Received for publication April 18, 2008.
Revised June 3, 2008.
Accepted for publication June 6, 2008.

LIGAND-RECEPTOR INTERACTIONS AT THE PARATHYROID HORMONE (PTH) RECEPTORS: SUBTYPE BINDING SELECTIVITY IS MEDIATED VIA AN INTERACTION BETWEEN RESIDUE 23 ON THE LIGAND AND RESIDUE 41 ON THE RECEPTOR

Abstract

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) bind and activate the PTH/PTHrP receptor (PTH-1R). However, while the related receptor PTH-2R responds potently to PTH, it is not activated by PTHrP. It is known that two hormone sites are responsible for these different potencies. Firstly, the absence of efficacy for PTHrP at PTH-2R is due to the presence of His-5 in PTHrP (Ile-5 in PTH) which interacts with the receptor's juxta-membrane (J) domain. Secondly, PTHrP has lower affinity than PTH for PTH-2R due to the presence of Phe-23 (Trp-23 in PTH) which interacts with the receptor's N-terminal extracellular (N) domain. We used these different receptor subtype properties to demonstrate that residue 41 in PTH-1R, when either the native Leu or substituted by Ile or Met, can accommodate either Phe or Trp at position 23 of the ligand. However, when Leu-41 is substituted by a smaller side chain, either Ala or Val (its equivalent residue in PTH-2R), the receptor becomes highly selective for those peptide ligands with Trp-23. Hence, despite the conservative nature of the substitutions found in the native ligands (Phe for Trp) and receptors (Leu for Val), they nevertheless enable a significant degree of selectivity to be achieved. Analysis of this functionally important ligand-receptor contact, within the context of the recent X-ray structure of the peptide-bound PTH-1R N domain, reveals the nature of the selectivity filter and how it is by-passed in PTH-1R.

Key words: Structure-activity relationships and modeling, Mutagenesis/Chimeric approaches, Receptor binding studies, Peptide hormones