Investigating the chromophore and binding pocket structure of a novel inward proton pump, GSS AntR, using biosynthetic 13C-retinal
Microbial rhodopsins are light-activated proteins with diverse physiological roles across all domains of life. A new member of the schizorhodopsin family, Antarctic rhodopsin (AntR) was found in freshwater lakes and acts as an inward proton pump. Its structure and biophysical properties are established, but its mechanism of action remains elusive. We optimized the biosynthetic production of 13C-all-trans-retinal for investigation of an AntR homolog with a GSS motif on helix C using Magic Angle Spinning solid-state NMR spectroscopy. Chemical shift values for all twenty of retinal’s carbon atoms were assigned and compared with bacteriorhodopsin, an extensively studied outward proton pump. Raman spectroscopy and molecular dynamics simulations confirmed that retinal adopts a unique conformation due to electrostatic interactions with neighbouring residues. Understanding activation mechanisms elucidates precise evolutionary steps taken to develop unconventional protein functions necessary for host survival and how they may advance optogenetic progress.