Deep Mutational Scanning of the GPCR Rhodopsin Reveals Pleiotropic Mutational Effects and Their Roles in Modulating Light Sensitivity
Biophysical pleiotropy, the phenomenon in which a mutation affects multiple protein properties, underlies many genetic diseases and shapes protein evolution, yet remains poorly understood, limiting synthetic biology and therapeutic development. Although extensively studied in soluble proteins, little is known about how pleiotropic effects shaped the evolution of receptor protein functions. Here, we developed a cell-based assay designed to assess pleiotropic effects in G protein-coupled receptors (GPCRs), the largest receptor class in eukaryotes. Because our assay design allows for multiple GPCR molecular phenotypes (basal activity, ligand-dependent signaling, and cellular receptor abundance) to be simultaneously assessed, we applied it to study rhodopsin, a visual GPCR that evolved high light sensitivity by maximizing light-driven responses while minimizing thermally driven noise. To investigate this, we integrated our assay with deep mutational scanning of relevant rhodopsin domains to test the impacts of [~]2,000 single-residue substitutions in rhodopsin across all three molecular phenotypes ([~]6,000 measurements). By analyzing these data in the context of rhodopsins protein structures, we discovered complex pleiotropic effects that act asymmetrically to impose joint constraints, restricting mutational tolerance in rhodopsins retinal binding pocket and G protein interaction interface. A dose-response analysis of rhodopsin signaling revealed that these pleiotropic constraints arise from the dual functions of its chromophore, which acts as an agonist upon light-driven isomerization but also an inverse agonist in darkness. Because these constraints reflect mechanistically driven limitations in the evolution of receptor signaling, these findings reveal a role for biophysical pleiotropy in shaping the sensory capabilities of receptor proteins. Significance StatementUnderstanding how genetic variation impacts protein function is an important but challenging area of research as many mutations are pleiotropic, simultaneously affecting multiple protein properties, including structure, stability, and activity. Previously, genetic screens have systematically mapped pleiotropic effects in soluble proteins, but transmembrane receptor proteins embedded in cellular membranes are much more difficult to assay. These are key receptors that convert environmental stimuli into appropriate physiological responses. Here, we present a systematic study of pleiotropic effects within the receptor responsible for dim-light vision in vertebrates, revealing how light sensitivity can be mediated through differential conformational states of its chromophore. This is important not only for understanding how receptors evolved enhanced capabilities, but also for the development of tunable protein systems.