| Background: G protein-coupled receptors (GPCRs) are abundant, activate complex signalling and
represent the targets for up to ~60% of pharmaceuticals but there is a paucity of structural data. Bovine
rhodopsin is the first GPCR for which high-resolution structures have been completed but significant
variations in structure are likely to exist among the GPCRs. Because of this, considerable effort has been
expended on developing in silico tools for refining structures of individual GPCRs. We have developed
REPIMPS, a modification of the inverse-folding software Profiles-3D, to assess and predict the rotational
orientation and vertical position of helices within the helix bundle of individual GPCRs. We highlight the
value of the method by applying it to the Baldwin GPCR template but the method can, in principle, be
applied to any low- or high-resolution membrane protein template or structure.
Results: 3D models were built for transmembrane helical segments of 493 GPCRs based on the Baldwin
template, and the models were then scored using REPIMPS and Profiles-3D. The compatibility scores
increased significantly using REPIMPS because it takes into account the physicochemical properties of the
(lipid) environment surrounding the helix bundle. The arrangement of helices in the helix bundle of the
493 models was then altered systematically by rotating the individual helices. For most GPCRs in the set,
changes in the rotational position of one or more helices resulted in significant improvement in the
compatibility scores. In particular, for most GPCRs, a rotation of helix VII by 240–300° resulted in
improved scores. Bovine rhodopsin modelled using this method showed 3.31 Å RMSD to its crystal
structure for 198 C
α
atom pairs, suggesting the utility of the method even when starting with idealised
structures such as the Baldwin template.
Conclusion: We have developed an in silico tool which can be used to test the validity of, and refine,
models of GPCRs with respect to helix rotation and vertical position based on the physicochemical
properties of amino acids and the surrounding environment. The method can be applied to any multi-pass
membrane protein and potentially can be used in combination with other high-throughput methodologies
to generate and refine models of membrane proteins. |