Biophysical Society Conference | Tahoe 2023

Proton Reactions: From Basic Science to Biomedical Applications

Wednesday Speaker Abstracts

THE ROLE OF WATER MOLECULES IN THE ACTIVATION MECHANISM OF RETINAL-BINDING MEMBRANE PROTEINS AND G PROTEIN-COUPLED RECEPTORS

Gebhard F.X. Schertler 1 ; Xavier Deupi 1 ; 1 Paul Scherrer Institute, Villigen, Switzerland

G protein coupled receptors (GPCRs) are a large family of membrane proteins present in all eukaryotic cells. They are key in cellular physiology and are also important drug targets for medical intervention. The visual receptor rhodopsin of vertebrates is a prime example of a GPCR, whose structure and function have been extensively studied and characterized in the past decades.Rhodopsin achieves light sensitivity by means of a chromophore ligand (retinal) that is covalently bound to a lysine residue (forming a protonated and positively charged Schiff base) in the receptor binding pocket. Close to the Schiff base, several water molecules contribute to organize functionally important hydrogen bond networks that undergo significant changes during protein activation. Interestingly, this is also the case in other non-GPCR microbial retinal binding proteins –such as the light-driven proton pump bacteriorhodopsin– despite there is no phylogenetic relationship between GPCRs and microbial retinal-binding proteins.Some water molecules have a specific role in protein activation, by appropriately orienting specific residues relative to the ligand, and by subtly modulating the structure of the transmembrane bundle. While the details of these mechanisms are still missing, recent developments in time-resolved serial crystallography in free electron lasers are enabling new mechanistic insights on the role of internal water molecules in the activation of retinal-binding membrane proteins.In this talk, I will describe recent studies on rhodopsin in which we observe, for the first time, changes in the position of waters and relevant receptor side chains shortly after activation of rhodopsin at unprecedented spatial and temporal resolution. I will also discuss the general nature and role of water-mediated networks in vertebrate and invertebrate rhodopsins and in non-light sensitive GPCRs. These also contain partially conserved water-mediated hydrogen bond networks that stabilize the ground state of the receptor and rearrange upon activation to stabilize the active state. Thus, such water-mediated networks are critical for ligand binding and receptor activation, and they are becoming increasingly important in drug discovery.References1. Lesca, E., Panneels, V. & Schertler, G. F. X. The role of water molecules in phototransduction of retinal proteins and G protein-coupled receptors. Faraday Discuss 207, 27–37 (2017). DOI: 10.1039/C7FD00207F2. Varma, N. et al. Crystal structure of jumping spider rhodopsin 1 as a light sensitive GPCR. Proc National Acad Sci 116, 14547–14556 (2019). DOI: 10.1073/pnas.19021921163. Bertalan, E., Lesca, E., Schertler, G. F. X. & Bondar, A.-N. C Graphs Tool with Graphical User Interface to Dissect Conserved Hydrogen-Bond Networks: Applications to Visual Rhodopsins. J Chem Inf Model 61, 5692–5707 (2021). DOI: 10.1021/acs.jcim.1c00827 4. Gruhl, T. et al. Ultrafast structural changes direct the first molecular events of vision. Nature 615, 939–944 (2023). DOI:10.1038/s41586-023-05863-6

36