GPCR interaction with other membrane proteins

RAMPs (receptor activity modifying proteins) constitute a well-known example. RAMP1 is a 148 amino acid protein that is predicted to have a single membrane-spanning domain, a large extracellular domain and a short cytoplasmic domain (Figure 149). RAMP2 and RAMP3 share ± 30% sequence identity with RAMP1. They form a family of chaperone proteins which interact with the calcitonin receptor-like receptor (CRLR), the related the calcitonin receptor and presumably with several other family B GPCRs.

The most striking consequence of RAMP-GPCR associations is the altered pharmacological profile/ phenotype of those receptors (Figure 152):

• The heterodimer between CRLR and RAMP1 behaves pharmacologically like a CGRP (calcitonin gene related peptide) receptor, which is characterized by a higher affinity for the 37 amino acid vasodilatory peptide CGRP than for adrenomedullin (Figure 150). CRLR itself does not display high affinity for CGRP and this explains why transfection with CRLR-coding DNA was unable to yield CGRP-responsive cells if they did not contain sufficient amounts of endogenous RAMP1.

• CRLR interaction with RAMP2 or RAMP3 generates an adrenomedullin receptor.

• The calcitonin receptor also interacts with individual RAMPs to generate different amylin receptors.

Figure 149 Structure of RAMP1. Reprinted from Trends in Pharmacological Science, 24, Morfis, M., Christopoulos, A. and Sexton, P, M., RAMPs: 5 years on, where to now?, 596-601. Copyright (2003), with permission from Elsevier.

The pharmacological profile of the RAMP-GPCR heterodimers is determined by specific interactions. Extracellular domains of both proteins appear to play a primordial role in this process. CGRP, adrenomedullin and related peptide ligands are supposed to bind within the central cleft of the RAMP-associated GPCR. Recently, non-peptide antagonists such as BIBN4096BS have been found to display high affinity for the CGRP receptor when human RAMP1 makes up part of it, but not in the case of rat or porcine RAMP1. Mutation studies revealed that this difference could be attributed to the presence of Trp at location 74 in human RAMP1 as compared to a basic amino acid in rat or porcine RAMP1 (Figure 151). Because BIBN4096BS is much more sensitive

ADM 5f RQ SMMF QGLRSFGC RF ST C TVQKLAHQIYQFTDKDKDNVAPRSKI5 PQGIi CGRP1 ACD TATC VT HRLAGLLSRSGG WKNNF VPTN VGSKAF CGRP2 ACN TATCVT H RLAGLL S RS GGMVKS NF VP TNVGS KAF AM Y LIN KCNTATCATQ RLAHFLVH S S NN F G AIL 5 S TNVG S NTY CALCITONIN CGNLSTCMLGTYTQD-----FNKFHTFPQTAIGVGAP

Figure 150 The amino acid sequences of the calcitonin family peptides (aligned for comparison).

NH2

Kt BIBN4096BS

TIM

hCRLR/hRAMPl

0.018

rCRLR/rRAMP 1

2.1

rCRLR/hRAMPl

0.032

hCRLR/rRAMP 1

1.8

SK-N-MC

0.004

Rat brain

1.5

Figure 151 Chemical structure of BIBN4096BS (reprinted from Trends in Pharmacological Science, 21, C. Juaneda, Y. Dumont and R. Quirion, The molecular pharmacology of CGRP and related peptide receptor subtypes, 432-438. Copyright (2000), with permission from Elsevier) and affinity for heterodimers between CRLR and different RAMPs (r = rat, h = human) (Mallee et al., 2002, reproduced by permission of the American Society for Biochemistry and Molecular Biology).

to the nature of the amino acids at location 74, it is currently believed that this and related antagonists act at the interface between RAMP 1 and CRLR.

RAMPs also assist/chaperone in the transport of CRLR to the cell surface (Figure 152). In this respect, CGRPs and their RAMP partners already form stable

. crlr crlr v7 crlr

0 0

Post a comment