Surface Density-Dependent Interactions between Photoactivated Sensory Rhodopsin 2 and Its Transducer

Sensory rhodopsin 2 from Natronomonas pharaonis (pSRII) is a heptahelical transmembrane protein that functions as a photosensor of a microbe with its cognate transducer protein, pHtrII, which is a bihelical transmembrane protein with a long cytoplasmic domain for regulation of flagella rotation. It was revealed that pSRII and pHtrII form a 2:2 complex, and the photoreaction induces tilting of the sixth helix (the F helix) in pSRII, which rotates a transmembrane helix of pHtrII. The conformational change is presumably transferred to the cytoplasmic domain through HAMP domains, which are ubiquitous for chemosensory systems. X-ray crystallography on the pSRII complex with pHtrII truncated C-terminal domain did not resolve a HAMP domain because of its structural flexibility. Thus, the involvement of the HAMP domain in the protein-protein interactions between the pSRII and pHtrII complexes is still elusive. Here, we applied surface-enhanced infrared spectroscopy to pSRII and pSRII fused with pHtrII-(1-159) including a HAMP domain or pHtrII-(1-83) excluding the domain to study structural changes under a physiological membrane orientation. Interestingly, the light-induced difference spectrum of pSRII-pHtrII-(1-159) was strongly attenuated in the amide I region in a surface-density-dependent manner. The suppression was observed almost similarly in pSRII-pHtrII-(1-83), suggesting that the conformation change of pSRII-pHtrII in the membrane region would be a primary key factor for the initial signal transduction from pSRII to pHtrII without interaction with the HAMP domain.