Abstract
Microbial rhodopsins and G-protein coupled receptors (GPCRs, which include animal rhodopsins) are two distinct (super) families of heptahelical (7TM) membrane proteins that share obvious structural similarities but no significant sequence similarity. Comparison of the recently solved high-resolution structures of the sodium-translocating bacterial rhodopsin and various Na+-binding GPCRs revealed striking similarity of their sodium-binding sites. This similarity allowed us to construct a structure-guided sequence alignment for the two (super)families, which highlighted their evolutionary relatedness. Our analysis supports a common underlying molecular mechanism for both families that involves a highly conserved aromatic residue playing a pivotal role in rotation of the 6th transmembrane helix.ReviewersThis article was reviewed by Oded Beja, G. P. S. Raghava and L. Aravind.Electronic supplementary materialThe online version of this article (doi:10.1186/s13062-015-0091-4) contains supplementary material, which is available to authorized users.
Highlights
Microbial rhodopsins and G-protein coupled receptors (GPCRs, which include animal rhodopsins) are two distinct families of heptahelical (7TM) membrane proteins that share obvious structural similarities but no significant sequence similarity
In most of the studied GPCRs, binding of the sensed ligand molecule causes a conformational change in the helical bundle that promotes an interaction with a GTP/GDP binding protein, which triggers the intracellular signal cascade [4, 5]
The GPCRs are divided into several families, the major of which are rhodopsin-like receptors, secretin receptors, glutamate receptors, fungal mating pheromone receptors, 3′–5'-cyclic adenosine monophosphate (cAMP) receptors, and frizzled receptors [10]
Summary
Microbial rhodopsins and G-protein coupled receptors (GPCRs, which include animal rhodopsins) are two distinct (super) families of heptahelical (7TM) membrane proteins that share obvious structural similarities but no significant sequence similarity. Binding of the Na+ ion by the stretches of polar residues of the 3d and 7th helices, which are conserved throughout MRs and GPCRs (see a structure-based sequence alignment, Additional file 1: Figure S1 and S4), strongly support a common origin of all these proteins.
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