Abstract
Sphingosine-1-phosphate (S1P) is a lipid mediator that can activate five cell membrane G protein-coupled receptors (GPCRs) which carry a variety of essential functions and are promising drug targets. S1P is composed of a polar zwitterionic head-group and a hydrophobic alkyl chain. This implies an activation mechanism of its cognate receptor that must be significantly different from what is known for prototypical GPCRs (ie receptor to small hydrophilic ligands). Here we aim to identify the structural features responsible for S1P agonism by combining molecular dynamics simulations and functional assays using S1P analogs of different alkyl chain lengths. We propose that high affinity binding involves polar interactions between the lipid head-group and receptor side chains while activation is due to hydrophobic interactions between the lipid tail and residues in a distinct binding site. We observe that ligand efficacy is directly related to alkyl chain length but also varies with receptor subtypes in correlation with the size of this binding pocket. Integrating experimental and computational data, we propose an activation mechanism for the S1P receptors involving agonist-induced conformational events that are conserved throughout class A GPCRs.
Highlights
By ensuring the conversion of extracellular stimuli into cellular responses, G protein-coupled receptors (GPCRs) modulate signaling pathways in a wide variety of biological processes
The hypothesis that agonism could depend on the deformation of the TM pocket induced by a hydrophobic volume is underpinned by the investigation of S1P1 interactions with a series of synthetic agonists[17], as well as structure-activity relationships work on FTY720 analogs[18, 19]
By combining computational and experimental approaches, we identify the structural and chemical features that are responsible for S1P agonism, leading to a comprehensive activation mechanism for the S1P receptor family
Summary
By ensuring the conversion of extracellular stimuli into cellular responses, G protein-coupled receptors (GPCRs) modulate signaling pathways in a wide variety of biological processes. While most of them recognize small polar agonists, GPCRs for lipid mediators are activated by hormone-like signaling molecules derived from lipid species, which possess long hydrophobic moieties[5] This subfamily is mostly composed of the sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) receptors (formerly grouped in the endothelial differentiation gene or EDG family) as well as the cannabinoid receptors. While the S1P head-group is required for its high affinity binding to the receptor, acting as an anchor, elements suggest that it is not key in receptor activation, but rather that the alkyl chain may be involved in agonist activity[16] This assumption is supported by the fact that an antagonist head-group can share closely related chemical functions with S1P and interact via the same residues, as exemplified by ML05612. By combining computational and experimental approaches, we identify the structural and chemical features that are responsible for S1P agonism, leading to a comprehensive activation mechanism for the S1P receptor family
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