Hapten stabilization of antibody conformation

Dynamic conformations compared for IgE and IgG1 in solution and bound to receptors

Dynamic conformations of two distinct immunoglobulin (Ig) isotypes, murine IgE and human IgG1, were examined with fluorescence resonance energy transfer measurements. The IgE mutant epsilon/C gamma 3* and the IgG1 mutant gamma/C gamma 3* each bind [5-(dimethylamino)naphthalen-1-yl]sulfonyl (DNS) in two identical antigen binding sites at the amino (N)-terminal ends of the Ig in the Fab segments. Eosin-DNS bound in these Fab sites served as the acceptor probe in these studies. Both Ig have a carboxy (C)-terminal domain (C gamma 3*) which contains genetically introduced cysteine residues. Modification of these cysteine sulfhydryls with fluorescein maleimide provided donor probes near the C-terminal ends of the Ig in the Fc segment. Energy transfer between the C-terminal and N-terminal ends was compared for these two Ig in solution and when they were found to their respective high-affinity receptors on plasma membranes: IgE-Fc epsilon RI on RBL cell membranes and IgG1-Fc gamma RI on U937 cell membranes. Previous energy-transfer measurements with these probes yielded an average end-to-end distance of 71 A for IgE in solution and 69 A for IgE bound to Fc epsilon RI, indicating that in both situations IgE is bent such that the axes of the Fab segments and the axis of the Fc segment do not form a planar Y-shape [Zheng, Shopes, Holowka, & Baird (1991) Biochemistry 30, 9125]. In the current study we found the average end-to-end distance for IgG1 in solution is 75 A and greater than or equal to 85 A for IgG1 bound to Fc gamma RI, suggesting an average bend conformation for IgG1 as well. The contributions of segmental flexibility to the average distances were assessed directly by measuring the efficiency of energy transfer as a function of variations in donor quantum yield caused by a collisional quencher and using these data to extract a Gaussian distribution of end-to-end distances. The distribution average (rho) and half-width (hw) were determined to be as follows: rho = 75 A, hw = 24 A for IgE in solution; rho = 71 A, hw = 12 A for IgE bound to Fc epsilon RI; and rho = 100 A, hw = 88 A for IgG in solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Dual conformations of an immunoglobulin light-chain dimer: heterogeneity of antigen specificity and idiotope profile may result from multiple variable-domain interaction mechanisms

The structure of an immunoglobulin antigen-binding fragment (Fab) has been thought to be invariantly defined by well-conserved amino acid residues in the variable domains of the heavy and light chains. These conserved residues enable folding of the polypeptide segments into the characteristic immunoglobulin fold domains and are the major controllers of interactions between domains. However, crystallographic studies of some immunoglobulin light-chain dimers have suggested and the crystallographic structure of the Fab in an Fab-neuraminidase complex may have proven that antibodies are not restricted to a single, invariant relative positioning of the two variable domains. We propose that in some cases the detailed quaternary structural relationships between the variable domains of heavy and light chains are not restricted to those of the canonical Fab. It is unclear whether alterations of these relationships occur only after complex formation with antigen or, if in ligand-free solution, Fab conformers might coexist in relative concentrations determined by isomerization rates. In the latter case, antibody-presenting lymphocytes may be polyspecific, and the specificity of lymphocytes might be modulated by anti-idiotopic antibodies complexed to cell surface receptors. In either case, the idiotopic repertoire displayed by an antibody or lymphocyte surface receptor might be changed by the presence or absence of antigen.

The effect of dodecyl sulfate on immunoglobulin hapten binding

The instantaneous effect of dodecyl sulfate (DDS), in the mM concn range, on the binding of monovalent hapten by immunoglobulin was examined. Fluorescence measurements were utilized to study the effect of the detergent on sheep antiserum generated against thyroxin (T4) and against methamphetamine. Haptens were conjugated with the thiocyanate derivative of fluorescein in order to determine hapten binding on the basis of increased fluorescence polarization for the fluorescein-thiocarbamyl-hapten adducts (FT4 or FA) bound to immunoglobulin. Incubation of anti-T4-serum with DDS for 1 hr before the addition of FT4 resulted in diminished binding. The effect occurred at DDS concns greater than 0.1 mM and was essentially complete at a DDS conc of 1 mM. A kinetic study demonstrated a two stage process. An initial, rapid stage, with a half time less than 30 sec accounted for a reduction of immunoglobulin binding by 75%. The remaining 25% binding capacity was lost during a second, much slower phase with a half-time of about 11/2 hr. Prior hapten binding inhibited the effect of DDS. The degree of protection from combining site denaturation afforded by prior hapten binding was limited by the dissociation rate of bound hapten. The major, rapid phase was completely and immediately reversible by dilution. Prolonged incubation in DDS resulted in irreversible denaturation. The overall rate of DDS denaturation of the entire immunoglobulin molecule, as revealed by changes in the circular dichroism spectrum of a sheep gamma globulin fraction, was considerably slower than the denaturation rate of the combining site.

