Exploring the Structural Basis for GPCR Signal Transduction

G-protein coupled receptors (GPCRs) constitute the major class of cell surface receptors regulating a variety of signal transduction in the cells. However, the mechanism for molecular recognition at the extracellular part and signal activation at the intracellular side as well as how these two functions are structurally integrated remain largely unknown. The key question that was explored in my studies centers on how receptors bind to a variety of ligands and convert these diverse external stimuli to an internal signal through molecular changes that occur during binding. A better understanding of how receptors mediate cell signaling can benefit from looking at a number of these ligand-binding complexes. To achieve the goal of defining ligand binding properties of GPCRs and to develop new tools, protocols, and strategies to overcome the challenge of inherent structural flexibility of GPCRs and to produce high quality crystals that can diffract to high resolution, my studies included development of the HT LCP-FRAP approach to characterize protein samples before crystallization trials (Chapter 2) and also the development of strategies to prepare receptor samples in complex with a variety of ligands (Chapter 3 & 4). These efforts attempted to create a platform for systematic study of multiple complexes for different GPCRs and as a successful example, led to discovery of complex structure of A2A adenosine receptor bound to a full agonist (Chapter 5). This structure reveals important insight into a general GPCR activation mechanism: how agonist binding at the outside of the cell triggers receptor conformational changes at the inside of the cell where the signaling cascade is initiated.