Methods for studying interactions of detergents and lipids with α-helical and β-barrel integral membrane proteins

Methods for studying interactions of protein with lipids and detergents are described for representatives of two major classes of membrane proteins: (1) the α-helical hetero-oligomeric integral cytochrome b6 f complex of oxygenic photosynthesis from cyanobacteria, and (2) the outer membrane β-barrel proteins BtuB and OmpF from Gram-negative Escherichia coli bacteria. Details are presented on the use of detergents for purification and crystallization of the b6 f complex as well as a method for lipid exchange. The positions of detergent and lipid molecules, which define eight potential lipid-binding sites in the b6 f complex, are described. Differences in detergent strategies for isolation and crystallization of β-barrel proteins relative to those for oligomeric helical membrane proteins are discussed, and purification and assessment of protein quality by circular dichroism (CD) is presented.