Novel Function for Receptor Activity-modifying Proteins (RAMPs) in Post-endocytic Receptor Trafficking

Jennifer M Bomberger,Narayanan Parameswaran,Carolyn S Hall,Nambi Aiyar,William S Spielman

doi:10.1074/jbc.m413786200

Abstract

RAMPs (1-3) are single transmembrane accessory proteins crucial for plasma membrane expression, which also determine receptor phenotype of various G-protein-coupled receptors. For example, adrenomedullin receptors are comprised of RAMP2 or RAMP3 (AM1R and AM2R, respectively) and calcitonin receptor-like receptor (CRLR), while a CRLR heterodimer with RAMP1 yields a calcitonin gene-related peptide receptor. The major aim of this study was to determine the role of RAMPs in receptor trafficking. We hypothesized that a 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. Employing adenylate cyclase assays, radioligand binding, and immunofluorescence microscopy, we observed that in HEK293 cells 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. These findings provide the first functional difference between the AM1R and AM2R at the level of post-endocytic receptor trafficking. These results indicate a novel function for RAMP3 in the post-endocytic sorting of the AM-R and suggest a broader regulatory role for RAMPs in receptor trafficking.

Highlights

The recent discovery of receptor activity-modifying proteins (RAMPs)1 has raised new possibilities for modes of regulation
The role of RAMP3 and N-ethylmaleimide-sensitive factor (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 agoniststimulated desensitization
We show here that while calcitonin receptor-like receptor (CRLR)-RAMP1 and CRLR-RAMP2 complexes do not interact with NSF, CRLR-RAMP3 complex interacts with NSF via the PSD-95/Discslarge/ZO-1 homology (PDZ) motif of RAMP3

Summary

Introduction

The recent discovery of receptor activity-modifying proteins (RAMPs) has raised new possibilities for modes of regulation. Factors influencing the sorting of receptors in the early endosomes are largely unknown, but some of the critical players are beginning to be identified for the GPCRs. Factors influencing the sorting of receptors in the early endosomes are largely unknown, but some of the critical players are beginning to be identified for the GPCRs It has been shown in other GPCR systems that interactions with PSD-95/Discslarge/ZO-1 homology (PDZ) domain proteins are responsible for altering the receptor-targeting after internalization (9 –11). NSF is a hexameric ATPase that plays a chaperoning role for soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) in the majority of membrane fusion events in a cell, but when targeting membrane receptors for recycling, NSF acts independently of the SNARE complex to promote rapid resensitization of the receptors at the plasma membrane (14 –16)

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