Membrane proteins play crucial roles in many cellular and physiological processes. The manner by which detergents affect lipid composition, functionality, and stability of solubilized membrane proteins is a poorly understood aspect of the structural biology of membrane proteins. For the past several decades, the selection of detergents for membrane protein solubilization and/or crystallization has been a random experiment. The main question that we will address in this application is whether a comprehensive approach for a systematic (non-random) selection of a detergent for a particular membrane protein can be developed. Although one of the most widely studied ion channels, the nicotinic acetylcholine receptor (nAChR) is a prime example of a multimeric membrane complex whose high-resolution structure has remained elusive. We have recently carried out lipidomics/activity studies with nAChR solubilized in different detergents (Asmar-Rovira et al., 2008) indicating that a comprehensive approach focused on detergent structure and solubilization conditions could provide important information on the preparation of functionally active complex membrane proteins. We propose to significantly expand our lipid-based approach in assessing the function and stability of the nAChR- detergent complex (nAChR-DC) with the primary goal of defining the manner in which detergent structure affects the lipid composition, ion channel function, agonist binding, state of aggregation of the nAChR-DC, and ultimately the ability to form membrane protein crystals. The central hypothesis is that certain detergents can alter the native lipid composition of solubilized membrane proteins in a manner that results in structural alterations that lead to functional loss, irreversible denaturation, aggregation, and inability to form crystals. We propose four Specific Aims: (1) Determine how the chemical nature of detergents affects the lipid composition of detergent-extracted nAChR from Torpedo californica tissue, (2) Determine how the chemical nature of detergents affects the ligand binding and ion channel function of the nAChR, (3) Examine the degree of aggregation of the nAChR in different detergents, and (4) Estimate the effect of lipid composition on mobile fraction and diffusion coefficient of nAChR-DC in the lipidic cubic phase (LCP). Our lipid-based analysis of the nAChR-DC has revealed an innovative perspective on the preparation of high-quality nAChR crystals. For the first time, we propose to perform biophysical studies of the nAChR in the LCP. Our preliminary data demonstrated that the LCP in combination with the lipid-based approach could lead to high-quality nAChR crystals. This project is technically significant because it will concurrently and systematically assess the lipid composition nAChR-DCs, and it is innovative in its use of the fluorescent recovery after photobleaching (FRAP) approach in the LCP to estimate nAChR mobile fraction and diffusion coefficient to correlate with receptor stability and/or aggregation. An implicit assumption of this lipid-based approach is that principles discovered about the nAChR-DC with respect to lipid composition, functionality, stability, and state of aggregation of the nAChR will be relevant to other important membrane receptor systems, ion channels, and membrane proteins.
The proposed research is relevant to public health because a high-resolution structure of the nAChR is of crucial importance for the design of novel agents that target defined nervous system pathologies such as Alzheimer's disease, schizophrenia, depression, attention deficit hyperactivity disorder and tobacco addiction. Thus, the proposed research is relevant to NIH's mission in that it will increase current knowledge about an important ligand-gated ion channel which could help extend healthy life and reduce the burdens of illness.
