The power of SANS, combined with deuteration and contrast variation, for structural studies of functional and dynamic biomacromolecular systems in solution

Small-angle neutron scattering (SANS), combined with macromolecular deuteration and solvent contrast variation (H2 O/D2 O exchange) allows focussing selectively on the signal of specific proteins in multi-protein complexes or mixtures of isolated proteins. We illustrate this unique capacity by the example of a functional protein-degradation system in solution, the PAN-20S proteasome complex in the presence of a protein substrate, ssrA-tagged GFP. By comparing experimental SANS data with synthetic SAXS (small-angle X-ray scattering) data, predicted for the same system under identical conditions, we show that SANS, when combined with macromolecular deuteration and solvent contrast variation, can specifically focus on the conformation of the PAN unfoldase, even in the presence of very large GFP aggregates. Likewise, structural information of native GFP states can be visualized in detail, even in the presence of the much larger PAN-20S unfoldase-protease oligomers, which would dominate the overall scattering signal when using X-rays instead of neutrons.