Jan 8, 2014
Organisms of all domains of life use photoreceptor proteins to sense and respond to light. The light-sensitivity of photoreceptor proteins arises from bound chromophores such as retinal in retinylidene proteins, bilin in biliproteins, and flavin in flavoproteins. Rhodopsins found in Eukaryotes, Bacteria, and Archaea consist of opsin apoproteins and a covalently linked retinal which is employed to absorb photons for energy conversion or the initiation of intra- or intercellular signaling. Both functions are important for organisms to survive and to adapt to the environment. While lower organisms utilize the family of microbial rhodopsins for both purposes, animals solely use a different family of rhodopsins, a specialized subset of G-protein-coupled receptors (GPCRs). Animal rhodopsins, for example, are employed in visual and nonvisual phototransduction, in the maintenance of the circadian clock and as photoisomerases. While sharing practically no sequence similarity, microbial and animal rhodopsins, also termed type-I and type-II rhodopsins, respectively, share a common architecture of seven transmembrane .-helices (TM) with the N- and C-terminus facing out- and inside of the cell, respectively. Retinal is attached by a Schiff base linkage to the .-amino group of a lysine side chain in the middle of TM7. The retinal Schiff base (RSB) is protonated (RSBH+) in most cases, and changes in protonation state are integral to the signaling or transport activity of rhodopsins.
Figure: Conformational changes upon rhodopsin activation leading to the Meta II activated state. 1. Photon absorption causes retinal cis → trans isomerization and small scale changes in structure in the immediate vicinity of the retinal, driving all subsequent activation steps. 2. Deprotonation of the RSBH+ along with further small-scale changes within the TM region. 3. Signal propagation to two regions almost universally conserved in class A GPCRs, the (D/E)RY and NPxxY(x)5,6F motifs. Changes in the (D/E)RY motif (in TM3; Glu134, Arg135, Tyr136 of bovine Rho), resulting in disruption of the .ionic lock. between Arg135 and Glu247 (on TM6), and changes in the NPxxY(x)5,6F region (TM7/H8) which rearranges. 4. Proton uptake from the cytoplasm onto Glu134. The TM helices are depicted in the following colors: TM1, blue; TM2, teal; TM3, green; TM4, lime green; TM5, yellow; TM6, orange; TM7, red; and H8, purple.
Ernst OP, Lodowski DT, Elstner M, Hegemann P, Brown LS, Kandori H.
Microbial and animal rhodopsins: structures, functions, and molecular mechanisms.
Chem Rev. 2014 Jan 8;114(1):126-63. doi: 10.1021/cr4003769. Epub 2013 Dec 23.