3.E.1.8.1
Anion-specific light-gated channel rhodopsin of 438 aas, Acr1, lacking measurable cation transport capability (Govorunova et al. 2015). The crystal structure of the light-gated anion channel, GtACR1, revealed a continuous tunnel traversing the protein from extracellular to intracellular sides (Li et al. 2021). The tunnel is the conductance channel closed by three constrictions: C1 in the extracellular half, mid-membrane C2 containing the photoactive site, and C3 on the cytoplasmic side. The crystal structure of bromide-bound GtACR1 revealed structural changes that relax the C1 and C3 constrictions, including a novel salt-bridge switch mechanism involving C1 and the photoactive site. Thus, substrate binding induces a transition from an inactivated state to a pre-activated state in the dark that facilitates channel opening by reducing free energy in the tunnel constrictions. The results provide direct evidence that the tunnel is the closed form of the channel of GtACR1 and shed light on the light-gated channel activation mechanism (Li et al. 2021).  The preferential transport of NO₃^- by full-length Guillardia theta anion channelrhodopsin 1, ACR1, is enhanced by its extended cytoplasmic domain (Ohki et al. 2023).