Structural Functional Studies of CRLR and RAMPs 1 &2

Diabetes, Digestive, Kidney Diseases

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Abstract

Receptor activity modifying proteins (RAMPS) are a family of three proteins having approximately 31% identity. RAMPs are approximately 150-177 amino acid integral membrane proteins with a cleavable signal peptide, predominant amino-terminal (N-terminal) extracellular domain, one transmembrane spanning domain and a 9 amino acid intracellular carboxy-terminal domain. At least one RAMP is expressed in all cells known to express CRLR (a 7 TM GPCR). RAMPS associate with either calcitonin receptor-like receptor (CRLR) or calcitonin receptors (CTR) to influence their ligand affinity and specificity. Both CRLR and CTR belong to the B family of GPCRs. The N-terminal domains of this B family all contain multiple highly conserved cysteine residues suggesting the presence of conserved secondary structure by virtue of disulfide linkage necessary for binding to their homologous family of peptide ligands. The N-terminal fragment of the human parathyroid hormone receptor 1 (PTHR1) and Glucagon-like peptide 1 receptor (also members of the class B family of GPCRs) has been purified from bacteria, renatured and shown to bind their respective ligands with significant affinity. Because transmembrane proteins and especially 7 TM proteins are difficult to separate from their lipid environment and maintain their native conformation, the unique features of CRLR/RAMP interaction with CGRP may allow structural studies of the extramembranous N-terminal fragment of the CRLR to be a feasible and still fruitful compromise toward the understanding of the structural basis of ligand-receptor interaction. We are investigating whether the N-terminal extracellular domains of RAMP1 and CRLR are sufficient for their stable association and whether the heterodimer is capable of binding CGRP. During the past year, we have succeeded in expressing high levels of N-terminal fragments of CRLR and RAMPs 1 and 2 in E. coli. Unfortunately, these fragments are directed to inclusion bodies in an incorrectly folded form. We have succeeded in purifying these proteins to homogeneity. We have also succeeded in refolding them. For CRLR, we have shown using enzymatic digestion, HPLC and mass spec, that the three disulfied linkages have been reformed in the correct pattern demonstrated for other B family members, (PTH, GLP-1, CRF1 and 2B receptors). Using similar methods, we have refolded RAMP2 and determined the disulfide linkage pattern for the first time for any member of the RAMP protein family. Protein-protein interaction studies are presently underway using a variety of methods including surface plasmon resonance, calorimetry, ultracentrifugation and radioligand binding.