Opioids remain the major medication for perioperative and chronic pain management, but their side effects, which include respiratory depression, constipation, addiction and others, relate to morbidity and mortality in our patients, as well as presenting a significant economic and political burden. Since both the analgesic and side effects are thought to arise from activation of the mu receptor (MUR), a better understanding of the structural and functional relationships of human MUR might provide clues for addressing this clinical dilemma. My long-term goal is to reveal the molecular recognition mechanism of opiates and opiate receptors by using biochemical, biophysical, protein engineering and the high resolution structural approaches. In this project, 4 specific aims are proposed. In aim 1, we will establish a novel structural model of the human MUR from a combination of rhodopsin structural data and the newly available experimental structures of human ?2 adrenergic receptor. This will permit opioid and MUR interaction studies at the computational level. In aim 2, we will engineer a variant of the water soluble, but structurally unaltered water soluble mu receptor (WSMUR) for dynamic opioid and receptor interactions in solution conditions. In aim 3, we will express the WSMUR for biophysical characterization studies and for various binding studies with opioids in solution conditions. In aim 4, we will crystallize the WSMUR for structural determinations in the presence or absence of opioids. As an anesthesiologist, formerly trained in academic centers in China, Japan, and United States, Dr. Liu is firmly committed to a career in academic anesthesia. His ultimate career goal is to achieve a better understanding of opioid and opioid receptor interactions, and use these data to develop optimal analgesics to ultimately improve patient care and satisfaction. This proposal will help him to reach his goals by providing further training and resources. His co-mentors have established records of nurturing junior faculty. Together with his current skill set, the advanced training enabled through the K-08 will help transition Dr. Liu towards an independent physician scientist in the area of perioperative medicine.