Allosteric Regulation of GPCR Rhodopsin by Soft Matter
Channel: Nipuna Weerasinghe
Date: 2020-10-10

G-protein-coupled receptors (GPCRs) are allosteric membrane proteins that mediate cellular signal transduction. They exist as dynamic conformational ensembles with multiple inactive and active conformations. Here we use the visual receptor rhodopsin as an archetypal GPCR to investigate how cellular soft matter (lipid membrane and cellular water) modulate the conformational dynamics of the GPCR activation, hypothesizing a flexible surface model. By using different polyethylene glycol (PEG) solutions, the osmotic pressure on rhodopsin in native membranes and POPC recombinant membranes was varied.  Shifting of the metarhodopsin equilibrium due to the changing the lipid environment and osmotic pressure was probed using UV-Visible spectroscopy. The metarhodopsin equilibrium was shifted towards the inactive Meta-I state in POPC recombinant membranes compared to the native membrane environment.

Furthermore, the analysis of transducin peptide-binding isotherms reveals that the peptide's binding affinity is significantly decreased when the lipid environment is changed from the native lipids to POPC lipids. We further performed a series of experiments to study how the binding affinity of the transducin C-terminal peptide to the Meta-II state is affected by the osmotic pressure in recombinant membranes. The POPC lipid membrane has a zero-spontaneous curvature that shifts the equilibrium towards the more compact, inactive Meta-I state compared to the native lipid membrane environment that has a negative spontaneous curvature that favors the more expanded state of Meta-II.  Our results delineate the crucial role of soft matter (lipids and water) in regulating the metarhodopsin equilibrium. 
References 

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