Andrea C. Hamill, Serena Wang, and C. Ted Lee Jr.. University of Southern California, Los Angeles, CA
Proteins are the workhorses of biology. Beginning as chains of specific combinations of amino acids, proteins fold due to these amino acids having differing degrees of affinity for or aversion to water. It is this seemingly simple folding event, and the resulting shape that the protein adopts in solution, often called the native state, which gives rise to a functional protein. However, proteins are not static entities and instead regularly undergo conformational changes to intermediately-folded or associated states during the course of activity, particularly upon interaction with other molecules. As such, information on the native state alone will not provide complete understanding of the form-function relationship. Instead, precise knowledge of the structure of the partially-folded or oligomeric states that form in response to various stimuli is required. Thus, to properly investigate this phenomenon necessitates two complementary approaches: (1) a means to induce changes in protein structure in a controlled and preferably reversible manner, and (2) a method to determine the conformation of non-native proteins at relatively high resolution. To achieve these tandem goals, we have developed two novel techniques, namely the interaction of proteins with photoresponsive surfactants to allow photoreversible control of protein structure, and the use of small-angle neutron scattering (SANS) to study non-native protein conformations in response to photosurfactant and light. We advance the argument that SANS has the potential to be the technique of choice to study in vitro protein conformations, including novel structural characterization of difficult-to-crystallize states such as partially-folded proteins in non-native conformations, supramolecular complexes undergoing self- or hetero-aggregation, proteins in action or systems that undergo domain motions upon activation, and membrane proteins dispersed in solution with surfactants or lipid bilayers.