Extrinsic Cotton effect and immunological properties of the p-azobenzenearsonate hapten attached to a helical amino acid copolymer

Selection by site-directed antibodies of small regions of peptides which are ordered in water

High resolution nuclear magnetic resonance has been used to study a short peptide which corresponds to the antigenic region of a larger peptide immunogen. This work shows that there is a strong conformational preference for a Type II beta-turn in solution. This observation has implications for the nature of the immunogenicity and antigenicity of peptide antigens as well as for the more general question of protein-folding mechanisms.

Synthetic peptides with antigenic specificity for bacterial toxins

The attachment of a diphtheria toxin-specific synthetic antigenic determinant and a synthetic adjuvant to a synthetic polymeric carrier led to production of a totally synthetic macromolecule which provoked protective antibodies against diphtheria when administered in aqueous solution. When peptides related to the B subunit of cholera toxin were synthesized and attached to tetanus toxoid, antibodies produced against the conjugate reacted in some but not all cases with intact cholera toxin and (especially with peptide CTP 3, residues 50-64) neutralized toxin reactivity, as tested by permeability in rabbit skin, fluid accumulation in ligated small intestinal loops and adenylate cyclase activation. Polymerization of the peptide without any external carrier, or conjugation with the dipalmityl lysine group, had as good an effect in enhancing the immune response as its attachment to tetanus toxoid. Prior exposure to the carrier suppressed the immune response to the epitope attached to it, whereas prior exposure to the synthetic peptide had a good priming effect when the intact toxin was given; when two different peptides were attached to the same carrier, both were expressed. Antisera against peptide CTP 3 were highly cross-reactive with the heat-labile toxin of Escherichia coli and neutralized it to the same extent as cholera toxin, which is not surprising in view of the great homology between the two proteins. A synthetic oligonucleotide coding for CTP 3 has been used to express the peptide in a form suitable for immunization. It led to a priming effect against the intact cholera toxin.

The immunodominant site of a synthetic immunogen has a conformational preference in water for a type-II reverse turn

Many short synthetic peptides have now been shown to induce antibodies reactive with their cognate sequences in the intact folded protein. Aside from the usefulness of such antibodies as site-specific reagents, the frequency with which this recognition occurs has raised several theoretical issues, the central one being that of how an antibody to a short synthetic peptide, which represents one of the most disordered states of a site in a protein, can react with the more ordered version of the same sequence in the folded protein. This apparent paradox can be resolved if the target site on the protein approaches disorder or if the peptide in solution or on a carrier adopts, with significant frequency, a conformation compatible with that of the cognate site in the protein. Various studies already suggest that antigenic sites in proteins correspond to regions of high atomic mobility. We now show, using high-field nuclear magnetic resonance (NMR) spectroscopy, that a nonapeptide selected by several monoclonal antibodies as the immunodominant site of a 36-amino-acid immunogen (residues 75-110 of influenza virus haemagglutinin) adopts a highly populated type-II reverse-turn conformation in water. This suggests that in this case the antibodies have selected a sequence possessing a conformational preference. Apart from helping us to understand immunological recognition, anti-peptide antibodies may provide reagents of sufficient precision for an immunological approach to the problem of protein folding.

Changes in intrinsic circular dichroism of several homogeneous anti-type 3 pneumococcal antibodies on binding of a small hapten

Three homogeneous antibodies against the capsular polysaccharide of Type III pneumococci of similar specificities and affinities were purified from a single bleeding of an individual rabbit and fractionated by isoelectric focusing. A comparison of the circular dichroic spectra of the three antibodies revealed differences among them, although the spectra were generally similar to those obtained previously for heterogeneous rabbit antibodies [Cathou, R. E., Kulcycki, A., Jr. & Haber, E. (1968) Biochemistry 7, 3958]. On binding of the hexasaccharide, -[-->3)-beta-D-glucuronic acid-(1-->4)-beta-D-glucose-(1-](3)-->, significant changes in all three circular dichroic spectra were observed. Since the oligosaccharide alone shows no transitions above 220 nm, these spectral changes can be attributed to changes in the intrinsic optical activity of the antibodies. Calculated difference circular dichroism spectra (of antibody minus that of antibody-hapten complex) of the three antibodies are different from each other, and resemble spectra of tryptophan and tyrosine derivatives. These changes in optical activity can be ascribed to changes in the asymmetric environments of aromatic chromophores directly in the combining site and/or to changes in orientation inside or beyond the site. Since the hapten-antibody interactions are different in the three antibodies, as shown by the difference spectra, the structures of the combining sites are presumably also different. We have interpreted these observations to mean that a relatively simple ligand may be bound by several different complementary sites.

Antibody-induced conformational changes in proteins

Several cases of modifications of functional properties of proteins upon specific interaction with antibodies have been reported in the last few years. The purpose of this paper is to discuss evidence that these effects may be due to conformational changes in the protein molecule.

Extrinsic Cotton effects characteristic of specific hapten-antibody interactions

The reversible binding of the haptens 2,4-dinitrophenyllysine (DNP-lysine) and 2,4,6-trinitrophenyllysine (TNP-lysine) to either the nitrophenyl-binding myeloma protein MOPC-315 or to pooled mouse anti-DNP or anti-TNP antibodies produces large and characteristic extrinsic Cotton effects (in the circular dichroic spectra). Despite the similarities in binding characteristics of the three proteins, the circular dichroic spectra produced by the haptens bound to the active sites of these proteins were markedly different. Extrinsic Cotton effects, therefore, provide a powerful new probe of the structure of the reversible complex formed between a hapten and an antibody active site.

Extrinsic Cotton effects in hapten--carrier and hapten--antibody interactions

Two homogeneous univalent hapten-protein conjugates, prepared by the covalent attachment of a single 2,4-dinitrophenyl (DNP-) or 2,4,6-trinitrophenyl (TNP-) side chain to the cysteine-SH in the active site of the enzyme papain, have been found to exhibit large Cotton effects in the wavelength region of the absorption bands of the DNP or TNP groups. This indicates that the DNP or TNP groups are noncovalently bound to some asymmetric region(s) of the papain molecule. These homogeneous papain conjugates are antigens that can elicit anti-DNP or anti-TNP antibody production in mice or rabbits. It is suggested that the noncovalent binding of a hapten to the surface of its carrier protein may profoundly affect the characteristics of the anti-hapten antibodies that are elicited. It has also been observed that the specific binding of these papain conjugates, and of the simple haptens DNP-lysine or TNP-lysine, to anti-hapten antibodies produces characteristic extrinsic Cotton effects.