The dark side of channelrhodopsins: intrinsic ion conductance of apo-opsins

Opsins are known as proteins that bind retinal to form light-sensing rhodopsins, which mediate diverse forms of photoreception and are widely used for optogenetic control of cellular activity. Yet many opsins lack canonical retinal-binding features or, in their native biological contexts, are expressed where chromophore availability is limited, raising the question whether retinal-free opsins are silent. Here we show that retinal-free ion-transporting microbial opsins are not inert but adopt intrinsic, light-independent functional states alongside their canonical light-driven activity. Using developmental phenotyping in transgenic Drosophila melanogaster, electrophysiology in Xenopus oocytes and mammalian cells, together with structural and computational analyses, we demonstrate robust light-independent ion conductance in multiple channel-type opsins and an ion-pumping opsin. Structural modeling and molecular dynamics simulations reveal conformational rearrangements in transmembrane helices and internal water networks that are consistent with stabilization of apo-conductive states. Together, these findings establish apo-opsin activity as a defined functional state of microbial opsins, expand the current framework of rhodopsin biology, and highlight the need to consider chromophore-independent activity in both basic research and optogenetic applications.