Characterization of an archaeal photoreceptor/transducer complex from Natronomonas pharaonis assembled within styrene-maleic acid lipid particles

The styrene-maleic acid (SMA) copolymers enable a detergent-free extraction of membrane proteins from lipid bilayers yielding stable water-soluble nanocontainers with lipid-encased proteins. Here, we demonstrate the ability of the SMA copolymer to extract a complex of interacting membrane proteins from proteoliposomes, namely the archaeal sensory rhodopsin II from Natronomonas pharaonis, NpSRII, in complex with its transducer, NpHtrII. The NpSRII/NpHtrII complexes retained their structural integrity within SMA lipid particles as revealed by diverse biophysical techniques. Pulse electron paramagnetic resonance data showed the SMA-encased NpSRII/NpHtrII complexes as native-like 2 : 2 dimers with a distance between NpSRII sites 159 according to the V-shaped conformation found in a crystal structure.