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Submitted on April 30, 2003
Accepted on July 25, 2003
1 School of Biomedical Sciences and Institute for Molecular Bioscience, School of Molecular & Microbial Science, University of Queensland, Brisbane, QLD 4072, Australia, School of Life Science, Queensland University of Technology, Brisbane, QLD.
* To whom correspondence should be addressed. E-mail: m.waters{at}mailbox.uq.edu.au.
MAb 263 is a widely used monoclonal antibody which recognizes the extracellular domain (ECD) of the GH (GH) receptor. It has been shown to act as a GH agonist both in vitro and in vivo, and we report here that it must be divalent to exert its effect on the full length receptor. To understand the mechanism of its agonist action, we have determined the precise epitope for this antibody using a novel random PCR mutagenesis approach together with expression screening in yeast. A library of 5200 clones of rbGHR ECD mutants were screened both with MAb 263 and with an anti-carboxy-tag antibody to verify complete ECD expression. Sequencing for clones that expressed complete ECD but were not MAb 263 positive identified 20 epitope residues distributed in a discontinuous manner throughout the ECD. The major part of the epitope, as revealed after mapping onto the crystal structure model of the ECD molecule, was located on the side and upper portion of domain 1, particularly within the D-E strand disulfide loop 79-96. Molecular dynamics docking of an antibody of the same isotype as MAb 263 was used to dock the bivalent antibody to the 1528 Å2 epitope and to visualize the likely consequences of MAb binding. The minimised model enables the antibody to grasp two receptors in a pincer-like movement from opposite sides, facilitating alignment of the receptor dimerization domains in a manner similar to, but not identical with, GH.
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