Magic-angle-spinning solid-state NMR (MAS-SSNMR) has emerged as a powerful technique of structural biology. It is particularly attractive for its unique capability of providing structure and dynamic for membrane proteins in lipid bilayers. In this presentation I will introduce our recent progress in structure and dynamic characterization of Leptosphaeria rhodopsin (LR).
LR was the first discovered eukaryotic light-driven proton pump, which uses light energy to transport protons across the cell membranes. LR shares the typical heptahelical topology and has a retinal covalently bound to the protein core. However, the structure and the detailed mechanisms of LR are still unknown. The proteins were prepared by P. pastoris expression system. To determine the structure of LR, the 2D and 3D MAS-SSNMR spectra were collected to achieve backbone and side-chain assignments. Sparsely 13C labeled protocol for P. pastoris expression systems were developed to obtain long-range distances for structural illustrations. The LR forms homo-trimers in lipid environments. Paramagnetic relaxation enhancements were applied in characterization of the intermonomer interface.
The LR shares heptahelical transmembrane topology. The hydrogen-bonding networks formed by the proton of the protonated Schiff-base and critical Asp residues are the key elements for LR function. The chemical shift of Asp residues suggested the carboxyl sidechains of D139 and D266 are deprotonated, consistent with the common knowledge of microbial rhodopsin. On the other hand, the solid-state H/D exchange experiments suggested the rapid exchange between solvent water and the proton of the protonated Schiff-base. This indicated the participation of water molecule in the pathway of proton pump of LR.