The purpose of the proposed research is to develop a set of NMR strategies for membrane protein structure determination. Combined with our existing E. coli-based cell-free (CF) expression system, these strategies will then be used to solve several structures of human integral membrane proteins (hIMPs). The existing NMR structure determination methods are not effective for membrane proteins because of their intrinsic structural and dynamical properties. The internal mobility of transmembrane (TM) helical bundles causes strong broadening of the signals in NMR spectra and creates problems with signal assignment, spectra analysis, and detection of long-range interactions, which are necessary to build up the structure of the TM 1-helical domain. Therefore, we propose a set of NMR strategies utilizing the CF expression system, which will significantly speed up structure determination of membrane proteins. In particular, we will further develop the combinatorial dual-isotope labeling strategy in order to make the resonance assignment process fast and robust (Aim 1);incorporate unnatural amino acids that can be modified with a paramagnetic label to measure relaxation enhancement and paramagnetic chemical shift effects (Aim 2);implement 13C-methyl and 19F-labeling in the CF system to obtain additional long-range distance constraints for structure determination (Aim 3). These methods will be applied to preselected (Aim4) hIMPs to determine the high resolution NMR structures of 10 or more targets (Aim 5).
In the proposed study we aim to create innovative tools for structure determination of membrane proteins and use them to solve several human integral membrane protein structures. Each human membrane protein structure is a potential target for rational structure-guided drug design and the significance of new structures cannot be overstated. To achieve the proposed aims, we are going to utilize a novel in vitro expression system to modify the existing and create new high-speed NMR methods for the determination of membrane protein structures.
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