NOE-Derived Methyl Distances from a 360 kDa Proteasome Complex.

Nuclear magnetic resonance spectroscopy is the prime tool to probe structure and dynamics of biomolecules at atomic resolution. A serious challenge for that method is the size limit imposed on molecules to be studied. Standard studies are typically restricted to ca. 30-40 kDa. More recent developments lead to spin relaxation measurements in methyl groups in single proteins or protein complexes as large as a mega-Dalton, which directly allow the extraction of angular information or experiments with paramagnetic samples. However, these probes are all of indirect nature in contrast to the most intuitive and easy-to-interpret structural/dynamics restraint, the internuclear distance, which can be measured by nuclear Overhauser enhancement (NOE). Herein, we demonstrate time-averaged distance measurements on the 360 kDa half proteasome from Thermoplasma acidophilium. The approach is based on exact quantification of the NOE (eNOE). Our findings open up an avenue for such measurements on very large molecules. These restraints will help in a detailed determination of conformational changes upon perturbation such as ligand binding.