mg2+; however, the rates of reactivation are independent of the Mg2+ concentration. In addition, Mg2+ affects the fluorescence intensity, but the rate constants of refolding are independent of Mg2+ concentration. Although the reactivation of GdHCl-denatured creatine kinase is complete about 3 h after dilution with reactivation solutions, the conformational changes during refolding occur in a much slower reaction. Mg2+ can induce complex changes in the relative fluorescence intensity during refolding over a broad range of concentrations.

Reactivation and refolding of a partially folded creatine kinase modified by 5,5'-dithio-bis(2-nitrobenzoic acid)

Creatine kinase with its thiol groups modified by 5, 5'-dithio-bis(2-nitrobenzoic acid) has been shown to be partially folded in a monomeric state using fluorescence, circular dichroism, proteolysis, and size exclusion chromatography studies. In the presence of DTT, the partially folded modified creatine kinase can be reactivated and refolded following a biphasic course, suggesting the existence of a monomeric intermediate during the refolding of CK. The results provide evidence for our previously suggested model of the refolding pathway of urea-denatured creatine kinase.

Protease digestion studies of an equilibrium intermediate in the unfolding of creatine kinase

Protease digestion experiments have been used to characterize the structure of an equilibrium intermediate in the unfolding of creatine kinase (CK) by low concentrations (0.625 M) of guanidine hydrochloride (GdnHCl). Eighteen of the major products of digestion by trypsin, chymotrypsin and endoproteinase Glu-C have been identified by microsequencing after separation by SDS/PAGE and electroblotting on poly(vinylidene difluoride) membranes. The C-terminal portion (Gly215 to Lys380) was much more resistant to digestion than the N-terminal portion (Pro1 to Gly133), although the area most sensitive to proteolysis was in the middle of the CK sequence (Arg134 to Arg214). These experiments are consistent with the two-domain model for the CK monomer. The structure of the intermediate is proposed to consist of a folded C-terminal domain and a partly folded N-terminal domain separated by an unfolded central linker. Protease susceptibility is clustered within two N-terminal regions and one central region. These regions are evidently exposed as a result of the partial unfolding and/or separation of the N-terminal domain. Further evidence for the structure of this intermediate comes from gel filtration studies. Treatment of CK with 0.625 M GdnHCl resulted in slow aggregation at 37 degrees C, but not at 12 degrees C, a phenomenon previously reported for phosphoglycerate kinase. The aggregation did not occur at higher GdnHCl concentrations and was unaffected by a reducing agent. It is proposed that aggregation is a consequence of non-specific interactions between hydrophobic regions, possibly domain/domain interfaces, which become exposed in the intermediate.

Dystrophin expression in the developing conduction system of the human heart

Duchenne muscular dystrophy (DMD) is frequently associated with myocardial involvement. Dystrophin, the DMD protein, is found at the plasmamembrane of striated muscle fibers. Although dystrophin is missing in most or all muscle fibers of DMD patients, cardiac muscle is not as severely affected as skeletal muscle. Therefore it is of great importance to study the expression of dystrophin in normal cardiac muscle. We performed immunohistochemical studies and examined cardiac muscle of fetuses of 8 to 13 weeks of development on dystrophin expression. At these stages dystrophin is observed in the myocytes of the developing ventricular conduction system and in the atrial cardiomyocytes. Dystrophin was absent from the heart of a 12-week-old DMD fetus.

Immunohistochemical delineation of the conduction system. I: The sinoatrial node

We have raised a mouse monoclonal antibody that reacts specifically with the myocytes of the sinoatrial node of the bovine heart. By use of this antibody (445-6E10) and antibodies against the gap junction protein connexin43, the periphery of the sinoatrial node and the distribution of gap junctions in the nodal region were studied. The reaction patterns of 445-6E10 and anti-connexin43 are exactly complementary; ie, connexin43 was not detected in the nodal myocytes but was clearly present in the atrial myocytes. Both reaction patterns demonstrate that nodal myocytes and atrial myocytes can unambiguously be distinguished by their characteristic molecular phenotype. The transitional nodal myocytes at the periphery of the node that have intermediate morphological and electrophysiological characteristics could now clearly be defined as nodal by our immunohistochemical criteria. The center of the node is surrounded by a region of interdigitating nodal and atrial bundles. Nodal bundles, coming from the center of the node, penetrate the atrial myocardium aligned at atrial bundles, forming histological connections between nodal and atrial myocytes at regular distances. This interdigitating arrangement of bundles of connexin43-negative nodal and connexin43-positive atrial myocytes is also found in the human and rat heart. We hypothesize that the architecture of the periphery of the node is important to prevent silencing of the pacemaking nodal myocytes by the atrium while ensuring a sufficient source loading of the nodal myocytes.

