Bacterial sensory rhodopsin
Anabaena sp. (cyanobacteria)
All organisms use light as a stimulus for various activities and hence need proteins that are sensitive to the presence or absence of light, as well as its various wavelengths or colors. Rhodopsins are transmembrane (embedded in the cell membrane) proteins that are sensitive to light. They are a family of photoreceptors with 7 alpha-helices embedded in the membrane and are present in eubacteria, archaea and eukaryotes. Rhodopsins are found in the photo-sensitive cells of the eyes in vertebrates, conjugate to G protein, and control the function of visual sences. Rhodopsins in archea, also known as bacteriorhodopsins, function as proton pumps activated by light. Each bacteriorhodopsin molecule contains a single light-absorbing group called chromophore or retinal bound to it. When exposed to light, the chromophore becomes activated upon the absorption of a photon and changes shape. This results in a change in conformation of the bacteriorhodopsin and the transfer of one proton (H+) from the intracellular space to the outside. Until recently, the structure of bacteriorhodopsin has been the primary source of knowledge about rhodopsins.
The structure shown here is that of a eubacterial rhodopsin from Anabaena. It is similar to bacteriorhodopsin from archaea in that it has 7 alpha-helices which traverse the cell membrane and a chromophore that is sensitive to light. However, the chromophore in this rhodopsin interconverts between two forms (13-cis and trans) depending on the wavelength of light. In the presence of orange light, more than 80% of the rhodopsin shows the 13-cis form of the chromophore, whereas in the presence of blue light the rhodopsins predominantly contain chromophores in the trans form. Thus the eubacterial rhodopsin is a color sensitive photoreceptor. The eubacterial rhodopsin also shows a marked difference in its structure at the cytoplasmic end from that of bacteriorhodopsin in the form of highly ordered loops at the cytoplasmic end of the protein preceded by a hydrophilic (able to form interactions with water) path that leads to the chromophore near the middle of the membrane. Though the function of this eubacterial rhodopsin is not yet known, it is possible that such color sensitivity could be used by the bacteria to control the expression of proteins that are required under different forms of illumination.
Protein Data Bank (PDB)
author: Ashwini Patil