Conformational Dynamics of GPCR Dimers is Dependent on Membrane Cholesterol

Abstract

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins involved in signal transduction across cell membranes. Increasing evidence points towards a large role of membrane constituents, especially cholesterol, in GPCR structure and function. Here, we analyze the molecular basis of cholesterol modulated GPCR association by coarse-grain molecular dynamics simulations. Several cholesterol interaction sites have been identified on the serotonin1A receptor that are highly dynamic and have a microsecond time scale of exchange with the bulk lipids. We believe that these cholesterol hot-spots are indicative of ‘high occupancy sites’ rather than ‘binding sites’. These specific interactions, along with general membrane effects, have been observed to modulate GPCR organization in a receptor-dependent manner. The plasticity and flexibility of the serotonin1A receptor dimers increase with increased cholesterol concentration. Interestingly, a dimer interface formed by transmembrane helices I was found to be sensitive to cholesterol. The modulation of dimer interface appears to arise from a combination of direct cholesterol occupancy and indirect membrane effects. Importantly, the presence of cholesterol at the dimer interface is correlated with increased dimer plasticity and flexibility. The increased plasticity of the dimer interfaces with increased cholesterol suggests that the formation of higher-order clusters could be facilitated with increased cholesterol. These results represent an important step in characterizing the molecular interactions in GPCR organization with potential relevance to therapeutic interventions.