Molecular Architecture of G Protein-Coupled Receptors

Abstract

This review of the current literature on mutations in G protein-coupled receptors (GPCRs) of the rhodopsin-related family intends to draw inferences from amino acid sequences for single receptors and multiple sequence alignments with regard to the molecular architecture of this class of receptors. For this purpose a comprehensive list of mutations within the transmembrane helical regions (TMs; over 390 mutations from 38 different receptor subtypes) and their effects on function was compiled, and an alignment of known GPCR sequences (over 150 separate sequences) was made. Regions most prominently involved in ligand binding are located in TMs 3, 5, 6, and 7. Position 3.32 in TM3 is occupied by a D in all biogenic amine receptors (sequence conservation) but may be occupied by uncharged residues in other receptors while its role in ligand binding is analogous (function conservation). TMs 5, 6, and 7 display considerable sequence conservation throughout the majority of GPCRs investigated, but not necessarily at those positions involved in ligand binding. However, considerable function conservation is observed for positions 5.42 (frequently hydrophilic), 5.46 (small amino acids required for agonist binding to "small ligand" receptors), 6.52 and 7.39 (high variability), and 7.43 (frequently aromatic). A general conclusion of this review is that there is overwhelming conservation of structure-function correlates among GPCRs. Thus, it is now possible to cross-correlate the results of mutagenesis studies between GPCRs of different subfamilies, and to use those results to predict the function of specific residues in new GPCR sequences.

Figure 1

Schematic presentation of the general topology of GPCRs. NT = N-terminus; CT = C-terminus; Ix = intracellular loop x; Ex = extracellular loop x; α indicates a supposed α-helical fragment of I3; β indicates a supposed β-pleated sheet substructure in I2; S-S indicates a possible cystine bond between TM3 and TM5; stacked circles indicate tentative ribosylation sites. Glycosylation has been shown to occur at either or both NT and E2; S-CO denotes a possible acylation site in CT. The numbering of the TMs is shown in the inset in the upper right corner.

Figure 2

Amino acid distribution throughout the GPCRs used in Table 3.

Figure 3

Mutation distribution in the transmembrane regions of GPCRs from Table 3. Open bars (□) represent the number of ineffective mutants reported; shaded bars (▧) represent the number of reports where agonist affinity (AG) is affected; closed bars (■) represent the number of reports where antagonist affinity (AN) is affected.