ИСТИНА

Natural anion channelrhodopsins (ACRs), first discovered in the cryptophyte alga Guillardia theta, generate large hyperpolarizing currents at membrane potentials above the Nernst equilibrium potential for chloride and thus constitute inhibitory tools for optogenetics that are more efficient than rhodopsin ion pumps and engineered chloride-conducting mutants of cation channelrhodopsins (CCRs). The primary sequences and gating mechanisms of ACRs differ from those of CCRs, and the residue determinants of the ACRs’ anion selectivity remain unclear. As a first step towards their identification we carried out an electrophysiological screen of homologous sequences found in other cryptophyte species and expressed in cultured mammalian cells. All tested proteins exhibited anion conductance, but differed in the amplitudes, kinetics and spectral sensitivity of their photocurrents. Sequence comparisons confirmed that the selectivity filter of natural ACRs is radically different from that of engineered chloride-conducting mutants of CCRs.