G-protein coupled receptors (GPCRs) are an important superfamily of membrane proteins that have been a target of nearly 40% of all commercially available pharmaceuticals due to their localization at the cell surface, making them easily accessible to interact with small molecule drugs. Despite consistent funding for new drugs, only 4 of the 24 new drugs approved by the FDA in 2013 targeted the GPCR superfamily. This low percentage is likely due, in part, to a multitude of side effects that often accompany treatment, which arise in part from a lack of structural data for GPCRs and a generally poor understanding of functional consequences of GPCR oligomerization. It has become increasingly evident that GPCRs associate with each other in membranes to form (homo- or hetero-) oligomeric complexes and that these oligomers broaden the range of cell signaling. A better understanding of the factors that drive oligomerization would potentially be key for targeted therapies to, first, understand the consequence of, and then, to modulate receptor-receptor association, e.g. by designing structure-based drugs to target an oligomer population. To pursue such ambitious goals of designing and rationalizing therapeutics that target a specific GPCR oligomer, knowledge gaps in structure-dynamics-function relationships must first be targeted, requiring a number of technological and methodological innovations, as well as the identification of viable and effective GPCR models to address basic questions regarding the functional impact of oligomerization, the lipid membrane environment, and hydration. We identify two 7TM receptors, the bacterial proteorhodopsin (PR) and the full-length human adenosine A2A GPCR that serve as excellent systems to develop and test the proposed tools to determine their oligomeric state and structure in detergent and lipid membranes. Crucially, both the PR and A2A receptors have been shown to oligomerize in native lipid or cell membrane environments, making it highly significant to test key hypotheses on their structure-dynamics-function relationships. The innovation of the proposed work lies in the choice of unique biophysical tools, many of which were developed by the PI and collaborators. They include electron paramagnetic resonance methods of Gd3+ spin-based labels to sensitively capture multiple distances in the 2-6 nm regime, Overhauser dynamic nuclear polarization to directly map out membrane and protein surface hydration dynamics, and effective Yeast expression protocols for synthesizing mg quantities of A2A receptors. The combination of these unique tools permits us to cast broadly important questions, such as: (1) What is the oligomeric state of the 7TM PR and A2A in lipid membranes? (2) Do lipid membrane composition, dynamics and hydration tune oligomerization? (3) What is the functional role of oligomerization observed for PR and A2A? The emphasis of the proposed studies on elucidating 7TM oligomer structure in native lipid membrane environment, explicit comparison to structures obtained in detergent complexes and the dynamics-based approach to reading out and evaluating protein function is novel and critically important for GPCR studies.
G-protein coupled receptors (GPCR) constitute 40% of all drug targets, yet a lack of structural data and a poor understanding of the functional consequences of GPCR associations to oligomers presents a major challenge to the efficient design of GPCR-targeting drugs with an acceptable level of side effects. The proposed research pursues basic method developments that will contribute to addressing this dilemma by studying the oligomer structure of two GPCR models in native lipid membrane environments using a series of innovative biophysical tools and approaches.
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