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

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.

Epitope mapping of recombinant antigens by transposon mutagenesis

We describe a method for generating a plasmid library expressing random truncations of a recombinant protein and for epitope mapping by screening the library with monoclonal antibodies. The key step is the random introduction of the transposon, Tn1000, which carries stop codons in all three reading frames, into a bacterial expression plasmid by using a simple bacterial mating procedure. Antibody-positive clones are then selected and the point of protein truncation is determined by sequencing the plasmid DNA at the point of transposon insertion. One advantage of the method is that no subcloning or in vitro manipulation of DNA is necessary.

Monoclonal antibodies for dystrophin analysis. Epitope mapping and improved binding to SDS-treated muscle sections

A group of 44 monoclonal antibodies (mAbs) raised against the central helical rod (25 mAbs) and C-terminal (19 mAbs) regions of dystrophin were prepared using trpE recombinant fusion proteins as immunogens. Some mAbs cross-react with the structurally related proteins, alpha-actinin and utrophin. Epitope mapping revealed uneven distribution of mAb-binding sites, no mAbs being produced against the C-terminal end of the helical fragment or the cysteine-rich region of the C-terminal dystrophin fragment. The failure of these large regions of the recombinant immunogens to elicit anti-dystrophin antibodies may be because of their inability to fold into the correct dystrophin-like conformation. The mAbs were selected for their ability to recognize 427 kDa dystrophin on Western blots after SDS/PAGE, and/or for immunostaining of the membrane in frozen muscle sections. Although some mAbs obtained by Western-blot screening failed to bind native dystrophin in frozen muscle sections, successful binding could be obtained after SDS or urea treatment of the tissue section to expose the epitopes. This increases the range of mAbs available for detection of dystrophin deletions in muscular dystrophy and evaluation of myoblast therapy.

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.

Identification by protein microsequencing of a proteinase-V8-cleavage site in a folding intermediate of chick muscle creatine kinase

We have identified by protein microsequencing a glutamic acid residue (Glu-166) in a folding intermediate of chick muscle creatine kinase that is very sensitive to cleavage by staphylococcal proteinase V8. Most other glutamic acid residues, including Glu-168, are already partly protected from proteolytic attack at this stage. After the final stages of protein refolding, when enzyme activity is recovered, Glu-166 is also resistant to proteolysis.

Rapid mapping by transposon mutagenesis of epitopes on the muscular dystrophy protein, dystrophin

Antibody-binding epitopes in the central helical region of the muscular dystrophy protein, dystrophin, have been mapped using a new strategy of transposon mutagenesis. Tn1000 transposons carrying translation termination codons were introduced randomly by bacterial mating into a large fragment of dystrophin cDNA in a pEX2 plasmid to produce a library of transformants expressing truncated dystrophin fusion proteins. Epitopes were progressively lost as the expressed sequences were shortened, enabling the epitopes recognised by 22 monoclonal antibodies to be placed in order along the dystrophin molecule without in vitro manipulation of DNA. The C-terminus of each truncated fusion protein was precisely located within the dystrophin sequence by direct sequencing of pEX2 transformants using transposon-specific primers. Sequences as short as 7 and 17 amino-acids have been identified as essential for antibody binding in this way. Nineteen of the 22 monoclonal antibodies had been selected for their ability to bind both native and SDS-denatured dystrophin and 15 of these bind to one sequence of 74 amino-acids (residues 1431-1505 of the 3684 residue sequence). This may be an area of high immunogenicity or of close structural similarity between native dystrophin and the SDS-treated recombinant fragment used for immunization.