Circular dichroism studies on the interactions of haptens with MOPC-315 and MOPC-460 mouse myeloma proteins and specific antibodies

Spectroscopic properties of light-chain derivatives of murine MOPC-315 immunoglobulin A

Three light-chain derivatives of the homogeneous IgA, secreted by the mouse myeloma MOPC-315, were studied employing circular dichroism and thermal-perturbation spectroscopy: (a) the light-chain dimer with intact native inter-chain disulfide bond, L2,cov; (b) the light-chain dimer with this bond reduced and alkylated, L2,ncov; and (c) the dimer of only the variable regions of the light chains, (VL)2. Comparison of the well resolved circular dichroism spectra of these derivatives allowed the assignments of the bands above 290 nm to the following chromophores: Trp-35L and Trp-91L in the variable domains, and Trp-148L, Trp-185L and the disulfide of Cys-214L in the constant domains. The differences in the spectral characteristics of L2,cov as compared to those of L2,ncov and (VL)2 illustrate the significant influence of the disulfide bridge on the conformation of the L2,cov. Pronounced differences are found between these light-chain derivatives ant the light chain--heavy chain associates, namely the intact protein M-315 and FV fragment. The comparison between the CD spectra of the free and the hapten-bound L2,cov, L2,ncov and (VL)2 directly demonstrates the existence of the conformational transitions in these proteins induced by hapten binding.

Proximity relationships within the Fc segment of rabbit immunoglobulin G analyzed by resonance energy transfer

Resonance energy transfer analysis has been carried out with a noncovalent rabbit hybrid of immunoglobulin G (IgG) composed of normal rabbit IgG and rabbit anti-lactose IgG. The hybrid IgG was prepared from proteins in which the single inter-heavy-chain disulfide linkage was specifically reduced and alkylated. Normal rabbit IgG was alkylated with the iodoacetyl derivative of N-(aminoethyl)-5-naphthylamine-1-sulfonic acid while the rabbit anti-lactose IgG was alkylated with either iodoacetamide or the iodoacetyl derivative of p-[[p-(dimethylamino)phenyl]azo]aniline. Fractionation with an anti-lactose-specific immunoadsorbent yielded a population in which each fluorescent donor [N-[(acetylamino)ethyl]-5-naphthylamine-1-sulfonic acid] was adjacent to a nonfluorescent acceptor [N-acetyl-p[[p-dimethylamino)phenyl]. Data on fluorescence quantum yield, excited-state lifetime, time-resolved emission anisotropy, and steady-state fluorescence polarization revealed a distribution of distances between the donor and acceptor. In the native molecule, the hinge regions are known to be covalently liked by a single disulfide bond. In the absence of this linkage, the hinge regions were separated such that for a majority of the molecules in solution (approximately 60%) the separation was 50-60 A. In the context of current knowledge of the IgG molecule, it is evident that the principle forces maintaining the integrity of the native, functional Fc segment are the strong noncovalent interactions of the CH3 domains and the single inter-heavy-chain disulfide bond.

Conformational transitions of antibodies induced by hapten binding

The conformational changes of antibody structure induced by hapten molecule binding were investigated by means of thermal perturbation difference spectroscopy. The studies of the free rabit anti-dinitrophenyl antibodies show the conformational transition at temperatures between 25 and 35 degrees C. The changes occurring at the higher temperature are accompanied by the screening of the significant part of exposed tyrosine residues. Binding of the hapten molecules induces a similar transition to that which occurs between the two temperature dependent states of the free antibody. In contrast to our previous results with anti-dansyl rabbit antibodies the dinitrophenyl lysine stabilizes the "low temperature" native state of the protein. The investigation of the MOPC-315 mouse immunoglobulin A myeloma protein possessing anti-dinitrophenyl activity indicates no conformational transition at temperatures between 25 and 35 degrees C and only a small decrease of tyrosine exposure induced by the hapten binding. Our present and previous results indicate that most of the free immunoglobulins exist in two native conformational states which have a small difference in free energy. Hapten binding causes the transition in equilibrium between the two states towards the one of better binding. It is possible that this transition is necessary but not sufficient step for inducing the effector function of antibodies.

The combining site of the dinitrophenyl-binding immunoglobulin A myeloma protein MOPC 315

Magnetic-resonance techniques are used to refine the model of the combining site of the Fv fragment of the dinitrophenyl-binding mouse myeloma protein MOPC 315 constructed by Padlan, Davies, Pecht, Givol & Wright (1976) (Cold Spring Harbor Symp. Quant. Biol.41, in the press). Light-absorption studies indicate a dinitrophenyl-tryptophan interaction in the Fv fragment of the type occurring in free solution. The Dnp-aspartate-tryptophan complex is therefore used as a starting point for the n.m.r. (nuclear-magnetic-resonance) analysis of the dinitrophenyl-Fv fragment interaction. Ring-current calculations are used to determine the geometry of the complex. The specificity of complex-formation between dinitrophenyl and tryptophan is confirmed by the lack of ring-current shifts of the dinitrophenyl resonances when tryptophan is replaced by any other aromatic amino acid. Proton n.m.r. difference spectra (at 270MHz), resulting from the addition of a variety of haptens to the Fv fragment, show that the combining site is highly aromatic in nature. Calculations on the basis of ring-current shifts define the geometry of the combining site, which involves a dinitrophenyl ring in van der Waals contact with four aromatic amino acid residues on the protein. The observation of a nuclear Overhauser effect on the H((3)) resonance of the dinitrophenyl ring provides additional constraints on the relative geometry of the H((3)) proton and an aromatic amino acid residue on the Fv fragment. The specificity of the Fv fragment for dinitrophenyl ligands arises from a stacking interaction of the dinitrophenyl ring with tryptophan-93(L), in an ;aromatic box' of essentially tryptophan-93(L), phenylalanine-34(H) and tyrosine-34(L); asparagine-36(L) and tyrosine-34(L) also contribute by forming hydrogen bonds with the nitro groups on the dinitrophenyl ring. The n.m.r. results also confirm that the antibody-hapten reaction may be visualized as a single encounter step. An Appendix shows the method of calculation of ring currents for the four aromatic amino acids and their use in calculating structures.