Changes at the N-terminus of human brain creatine kinase during a transition between inactive folding intermediate and active enzyme

CK-STAR, a monoclonal antibody against human brain creatine kinase (CK), can be shown by chemical cleavage mapping and peptide synthesis to recognize an epitope at the free N-terminus of the enzyme. The epitope could be largely reproduced by a synthetic peptide based on the first 18 amino acids and could be partly formed by the first 11 amino acids. The antibody did not bind to native CK, but it did bind to CK in various partially denatured forms and to an enzymically inactive intermediate in the refolding process. Competitive binding studies have shown that the N-terminal conformations of both the refolding intermediate and the free peptide resemble that of CK partially denatured by attachment to plastic. The results suggest that the final stages of CK refolding and reactivation involve a structural change at the N-terminus or its interaction with some other part of the CK molecule, thus masking the CK-STAR epitope.

Structural changes in the C-terminal region of human brain creatine kinase studied with monoclonal antibodies

Epitopes on human brain creatine kinase (B-CK) recognized by three monoclonal antibodies have been located by chemical cleavage methods, followed by peptide synthesis or analysis of specificity for natural variants (isoforms). One antibody, CK-HTB, recognizes a conformational, or assembled, surface epitope on native CK which is also present on partially unfolded forms. It requires an Asn residue at position 300 in the amino acid sequence and will not recognize variants with Lys or His in this position. This results in a striking specificity of the antibody, which binds to B-CK only in chicken and man, but to muscle-form (M-CK) only in the rat. The results suggest that Asn-300 is exposed on the enzyme surface as part of a relatively denaturation-resistant region. Two monoclonal antibodies, CK-END1 and CK-END2, recognise epitopes within 53 amino acids of the C-terminus and bind to a synthetic hexapeptide representing the last six amino acids of human B-CK (Leu-375-Lys-380). The two antibodies show overlapping, but distinct, specificities in their binding to CK variants. CK-END1 requires Met-376 and will not tolerate Ile in this position, whereas CK-END2 requires Leu-375 and will not tolerate Met. Neither antibody binds to native CK, though both will bind to a folding intermediate and to partially unfolded states. This shows that the C-terminus of CK becomes inaccessible to the antibodies during those later stages of protein folding associated with recovery of enzyme activity and suggests that the protein may 'tuck in its tail' during one of the final steps.

Different localization of dystrophin in developing and adult human skeletal muscle

Duchenne and Becker muscular dystrophy are caused by defects in dystrophin synthesis. Using affinity-purified polyclonal anti-dystrophin antibodies, we have studied immunohistochemically the subcellular localization of dystrophin in embryonic, fetal, and adult human skeletal muscle. In the embryonic stages dystrophin first appears in the sarcoplasm at the peripheral ends of the myotubes, immediately adjacent to the tendons, whereas in fetal stages dystrophin is found throughout the entire myofibers. In agreement with literature data, in adult muscle dystrophin expression was found to be restricted to the sarcolemma. The sarcoplasmic localization in embryonic and fetal tissue and the sarcolemmal localization of dystrophin in mature muscle suggests the accumulation of dystrophin in the cytoplasm prior to its integration into the membrane. These results increase our knowledge of the ontogenesis of dystrophin and may lead to a better understanding of the great diversity in pathological cases of Duchenne and Becker muscular dystrophy.

Spatial distribution of "tissue-specific" antigens in the developing human heart and skeletal muscle. I. An immunohistochemical analysis of creatine kinase isoenzyme expression patterns

Using monoclonal antibodies against the M and B subunit isoforms of creatine kinase (CK) we have investigated their distribution in developing human skeletal and cardiac muscle immunohistochemically. It is demonstrated that in skeletal muscle, a switch from CK-B to CK-M takes place around the week 8 of development, whereas in the developing heart, CK-M is the predominant isoform from the earliest stage examined onward (i.e., 4 1/2 weeks of development). In all hearts examined, local differences in concentration of the CK isoforms are observed. The CK-M expression in the developing outflow tract (OFT) and conduction system is described in detail. Between the weeks 5 and 7 of development, the distal portion of the OFT is characterized by low CK-M expression, whereas around the week 8-10 of development the myocardium around the developing semilunar valves in the OFT expresses a very high level of CK-M. At all stages examined, a relatively low CK-M level is observed in those regions in which the "slow" components of the conduction system do develop (e.g., the sinoatrial junction and atrioventricular junction), whereas a relatively high concentration of CK-M is observed in those areas that are destined to become the "fast" components, i.e., the subendocardial myocardium of the ventricles. The high expression of CK-M in the developing "fast components" of the conduction system contrasts with the relatively low expression of CK-M in the force-producing myocardium of the interventricular septum and free ventricular wall.

