Identification of the Human Receptor Activity-modifying Protein 1 Domains Responsible for Agonist Binding Specificity

• Published in: The Journal of biological chemistry 2003-06, Vol.278 (25), p.22623-22630
• Main Authors: Kuwasako, Kenji, Kitamura, Kazuo, Nagoshi, Yasuko, Cao, Yuan-Ning, Eto, Tanenao
• Format: Article
• Language: English
• Subjects: Adrenomedullin; Amino Acid Sequence; Binding Sites; Calcitonin Gene-Related Peptide - metabolism; Cell Line; Flow Cytometry; Gene Expression Regulation; Humans; Intracellular Signaling Peptides and Proteins; Membrane Proteins - chemistry; Membrane Proteins - genetics; Membrane Proteins - metabolism; Molecular Sequence Data; Mutagenesis; Peptides - metabolism; Receptor Activity-Modifying Protein 1; Receptor Activity-Modifying Proteins; Recombinant Proteins - chemistry; Recombinant Proteins - metabolism; RNA, Messenger - genetics; Sequence Deletion; Transfection
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M302571200

When co-expressed with receptor activity-modifying protein (RAMP) 1, calcitonin receptor-like receptor (CRLR) can function as a receptor for both calcitonin gene-related peptide (CGRP) and adrenomedullin (AM). To investigate the structural determinants of ligand binding specificity, we examined the extracellular domain of human (h) RAMP1 using various deletion mutants. Co-expression of the hRAMP1 mutants with hCRLR in HEK-293 cells revealed that deletion of residues 91–94, 96–100, or 101–103 blocked [ 125 I]CGRP binding and completely abolished intracellular cAMP accumulation normally elicited by CGRP or AM. On the other hand, the deletion of residues 78–80 or 88–90 significantly attenuated only AM-evoked responses. In all of these cases, the receptor heterodimers were fully expressed at the cell surface. Substituting alanine for residues 91–103 one at a time had little effect on CGRP-induced responses, indicating that although this segment is essential for high affinity agonist binding to the receptors, none of the residues directly interacts with either CGRP or AM. This finding suggests that RAMPs probably determine ligand specificity by contributing to the structure of the ligand-binding pocket or by allosteric modulation of the conformation of the receptor. Interestingly, the L94A mutant up-regulated surface expression of the receptor heterodimer to a greater degree than wild-type hRAMP1, thereby increasing CGRP binding and signaling. L94A also significantly increased cell surface expression of the hRAMP1 deletion mutant D101–103 when co-transfected with hCRLR, and expression of a L94A/D101–103 double mutant markedly attenuated the activity of endogenous RAMP1 in HEK-293T cells.