Commentary and Perspective
Introduction of Session 8, “Structural mechanism of animal rhodopsins and GPCR”
2023 Volume 20 Issue Supplemental Article ID: e201015
Details
2023 Volume 20 Issue Supplemental Article ID: e201015
G protein-coupled receptors (GPCRs), seven helical transmembrane proteins, are responsible for signal transduction pathways by detecting diffusible signaling molecules such as hormones, neurotransmitters and odorants. Animal rhodopsins are highly specialized photoreceptor GPCRs for vision and function via isomerization by the light absorption of retinal chromophore (as a signaling molecule). The dynamic structural information of rhodopsin and GPCRs has recently been elucidated through several types of spectroscopies and structural analyses. In the session 8 chaired by me and Dr. Gebhard F. X. Schertler from Paul Scherrer Institute, five researchers presented cutting-edge results regarding ‘structural mechanism of rhodopsins and GPCRs’, as described below.
The first speaker was Dr. Hideaki Kato from the University of Tokyo, who discussed the ‘structural and functional diversity in pump-like cation channelrhodopsins’. To optically control cell excitability, optogenetics has revolutionized neuroscience. To broaden the application of optogenetics, increasing numbers of ion-translocating rhodopsins (ion pump-type rhodopsins and ion channel-type rhodopsins) have been engineered or discovered in nature. Recently, a new type of channelrhodopsins, pump-like cation-conducting channelrhodopsins or bacteriorhodopsin-like channelrhodopsins (PLCRs or BCCRs), have been discovered and attracted broad attention due to their unique properties (e.g., large photocurrents, high light sensitivity, high ion selectivity). In his talk, he presented the cryo-EM structures of PLCRs [1] and discussed their structure-function relationships.
The next speaker was Dr. Oliver P. Ernst from University of Toronto, who discussed ‘activation studies of opsin and adenosine A2a receptor’. In his talk, he presented studies on the activation of the apoprotein opsin by odorants. Three crystal structures of the active conformation of opsin, Ops*, with monoterpene odorants in the orthosteric retinal binding pocket, as well as biochemical, biophysical and electron paramagnetic resonance (EPR) [2] studies reveal how these odorants can act as agonists to promote conformational changes towards Ops*. He also presented the conformational equilibrium of adenosine A2a receptor.
The third speaker was Dr. Xavier Deupi from Paul Scherrer Institute, who talked about ‘structure and activation of light-activated G protein-coupled receptors’. A wealth of biophysical, functional, and structural data have made the mammalian visual low-light receptor rhodopsin a paradigm of the GPCR family. Recently, they discovered that such similarities are not limited to vertebrate rhodopsins but extend even to visual receptors from invertebrates [3]. In his talk, he presented (i) a novel position for the retinal counterion in the non-bistable box jellyfish rhodopsin that is discovered by employing computational structural models validated by mutagenesis and activity assays, and (ii) the structure of an early active intermediate of bovine rhodopsin (obtained at the SwissFEL free-electron laser) suggesting that the earliest structural rearrangements upon activation already appear in regions involved in later stages of the conserved class A GPCR activation mechanism.
The fourth speaker was Dr. Kota Katayama from Nagoya Institute of Technology, who talked about ‘spectroscopic study of photoactivation dynamics of cone opsin’. Structural studies of cone pigments (blue, green, and red pigments) have lagged far behind those of rhodopsins mainly due to difficulties in sample preparation, except for the spectroscopic studies of the only early intermediate, Batho state [4]. Recently, his group obtained light-induced FTIR difference spectra of Lumi intermediate of the monkey green (MG) pigment at >100 K, and found a large a-helical structural change coupled with retinal relaxation. In his talk, he presented MG conformational dynamics during activation using ATR-FTIR spectroscopy and discussed the differences in conformational changes and phototransduction mechanism between cone pigments and rhodopsins.
The last speaker was Dr. Valérie Panneels from Paul Scherrer Institute, who talked about ‘Mammalian rhodopsin dynamics using X-ray free electron laser’. The intramolecular initial events that transform the rhodopsin resting state into the transducin-binding activated state, Meta II, are not fully understood. In her talk, she presented structural snapshots of native bovine rhodopsin at room temperature using time-resolved ultrafast serial femtosecond crystallography at the SACLA and SwissFEL X-ray free electron lasers (XFELs): Thousands of rhodopsin microcrystals grown in the dark were injected in the light of a pump laser and probed after various time-delays from femtoseconds to milliseconds using XFEL [5]. Interestingly, some amino acids known to be key elements in the signal transduction are involved in the protein breathing motion. The same type of experiment was applied on later time-delays from 100 ps to early microseconds showing a relaxation of the whole structure followed by the first major retinal conformational change modifying its binding pocket.