Creatine kinase isozyme expression in prenatal rat heart

The distribution pattern of creatine kinase (E.C 2.7.3.2) isozymes in prenatal rat heart and skeletal muscle was studied by immunohistochemistry. Between embryonic day (ED) 12-18, creatine kinase M (CK-M) is heterogeneously expressed in the heart: a pronounced staining of CK-M is first observed in the outflow tract and the trabeculae of the right ventricle (ED12-14), and subsequently in the venous valves, the interatrial septum and the sinoatrial node. From ED18 onwards, a homogeneous expression of CK-M is observed due to an increase in isozyme concentration in the remaining part of the myocardium. By contrast, the developmental appearance of creatine kinase B (CK-B) occurs almost homogeneously throughout the heart between ED11-14. Thereafter, a decrease of the CK-B is first observed in the inflow tract (in particular in the sinoatrial node), in the inner part of those atrial walls that are adjacent to the atrioventricular junction, and temporarily in a band in the upper part of the interventricular septum. From ED18, a selective disappearance of CK-B is found in the papillary muscle of the left ventricle. At birth, a considerable amount of CK-B remains present in the ventricular walls. Although some of the stage-dependent regional differences in expression of the creatine kinase isozymes, in particular those of the M-subunit, are shared by other mammalian and avian species, their significance for the developmental changes in the physiology of the heart is speculative at present.

Monoclonal antibody studies suggest a catalytic site at the interface between domains in creatine kinase

We have located the epitopes recognized by four different monoclonal antibodies which bind to partially unfolded creatine kinase (CK) (ATP: creatine N-phosphotransferase, EC 2.7.3.2) but not to the native enzyme. The epitopes appear to be buried within the CK structure in its native, proteinase-resistant, state. When the epitopes are made accessible to antibody by mild denaturation, CK becomes enzymically-inactive and can be cleaved by proteinase V8 into two large fragments which retain the epitopes and may represent domains. Epitopes on each V8 fragment are associated with highly conserved sequences and are brought physically close to the active site of the enzyme during the later stages of CK refolding and reactivation. The results suggest a catalytic site formed at the interface between two domains which carry the epitopes on their interacting surfaces. Separation of loosely associated domains before or during immunization may account for the origin of antibodies against buried epitopes.

Treatment of human muscle creatine kinase with glutaraldehyde preferentially increases the immunogenicity of the native conformation and permits production of high-affinity monoclonal antibodies which recognize two distinct surface epitopes

Treatment of human muscle creatine kinase (MM-CK) with glutaraldehyde produced highly aggregated forms which retained the native antigenic structure. Immunization of BALB/c mice with CK aggregates instead of untreated CK produced over ten-fold higher titres of antibody against native CK without increasing the titres of antibody against denatured enzyme. Production of high-affinity monoclonal antibodies specific for both the muscle isoenzyme and the native conformation became possible where the use of untreated CK had failed. Four monoclonal antibodies have been characterized by an epitope mapping technique and compared with a commercially available monoclonal antibody. One antibody has a much higher affinity for MM-CK than the other three and the commercial antibody. Competition studies show that it also recognizes a different epitope on the CK surface from the other three monoclonal antibodies which bind to the same surface region as the commercial antibody. Immunoassays based on the high affinity antibody can easily measure less than 1 ng of CK, a sensitivity comparable to, or better than, standard enzymatic assays.

Monoclonal antibody studies of creatine kinase. The ART epitope: evidence for an intermediate in protein folding

Chemical cleavage at cysteine residues with nitrothiocyanobenzoic acid shows that the last 98 amino acids of the 380-amino-acid sequence of chick muscle creatine kinase are sufficient for binding of the monoclonal antibody CK-ART. Removal of the last 30 amino acids by cleavage at methionine residues with CNBr results in loss of CK-ART binding. CK-ART binding is also lost when these C-terminal methionine residues are oxidized to sulphoxide, but binding is regained on reduction. Proteinase K 'nicks' native CK at a single site near the C-terminus and two fragments of 327 amino acides and 53 amino acids can be separated by subsequent SDS or urea treatment. CK-ART still binds normally to 'nicked' CK, which is enzymically inactive. After treatment with either urea (in a competition enzyme-linked immunosorbent assay) or SDS (on Western blots), however, CK-ART binds to neither of the two fragments, although these treatments do not affect binding to intact CK. This suggests that parts of both CK fragments contribute to the CK-ART epitope. CK-ART is both species- and isoenzyme-specific, binding only to chick M-CK. The only C-terminal regions containing chick-specific sequences are residues 300-312 and residues 368-371, the latter group being close to the essential methionine residues. We suggest that one, or possibly both, of these regions is involved in forming the conformational epitope on the surface of the CK molecule which CK-ART recognizes. Native CK is resistant to trypsin digestion. The C-terminal half of urea-treated and partly-refolded CK is also resistant to trypsin digestion, whereas the N-terminal half is readily digested. The results suggest a C-terminal region which can refold more rapidly than the rest of the CK molecule and provide evidence for an intermediate in CK refolding.