Functional types of microbial rhodopsins. For molecules shown as ribbons high-resolution crystal structures have been obtained. Abbreviations: BRs, bacteriorhodopsins; PRs, proteorhodopsins; HRs, halorhodopsins; NaRs, Na+-pumping rhodopsins; SRs, sensory rhodopsins; ASR, Anabaena sensory rhodopsin; ER, enzymerhodopsins; CCRs, cation channelrhodopsins; ACRs, anion channelrhodopsins; eukar., eukaryotic; eubact., eubacterial; HK, histidine kinase; GC, gunanylyl cyclase; CD, cytoplasmic domain.