GPCRDB: GPCR article 1

A GPCR picture Copyright (C) 2006, GPCRDB.
This is a summary of an article published in TIPS (1994), vol 15, pp 170-172.

A model for G protein interaction in G protein coupled receptors.

L. Oliveira*, A.C.M. Paiva*, C. Sander+, G. Vriend+.

* Escola Paulista de Medicina, Sao Paulo, Brazil
+ BIOcomputing, EMBL, Heidelberg, Germany.

Abstract

G protein-coupled receptors (GPCRs) form a large superfamily of proteins that transduce signals across the cell membrane. At the external side they receive a ligand (a photon in case of opsins), and at the cytosolic side they activate a G protein. GPCRs consist of one single protein chain that crosses the membrane seven times, similar to the transmembrane seven- helix bundle of bacteriorhodopsin [1]. Most ligands bind between the membrane helices, but the periplasmic loops are sometimes also involved in ligand recognition. The second and third cytosolic loop and part of the (cytosolic) C-terminal end of the receptors are involved in G-protein recognition. Although hundreds of GPCRs exist, that can be activated by a multitude of agonists, G protein activation leads to activation of only a limited set of secondary messengers (GPCRs. Four distinct steps can be observed in the process of GPCR activation: Although these four phases clearly differ in the kind of processes taking place, they are not discrete and independent. For example, allostery between ligand binding and G protein binding has been observed for several GPCR's [28,29], as well as cation-dependent allosteric regulation of agonist and antagonist binding [8,10,11]. Many residues involved in this allosteric effect have been mutated. All mutations led to changes in ligand and/or G protein binding. Mutation of D-224 to asparagine abolished the cation effect on the allosteric site [9,10] and weakened the agonist binding to GPCR's without affecting antagonist binding or G protein activation [12,13]. In case of rhodopsin-transducin interaction, a synergistic competitive mechanism has been uncovered for peptides with the same sequence as these regions. G protein binding and activation by rhodopsins seems to be a two step process:
transducin binding primarily by the ERY (DRY) sequence motif, perhaps to the C-terminal helix of the a-subunit of the G protein [31,32];
G protein activation after binding to the cytosolic loops [27].

Results and discussion

The allosteric site
Although only indirect evidence exists, R-340 apears to be the key residue to initiate G protein binding; R-340 is one of the few residues entirely conserved in all rhodopsin-like GPCR's, it is near the allosteric site and near cytosol.
The residues in the allosteric site, D-224, N-130 and N-729, form a polar pocket to accomodate R-340 thereby preventing this residues interaction with the G protein. These residues are highly but not entirely conserved among the rhodopsin-like receptors and thus their roles on the basic signalling mechanism can only be accessory. Although they are not entirely conserved, the polar pocket is always observed. Two other residues that are in close spatial proximity, residues 725 and 726, are normally polar residues.
We propose that this effect is the result of weakening of a salt bridge between D-224 and R-340. 
Model for the mechanism
The signalling mechanism might be described as a two state process: ¥ OFF when the R-340 side chain sits in the polar pocket; ¥ ON when, upon ligand binding, the allosteric pocket is disturbed, the R- 340 side chain released and shifted towards the cytosol where it can bind the G protein.
Interestingly, this process can also work in reverse. A G protein can cause the R-340 side chain to move out of the polar pocket. This changes the allosteric site thereby modifying the affinity for the ligand. This two way signalling could explain the allostery seen for ligand and G protein binding in several GPCR's [19]. We believe that upon binding of the G protein to the receptor, first the R-340 side chain will interact with a negatively charged residue in the C-terminal helix of the G protein alpha-chain. In many cases D-339 or E-339 enhances this initial binding. Thereafter the other cytosolic loops complement the interaction, leading to the second stage of G protein activation: GDP dissociation and subsequent GTP association.
This hypothesis requires that ligand binding to the receptor promotes conformational changes of residues in or near the allosteric site, releasing the R-340 side-chain to rotate and reach the ideal position for G protein binding (see fig 2). Thus the common step for signalling in all rhodopsin- like GPCR's may simply be a re-orientation of the flexible side chain of R- 340. This idea is in agreement with much experimental data on G protein binding and with data on agonist and antagonist binding. It also provides a detailed mechanistic view of the allosteric effect observed for several receptors.

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