Role of the N-terminal domain of the calcitonin receptor-like receptor in ligand binding

Calcitonin receptor-like receptor (CRLR) is a seven-transmembrane (7-TM) domain class B G protein-coupled receptor (GPCR) which requires coexpression of different receptor activity modifying proteins (RAMP) to become a functional calcitonin gene-related peptide (CGRP) receptor or an adrenomedullin (AM) receptor. The N-terminal (Nt) extracellular region of class B GPCRs in ligand binding has been reported for receptors such as glucagon and parathyroid hormone. We hypothesize that the Nt-domain of CRLR (Nt-CRLR) is an autonomously folded unit possessing a well-defined structure and is involved in ligand binding and specificity. To obtain structural and functional information on the Nt-CRLR, we cloned and expressed the Nt-CRLR as a fusion protein in Escherichia coli. Overexpressed protein formed an inclusion body, which was refolded and purified, resulting in a soluble monomeric protein. Far-UV CD and fluorescence spectra of Nt-CRLR showed characteristics of a folded protein. The ability of Nt-CRLR to bind CGRP and AM independent of RAMPs was determined by studying inhibition of ¹²⁵I-CGRP and ¹²⁵I-AM binding to pregnant rat uterine membrane in the presence of Nt-CRLR protein. We observe that Nt-CRLR inhibits ¹²⁵I-CGRP and ¹²⁵I-AM binding to rat uterus in a dose-dependent fashion (IC₅₀ = 0.25 and 0.29 μM, respectively). Taken together, our data provide evidence that Nt-CRLR is structured and further that a significant part of the binding affinity comes from binding to the Nt-domain.