Conformation of immunoglobulin M. 2. Nanosecond fluorescence depolarization analysis of segmental flexibility in anti-epsilon-l-dimethylamino-5-naphthalenesulfonyl-L-lysine anti-immunoglobulin from horse, pig, and shark

The rotational motions of immunoglobulin M (IgM) were investigated by the nanosecond fluorescence depolarization technique. The fluorophore epsilon-1-dimethylamino-5-naphthalenesulfonyl-L-lysine (DNS-lysine) was specifically bound in the combining sites of anti-DNS IgM antibodies from the horse, pig, and nurse shark. Fluorescence lifetime analysis showed the presence of a long lifetime component (21-27 ns) with antibodies from all three species. With the mammalian antibodies, the fluorophore appeared to be rigidly bound in the combining sites as judged by the presence of induced circular dichroism of DNS-lysine (equine antibodies) and single exponential anisotropy decay of the isolated Fabmu fragments (equine and porcine antibodies). The small amount of available purified nurse shark antibody did not allow preparation of fragments or induced circular dichroism measurements to directly determine rigidity of fluorophore binding. However, at least some of the hapten must have been rigidly bound since long rotational correlation times were measured for the shark DNS-lysine-anti-DNS complexes. When the emission anisotropy of the fluorophore-anti-DNS IgM complexes was measured as a function of time, it was found that all three antibody species exhibited restricted segmental flexibility in the nanosecond time range. Moreover, when the equine anti-DNS IgM was exposed to 1 M acetic acid for 1 h, the antibody underwent a conformational change which resulted in an increase in its overall flexibility. Comparison of the rotational correlation times of native equine IgM and of proteolytic fragments indicated that flexibility of IgM consists of either hindered rotation of the Fab'mu segment or a combination of at least two modes of motion: rotation of Fabmu and/or Fab'mu and bending of the entire (Fab')2mu region as a unit. Similar modes of flexibility also occur in native porcine IgM. In acid exposed equine IgM, the major contribution to depolarization is from independent rotation or wagging of the Fab'mu segments. Thus, acid apparently causes a conformational change in or near the Cmu2 domains. In contrast, flexibility in nurse shark IgM appears to involve only bending of (Fab')2mu as a unit. Our results suggest that segmental flexibility is an essential functional feature of all IgM antibodies and that control of this flexibility through domain interactions may play an important role in such conformationally sensitive functions as complement fixation.

Circular dichroism of hapten--antibody complexes: characterization of the combining sites of native and reformed MOPC-315 protein, its isolated subunits, and its Fv fragment

Extrinsic Cotton effects generated by binding haptens to native and reformed MOPC-315 protein, its subunits, and its Fv fragment have been examined. The identity of the combining sites of native and reassociated proteins and Fv-315 was demonstrated by the identity of their circular dichroism (CD) difference spectra. The spectrum of TNP-aminocaproate complexed with L chains differed in maxima and minima and cross-over points and lacked the 495-nm CD peak of TNP-aminocaproate-MOPC-315 protein and Nalpha-TNP-tryptophan spectra. A negative 293-nm tryptophanyl CD band, present in spectra of MOPC-315 protins and Fv-315 but absent from spectra of L and H chains, was blue-shifted by haptens and may represent electronic interactions occurring within the MOPC-315 combining site between tryptophanyl and chromophoric residues of different subunits. This conclusion is supported by molecular models of the MOPC-315 combining site.

A functional examination of hapten-binding derivatives from a murine myeloma protein with immunoglobulin features

Various enzymatic derivatives of the murine myeloma protein IgA(MOPC-315) were subjected to hapten-binding quantitation and circular dichroic analysis in an attempt to ascertain the structural localization and functional definition of the immunoglobulin active site. The extrinsic Cotton effects observed with near saturation of the active site of the derivatives using epsilon-Dnp-L-lysine were qualitatively and quantitatively identical when normalized with respect to their macromolecular content of Dnp-binding sites; ellipticity maxima of 378 and 438 nm and an ellipticity minimum of 325 nm were recorded. Fluorescence quenching data confirmed the molecular nature of the derivation products of IgA(MOPC-315) but also introduced disparity with respect to binding kinetics, i.e., K values of 2.31 x 10(6), 6.62 x 10(6), and 2.06 x 10(7) M(-1) were determined for IgA(MOPC-315), Fab'(MOPC-315), and Fv(MOPC-315), respectively. We speculate that the results can be explained by possible modulating effects on haptenbinding contributed by novel conformations introduced through proteolysis.