Interaction of porcine immunoglobulin M with protein A of Staphylococcus aureus

Intrigued by reports that the mitogenic effect of protein A on B lymphocytes was due to a direct interaction of cell surface immunoglobulin with protein A, the binding of 19 S, 8 S, and Fab mu fragments of 125I-labeled IgM isolated from porcine serum was investigated. Approx. 60% of purified 19 S porcine IgM interacted specifically with Protein A-Sepharose. Mild reduction and alkylation of 19 S IgM to yield monomeric IgM did not appear to alter its ability to bind to protein A. Elution of either molecular species of IgM from protein A and subsequent repassage over Protein A-Sepharose resulted in nearly quantitative rebinding of the IgM to protein A. Fab mu fragments prepared by digestion of 19 S IgM with pepsin exhibited binding characteristics similar to that observed for intact and monomeric IgM. These results suggest that: (1) there are at least two populations of porcine serum IgM, one that binds to protein A and one that does not; (2) these populations are not interconverting; (3) the ability of IgM to bind to protein A is not dependent on the 19 S pentameric structure extant in sera, but rather is an intrinsic property of some and not all four chain IgM protomers; and (4) a binding site for protein A on porcine IgM is localized in the Fab mu (including the C mu 2 domain) regions of the molecule.

Thermodynamics of hapten-antibody interactions

Two schools of thought are currently prevalent regarding the thermodynamic mechanism(s) of hapten-antibody interaction(s). While one school is a proponent of the hapten-antibody reaction being driven predominantly by enthalpy, the second school rationalizes the mechanism as an enthalpy-entropy compensation, the magnitude of the latter being dependent on the temperature at which hapten-antibody interaction is carried out.

Crystallographic structure studies of an IgG molecule and an Fc fragment

The crystal structures of a human IgG antibody molecule Kol and a human Fc fragment have been determined at 4-A and 3.4-A resolution respectively, by isomorphous replacement. The electron-density maps were interpreted in terms of immunoglobulin domains based on the Rei and McPC 603 models (Kol) and by model-building (Fc). The Fab parts of Kol have a different quaternary structure from that observed in isolated crystalline Fab fragments, there being no longitudinal V-C contact in Kol. The Fc part C terminal to the hinge is disordered in the Kol crystals. It is suggested that the Kol molecule is flexible in solution, whereas fragments are rigid. In the Fc fragment both CH3 and CH2 show the immunoglobulin fold. The CH3 dimer aggregates as CH1-CL while CH2 are widely separated from each other. The carbohydrate bound to Fc is in fixed position. From these structures a hypothetical liganded antibody molecule has been constructed, which is assumed to be rigid.

Persistence of hapten-antibody complexes in the circulation of immunized animals after a single intravenous injection of hapten

To study the fate of a low molecular weight antigen (hapten) in the circulation of animals whose sera contain antibodies specific for that low molecular weight antigen, a single injection of digoxin-(3)H (0.4 mg/kg) was administered intravenously to 18 rabbits. Thirteen animals (nine nonimmunized and four immunized with bovine serum albumin) served as control animals. In five rabbits which had been immunized with a digoxin-bovine serum albumin conjugate and whose sera contained digoxin-specific antibodies, the mean 12-h serum digoxin concentration was 8,300 ng/ml (control: 92 ng/ml) and the mean serum concentration 12 mo after the single injection of digoxin-(3)H was 85 ng/ml. In digoxin-immunized rabbits, less than 10% of the digoxin-(3)H was excreted in the first 10 days (control: 77% recovered in urine and feces) and the mean biological half-life of digoxin, as calculated from serum digoxin-(3)H disappearance curves, was 72 days (control: 3.4 days). In sera of digoxin-immunized rabbits, more than 90% of the circulating digoxin-(3)H was immunoglobulin bound, as determined by the double-antibody and dextran-coated charcoal methods. The serum disappearance rate of (125)I-antidigoxin antibodies was similar in nonimmunized and in immunized animals and in the presence or absence of digoxin. It is concluded that the biological half-life of a hapten may be markedly prolonged when the hapten is bound to specific antibody. The persistence of antibody-hapten complexes in the circulation suggests that these complexes may not be deposited in tissues and raises the possibility that low molecular weight determinants may be capable of preventing or reversing the deposition of immune complexes, containing macromolecular antigens, in the tissues of experimental animals and man.

The dissociation of avidin-biotin complexes by guanidinium chloride

Avidin molecules in which a fraction of the four binding sites were occupied by biotin did not dissociate completely in 6.4m-guanidinium chloride. Only unoccupied subunits dissociated. The remainder recombined to form the tetrameric avidin-biotin complex. The rate at which unoccupied subunits were unfolded and dissociated was only decreased by one-half in species in which three of the four binding sites were occupied by biotin. These results can be explained by assuming that unfolding of unoccupied subunits followed by dissociation from the tetramer is initiated by penetration of guanidinium ions into the binding site and disorganization of this region of the subunit. When a site is occupied by biotin this pathway is blocked and the subunit does not unfold. Each subunit behaves independently and is not markedly stabilized when neighbouring subunits are occupied.