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

## TL;DR

This study investigates how a light-sensitive protein interacts with its transducer, revealing how signals are transmitted in microbial systems.

## Contribution

The study uses surface-enhanced infrared spectroscopy to show that the HAMP domain may not be essential for initial signal transduction.

## Key findings

- The light-induced structural changes in pSRII-pHtrII are strongly attenuated in a surface-density-dependent manner.
- The suppression of the amide I region is similar in both constructs with and without the HAMP domain.
- The conformational change in the membrane region appears to be key for initial signal transduction.

## Abstract

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.