A method for solution NMR structural studies of large integral membrane proteins: Reverse micelle encapsulation

The structural study of membrane proteins perhaps represents one of the greatest challenges of the post-genomic era. While membrane proteins comprise over 50% of current and potential drug targets, their structural characterization lags far behind that of soluble proteins. Nuclear magnetic resonance (NMR) offers great potential not only with respect to structural characterization of integral membrane proteins but may also provide the ability to study the details of small ligand interactions. However, the size limitations of solution NMR have restricted comprehensive structural characterization of membrane protein NMR structures to the relatively small β-barrel proteins or helical proteins of relatively simple topology. In an effort to escape the barriers presented by slow molecular reorientation of large integral membrane proteins solubilized by detergent micelles in water, we have adapted the reverse micelle encapsulation strategy originally developed for the study of large soluble proteins by solution NMR methods. Here we review a novel approach to the solubilization of large integral membrane proteins in reverse micelle surfactants dissolved in low viscosity alkane solvents. The procedure is illustrated with a 54 kDa construct of the homotetrameric KcsA potassium channel.