Structure and dynamics of plant cell walls and membrane peptides from solid-state NMR

Abstract

Solid-state nuclear magnetic resonance (SSNMR) is a powerful technique to study the structure, dynamics and interactions of bio-macromolecules. This thesis mainly focuses on the characterization of the architecture and loosening of primary plant cell walls and the interactions between membrane and peptides. Plant cell wall is a complex system mainly comprising three types of insoluble polysaccharides: cellulose, hemicellulose and pectin. The spatial arrangement of these macromolecules has been largely elusive due to the lack of high-resolution and sitespecific characterization techniques. Here, we introduce SSNMR to investigate the interactions of macromolecules in ¹³C-labeled plant primary cell walls with minimal treatment. Our multidimensional ¹³C spectra show intense cellulose-pectin correlations, suggesting subnanometer contacts between these polymers. The cellulose-pectin interaction is found to be an inherent feature of primary cell walls because it is independent of the hydration history and is caused by site-specific interactions instead of molecular crowding. By measuring water to polysaccharide spin diffusion in intact and sequentially digested walls, we are able to examine the three-dimensional structure of cell walls. Our results suggest a single network model, where cellulose microfibrils make physical contacts with both pectin and hemicellulose. We also investigated how this network was unlocked by expansin, a wall-loosening protein. Using differential isotopic labeling and dynamic nuclear polarization, we determined the binding sites of 0.2 mg expansin in cell walls. Cellulose microfibrils with entrapped hemicellulose were found to be the targets of expansins, thus shedding light on the mechanisms of wall elongation and plant growth. These results have deepened our understanding of plant cell walls, a smart material with both high mechanical strength and extensibility. In addition, we also developed new approaches to investigate the interactions between membranes and peptides. By measuring heteronuclear correlation spectra and proton relaxation times, we determined the localization of the Influenza M2 peptide in distinctly curved membrane domains. Using a rigid-solid heteronuclear correlation experiment, we were able to determine the depth of insertion of dynamically invisible peptides in gel-phase membranes. These studies provide new strategies to study the functionally relevant membrane-curvature induction by proteins and the partitioning and insertion of proteins into lipid membranes.