Sensory rhodopsin II/transducer complex formation in detergent and in lipid bilayers studied with FRET

The photophobic receptor from Natronomonas pharaonis (NpSRII) forms a photo-signalling complex with its cognate transducer (NpHtrII). In order to elucidate the complex formation in more detail, we have studied the intermolecular binding of both constituents (NpSRII and NpHtrII157; truncated at residue 157) in detergent buffers, and in lipid bilayers using FRET. The data for hetero-dimer formation of NpSRII/NpHtrII in detergent agrees well with KD values (∼ 200 nM) described in the literature. In lipid bilayers, the binding affinity between proteins in the NpSRII/NpHtrII complex is at least one order of magnitude stronger. In detergent the strength of binding is similar for both homo-dimers (NpSRII/NpSRII and NpHtrII/NpHtrII) but significantly weaker (KD  ∼ 16 μM) when compared to the hetero-dimer. The intermolecular binding is again considerably stronger in lipid bilayers; however, it is not as strong as that observed for the hetero-dimer. At a molar transducer/lipid ratio of 1:2000, which is still well above physiological concentrations, only 40% homo-dimers are formed. Apparently, in cell membranes the formation of the assumed functionally active oligomeric 2:2 complex depends on the full-length transducer including the helical cytoplasmic part, which is thought to tighten the transducer–dimer association.