Computer modelling of the interaction between human choriogonadotropin and its receptor

Structural evaluation of a mimicry-recognizing paratope: plasticity in antigen-antibody interactions manifests in molecular mimicry

Molecular mimicry manifests antagonistically with respect to the specificity of immune recognition. However, it often occurs because different Ags share surface topologies in terms of shape or chemical nature. It also occurs when a flexible paratope accommodates dissimilar Ags by adjusting structural features according to the antigenic epitopes or differential positioning in the Ag combining site. Toward deciphering the structural basis of molecular mimicry, mAb 2D10 was isolated from a maturing immune response elicited against methyl α-d-mannopyranoside and also bound equivalently to a dodecapeptide. The physicochemical evidence of this carbohydrate-peptide mimicry in the case of mAb 2D10 had been established earlier. These studies had strongly suggested direct involvement of a flexible paratope in the observed mimicry. Surprisingly, comparison of the Ag-free structure of single-chain variable fragment 2D10 with those bound to sugar and peptide Ags revealed a conformationally invariant state of the Ab while binding to chemically and structurally disparate Ags. This equivalent binding of the two dissimilar Ags was through mutually independent interactions, demonstrating functional equivalence in the absence of structural correlation. Thus, existence of a multispecific, mature Ab in the secondary immune response was evident, as was the plasticity in the interactions while accommodating topologically diverse Ags. Although our data highlight the structural basis of receptor multispecificity, they also illustrate mechanisms adopted by the immune system to neutralize the escape mutants generated during pathogenic insult.

Plasticity within the Antigen-Combining Site May Manifest as Molecular Mimicry in the Humoral Immune Response

Structural and physiological facets of carbohydrate-peptide mimicry were addressed by analyzing the Ab response to alpha-d-mannopyranoside. mAbs against alpha-d-mannopyranoside were generated and screened with the carbohydrate-mimicking 12 mer (DVFYPYPYASGS) peptide. Three mAbs, 2D10, 1H11, and 1H7, which were subjected to detailed analysis, exhibit diverse V gene usage, indicating their independent germline origins. Although the mAb 1H7 was specific in binding only to the immunizing Ag, the Abs 2D10 and 1H11 recognize the 12 mer peptide as well as the immunogen, alpha-d-mannopyranoside. The Abs that recognize mimicry appear to bind to a common epitope on the peptide and do not share the mode of peptide binding with Con A. Binding kinetics and thermodynamics of Ag recognition suggest that the Ab that does not recognize peptide-carbohydrate mimicry probably has a predesigned mannopyranoside-complementing site. In contrast, the mimicry-recognizing Abs adopt the Ag-combining site only on exposure to the sugar, exploiting the conformational flexibility in the CDRs. Although the mAb 1H7 showed unique specificity toward mannopyranoside, the mimicry-recognizing Abs 2D10 and 1H11 exhibited degenerate specificities with regard to other sugar moieties. It is proposed that the degeneracy of specificity arising from the plasticity at the Ag-combining site in a subset of the Ab clones may be responsible for exhibiting molecular mimicry in the context of Ab response.

Critical relationship between glycosylation of recombinant lutropin receptor ectodomain and its secretion from baculovirus-infected insect cells

The lutropin receptor ectodomain overexpressed under the control of the powerful polyhedrin promoter in baculovirus-infected Sf9 insect cells, is mainly found in an inactive, intracellularly-aggregated form. It is secreted in an active form under the control of the P10 promoter, a somewhat weaker and earlier promoter, at the price of a lower production. The apparent molecular masses of the two species encoded by the same cDNA are 48 kDa and 60-68 kDa, respectively. The relationship between the extent and type of glycosylation and the extracellular targeting for the recombinant lutropin receptor ectodomains was investigated precisely with endoglycosidases, lectins of various specificities, and a glycosylation inhibitor, and tested with monoclonal and polyclonal antibodies. The results indicate that the strong polyhedrin promoter probably overwhelms the processing capacity of the ER in Sf9 cells, so that only a high-mannose precursor is expressed in large amounts. Only a minute amount of protein is secreted, which has been processed by Sf9 exoglycosidases/glycosyltransferases and bears complex/hybrid oligosaccharides. The weaker P10 promoter allows secretion of a mature and active receptor ectodomain, bearing complex glycosylation. An important O-linked glycosylation is also added post-translationally on this species. In particular, beta-galactose and sialic acid residues were specifically detected in the secreted species, evidence of the induction of the corresponding glycosyltransferases or of their genes. These results suggest that Sf9 cells should eventually be engineered with chaperones and glycosyltransferases in order to improve the production of demanding glycoproteins such as the porcine lutropin ectodomain, so as to open the way to resolution of the three-dimensional structures of these receptors.

Ligand-induced receptor dimerization may be critical for signal transduction by choriogonadotropin

A mechanism of signal transduction by human choriogonadotropin (hCG) has been proposed. Competitive inhibition of the binding of hCG to its receptor by the serine protease inhibitors led to the identification of local structural homology of an extracellular region of the receptor with the reactive site loop of chymotrypsin inhibitor. Synthetic peptides from the extracellular domain of luteinizing hormone-choriogonadotropin (LH/CG) receptor, rationally designed on the basis of this homology, were found to affect hormone-receptor binding and bioactivity. A receptor peptide incorporating one complete structural unit of the leucine-rich repeats motif of the extracellular domain of the receptor significantly increased hCG-receptor binding in a dose-dependent manner. However, the testosterone production in a Leydig cell bioassay was inhibited in the presence of this peptide. The agonistic effect on the hCG-receptor binding and the antagonistic effect on the testosterone production of a receptor peptide suggests the possibility of more than one quasi-equivalent receptor-binding site on the hormone. Hormone-induced receptor oligomerization may therefore be involved in the mechanism of signal transduction by hCG.

Topological analysis of the functional mimicry between a peptide and a carbohydrate moiety

The shared surface topology of two chemically dissimilar but functionally equivalent molecular structures has been analyzed. A carbohydrate moiety (alpha-D-mannopyranoside) and a peptide molecule (DVFYPYPYASGS) bind to concanavalin A at a common binding site. The cross-reactivity of the polyclonal antibodies (pAbs) was used for understanding the topological relationship between these two independent ligands. The anti-alpha-D-mannopyranoside pAbs recognized various peptide ligands of concanavalin A, and the anti-DVFYPYPYASGS pAbs recognized the carbohydrate ligands, providing direct evidence of molecular mimicry. On the basis of differential binding of various rationally designed peptide analogs to the anti-alpha-D-mannopyranoside pAbs, it was possible to identify different peptide residues critical for the mimicry. The comparison of circular dichroism profiles of the designed analogs suggests that the carbohydrate mimicking conformation of the peptide ligand incorporates a polyproline type II structural fold. The concanavalin A binding activity of these analogs was found to have a direct correlation with the topological relationship between peptide and carbohydrate ligands.