Delineation of Tyrosine-Containing Epitopes within the beta Subunit of Bovine Thyrotropin

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Characterization of epitope regions of thyrotropin beta-subunit recognized by the monoclonal antibodies mAb279 and mAb299: a chimeric peptide approach

This investigation describes the design, synthesis and evaluation of chimeric peptides related to the bovine thyrotropin beta-subunit, bTSHbeta. The structures of these chimeric peptides were derived from investigations with linear peptides and sequence alignment studies, in association with a homology model of TSHbeta developed from the hCG X-ray crystallographic structure. The structures of these chimeric peptides comprised beta-turn regions of loop L1 [bTSHbeta(14-20)] and loop L3 [bTSHbeta(65-72)] held in close proximity by a bis-beta-alanine linker and the disulfide bond bTSHbeta[Cys16-Cys67]. Linear and cyclic chimeric peptides were evaluated in immunochemical assays for their ability to inhibit the binding of radio-iodinated bTSHbeta [125I-bTSHbeta] to the monoclonal antibodies, mAb279 and mAb299. Previously, mAb279 and mAb299 have been shown to recognize epitopes accessible on the surface of TSHbeta that lie in close proximity to the TSH receptor-binding site. The results indicate that these chimeric peptides can specifically inhibit in a dose-dependent manner the binding of 125I-bTSHbeta to mAb299, while having a lesser effect on the binding with mAb279. Based on these results, it can be concluded that the bTSHbeta-epitope recognized by mAb299 involves contributions from amino residues from the beta-turn regions of the L1 and L3 loops of TSHbeta, and that these loop regions flank part of the receptor binding site of the bTSH beta-subunit.

Evaluation of a pepscan approach to identify epitopes recognised by anti-hTSH monoclonal antibodies

In this study, several methodological aspects of the pepscan strategy have been investigated with the objective to delineate the amino acid sequences of peptide segments that form the epitopes of thyrotropin beta-subunit (TSHbeta) recognised by monoclonal antibodies. Hitherto, the pepscan strategy has found application as an effective method to identify linear sequence regions that constitute contiguous epitopes within the primary structure of some proteins. However, with heterodimeric glycoprotein hormones and their subunits such as TSHbeta, as well as for many other globular proteins, the majority of the epitopes recognised by anti-protein antibodies will be derived from discontinuous segments that collectively form the epitope. In these cases the pepscan technique will only be able to identify individual segments of the overall discontinuous epitope site as linear peptides, some of which may interact with relatively low binding affinity. Consequently, additional attention must thus be given to the optimisation of the specific binding and detection conditions. Knowledge of the structures of these peptide segments can, however, provide a valuable basis to develop peptide structures that more closely mimic the topographical features of the epitope in the mature, folded protein. In an attempt to identify functional segments involved in the epitopes recognised by the anti-hTSH monoclonal antibodies, mAb279 and mAb299, the impact of various experimental conditions on the efficacy of the pepscan strategy has been investigated. The strategy involved the synthesis of a series of overlapping pin-bound octapeptides with amino acid sequences derived from the TSH beta-subunit. The ability of these pin-bound octapeptides to bind to either mAb279 or mAb299 in ELISA-based assay was then determined under conditions involving different concentrations of the primary and/or secondary antibodies, and changes in buffer composition, incubation times and washing procedures. Theresults of this study illustrate some of the constraints and limitations of the pepscan technique when used to delineate discontinuous epitopes of globular proteins, as well as providing insight into potential avenues to optimise and refine this method.

Direct synthesis and characterisation of multi-dendritic peptides for use as immunogens

The direct synthesis and subsequent rapid characterisation of multiple antigen peptides (MAPs) for use as immunogens has presented difficulties, partly because of the formation of incomplete or truncated peptide sequences during the synthetic procedure. Therefore, many researchers have resorted to ligation procedures for the synthesis of MAP constructs. This article describes a method to improve the yield of MAP constructs by direct synthesis methods, as well as a general procedure that enables easier characterisation of the synthetic products. In particular, during the synthesis of MAP constructs, a capping procedure was introduced after each amino acid coupling step, thus improving significantly the yield of the desired multi-dendritic peptidic immunogens. Through the use of this capping procedure, problems arising from the incomplete amino acid residue coupling at the point of synthesis were minimised, and any deletion peptides which formed could be eliminated more readily during the subsequent purification procedures. In addition, previous difficulties in purification and characterisation of MAP construct by, e.g. electrospray mass spectroscopy (ES-MS), often led to the multi-dendritic peptidic immunogens being used without full characterisation after dialysis and recovery of the product(s). This article describes an enzymatic (tryptic) digestion method with the MAP construct, followed by characterisation of the enzymatic digest by reversed phase high-performance liquid chromatography-ES-MS. With this method, fragments of the MAP construct cleaved at specific amino acid residue sites (e.g. lysine or arginine) within the sequence of the parent peptide can be readily determined and the kinetics of the digestion easily followed. This enzymatic digestion procedure thus provides a facile approach to confirm that all of the multi-dendritic arms of the purified MAP construct have been equivalently elongated during the peptide synthesis and that consequently the purified construct structure contains the correct peptide sequence.

Contribution of Specific Disulphide Bonds to Two Epitopes of Thyrotropin beta-Subunit Associated with Receptor Recognition

In previous studies, we have shown that two epitopes of bovine thyrotropin beta-subunit that are recognised by the monoclonal antibodies designated mAb 279 and mAb 299 are also associated with the receptor-binding site of bovine thyrotropin. The present investigation has examined the role of the six disulphide bonds of bovine thyrotropin beta-subunit in the conformational stabilisation of these two epitopes, and hence assessed the relative contribution that these disulphide bonds make to the stabilisation of the receptor-binding region of the beta-subunit. The experimental procedure involved the production of several bovine thyrotropin beta-subunit-related derivatives in which an increasing number of the disulphide bonds were selectively reduced with dithiothreitol and alkylated with iodoacetic acid. Antibody-binding properties of these derivatives were then evaluated in thyrotropin beta-subunit-specific immunoassays based on the use of the well characterised mAb 279 and mAb 299, to determine the effect of disulphide bond reduction and alkylation on each epitopic specificity. In separate experiments, the residual disulphide bonds that remained intact following these selective partial reductive alkylation procedures were then fully reduced and alkylated with the fluorescent reagent 5-N-[(iodoacetamidoethyl)amino]naphthalene 1-sulphonic acid. The relative contribution of individual disulphide bonds in the stabilisation of each epitope could then be assessed after application of reverse-phase HPLC peptide mapping methods. Epitope recognition by mAb 279 was not dependent on the preservation of the so-called determinant loop Cys88-Cys95 disulphide bond nor directly involved binding interactions via the Cys2-Cys52, Cys27-Cys83, and Cys31-Cys85 disulphide bonds. However, the experimental results indicated that the mAb 279 epitope was stabilised by the Cys19-Cys105 and Cys16-Cys67 disulphide bonds, which is consistent with other data on the role of the C-terminal region of the thyrotropin beta-subunit in this epitope. In contrast, the presence of an intact Cys88-Cys95 disulphide bond was required for the stabilisation of the mAb 299 epitope, although the location of this disulphide bond is distal to the hairpin loop structure that constitutes the mAb 299 epitope. These results on the relative contribution of these disulphide bonds are also discussed in terms of their relationship to the stabilisation of the predicted region of bovine thyrotropin beta-subunit involved in receptor binding.