Effects of Solubilization on the Structure and Function of the Sensory Rhodopsin II/Transducer Complex

Lipid–protein interactions are known to play a crucial role in structure and physiological activity of integral membrane proteins. However, current technology for membrane protein purification necessitates extraction from the membrane into detergent micelles. Also, due to experimental protocols, most of the data available for membrane proteins is obtained using detergent-solubilized samples. Stable solubilization of membrane proteins is therefore an important issue in biotechnology as well as in biochemistry and structural biology. An understanding of solubilization effects on structural and functional properties of specific proteins is of utmost relevance for the evaluation and interpretation of experimental results. In this study, a comparison of structural and kinetic data obtained for the archaebacterial photoreceptor/transducer complex from Natronomonas pharaonis (NpSRII/NpHtrII) in detergent-solubilized and lipid-reconstituted states is presented. Laser flash photolysis, fluorescence spectroscopy, and electron paramagnetic resonance spectroscopy data reveal considerable influence of solubilization on the photocycle kinetics of the receptor protein and on the structure of the transducer protein. Especially the protein-membrane proximal region and the protein–protein interfacial domains are sensitive towards non-native conditions. These data demonstrate that relevance of biochemical and structural information obtained from solubilized membrane proteins or membrane protein complexes has to be evaluated carefully.