Epitope analysis and molecular modeling reveal the topography of the C-terminal peptide of the \beta-subunit of human chorionic gonadotropin

Venkatesh, Natarajan and Krishnaswamy, Sankaran and Meuris, Sylvain and Murthy, Gundulpet Satyanarayana (1999) Epitope analysis and molecular modeling reveal the topography of the C-terminal peptide of the \beta-subunit of human chorionic gonadotropin. In: European Journal of Biochemistry, 265 (3). pp. 1061-1066.

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Abstract

Human chorionic gonadotropin (hCG) belongs to a family of heterodimeric glycoprotein hormones that share a common \alpha-subunit and a hormone-specific \beta-subunit. Among the gonadotropin \beta-subunits, greater than 85% homology exists between lutropin (hLH)\beta and hCG\beta in their first 114 amino acid residues. However, unlike hLH\beta, hCG\beta contains a 31-amino acid hydrophilic stretch at its carboxyl end (CTP\beta: C-terminal peptide). Although the crystal structure of deglycosylated hCG has been solved, the topography of CTP\beta remains unknown. In this study, we have attempted to define the topology of CTP using mAb probes. We investigated three epitopes on hCG\alpha, which are hidden in the hCG\alpha\beta dimer. However, these epitopes are not hidden in hLH, which has a similar subunit interface to that of hCG, but lacks CTP\beta. This suggested that these epitopes are not masked at the subunit interface of hLH or hCG. Hence, we hypothesized that, in the case of hCG, these epitopes are masked by the CTP\beta. Consistent with this view, several treatments of hCG that removed CTP\beta unmasked these epitopes and enhanced their reactivity with the corresponding mAbs. In order to localise the position of CTP\beta on the \alpha-subunit, we used an epitope-mapping strategy [N. Venkatesh & G. S. Murthy (1997) J. Immunol. Methods 202, 173-182] based on differential susceptibility of epitopes to covalent modifications. This enabled us to predict the possible topography of CTP\beta. Further, we were also able to build a model of CTP\beta, completely independently of the epitope-mapping studies, using a homology-based modeling approach [S. Krishnaswamy, I. Lakshminarayanan & S. Bhattacharya (1995) Protein Sci. 4 (Suppl. 2), 86-97]. Results obtained from these two different approaches (epitope analysis and homology modeling) agree with each other and indicate that portions of CTP\beta are in contact with hCG\alpha in the native hCG dimer.

Item Type:	Journal Article
Publication:	European Journal of Biochemistry
Publisher:	Blackwell Science Ltd
Additional Information:	Copyright for this article belongs to Blackwell Science Ltd.
Keywords:	carboxy-terminal peptide;chemical modification;epitope mapping;glycoprotein hormones;human chorionic gonadotropin
Department/Centre:	Division of Biological Sciences > Molecular Reproduction, Development & Genetics
Date Deposited:	23 Jan 2007
Last Modified:	19 Sep 2010 04:18
URI:	http://eprints.iisc.ac.in/id/eprint/2918

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