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Our understanding of the rules controlling the spectral tuning of light absorbing proteins is limited. When looking at rhodopsins as canonical examples, the fact that the cavity incorporates the chromophore counterion in different positions and polar residues with different orientations, leads to patterns of electrostatic potential whose effect is not obvious. In this work we use a model of the optogenetic reporter Arch-3 capable to describe the effect of the diffusion of its counterion charge on both excitation energies (lambda-max) ) and chromophore geometry. By optimizing such charge for a set of increasing lambda values, we show that progression towards redder values occurs along two distinct paths featuring a “compact” or an “extended” charge diffusion respectively. These results are validated by showing that both paths replicate the experimentally observed relationships between lambda-max and chromophore isomerization in different sets of Arch-3 variants, NeoR variants and other microbial rhodopsins from 16 different organisms. 

Melanopsin is a photopigment belonging to the G Protein-Coupled Receptor (GPCR) family expressed in a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) and responsible for a variety of processes. The bistability and, thus, the possibility to function under low retinal availability would make melanopsin a powerful optogenetic tool. Here, we aim to utilize mouse melanopsin to trigger macrophage migration by its subcellular optical activation with localized blue light, while simultaneously imaging the migration with red light. To reduce melanopsin’s red light sensitivity, we employ a combination of in silico structure prediction and automated quantum mechanics/molecular mechanics modeling to predict minimally invasive mutations to shift its absorption spectrum towards the shorter wavelength region of the visible spectrum without compromising the signaling e"ciency. The results demonstrate that it is possible to achieve melanopsin mutants that resist red light-induced activation but are activated by blue light and display properties indicating preserved bistability. Using the A333T mutant, we show that the blue light-induced subcellular melanopsin activation triggers localized PIP3 generation and macrophage migration, which we imaged using red light, demonstrating the optogenetic utility of minimally engineered melanopsins.