8.A.127. The Receptor Activity-Modifying Protein (RAMP) Family
RAMPs (1-3) in humans are proteolytically processed, single transmembrane, accessory proteins, crucial for plasma membrane expression, but they also determine receptor phenotype of various G-protein-coupled receptors (Bomberger et al. 2005). For example, adrenomedullin receptors are comprised of RAMP2 or RAMP3 (AM1R and AM2R, respectively) and the calcitonin receptor-like receptor (CRLR; Q16602; TC# 9.A.14.4.12), while a CRLR heterodimer with RAMP1 yields a calcitonin gene-related peptide receptor. The PDZ type I domain, present in the C terminus of RAMP3, but not in RAMP1 or RAMP2, leads to protein-protein interactions that determine receptor trafficking. The CRLR-RAMP complex undergoes agonist-stimulated desensitization and internalization and fails to resensitize (i.e. degradation of the receptor complex). Co-expression of N-ethylmaleimide-sensitive factor (NSF) with the CRLR-RAMP3 complex, but not CRLR-RAMP1 or CRLR-RAMP2 complex, altered receptor trafficking to a recycling pathway. Mutational analysis of RAMP3, by deletion and point mutations, indicated that the PDZ motif of RAMP3 interacts with NSF to cause the change in trafficking. The role of RAMP3 and NSF in AM2R recycling was confirmed in rat mesangial cells, where RNA interference with RAMP3 and pharmacological inhibition of NSF both resulted in a lack of receptor resensitization/recycling after agonist-stimulated desensitization. Thus, functional differences between the AM1R and AM2R occur at the level of post-endocytic receptor trafficking (Bomberger et al. 2005).