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Heliorhodopsin Heliorhodopsins, distantly related to type-1 rhodopsins, are embedded in the membrane with their N termini facing the cell cytoplasm, an orientation that is opposite to that of type-1 or type-2 rhodopsins. Heliorhodopsins show photocycles that are longer than one second, which is suggestive of light-sensory activity. Heliorhodopsin photocycles accompany retinal isomerization and proton transfer, as in type-1 and type-2 rhodopsins, but protons are never released from the protein.The structures of several heliorhodopsins have been solved displaying seven transmembrane helices (TM), six loops and short N and C termini. Heliorhodopsins share a common fold with the type-1 rhodopsins, however there are clear structural differences, particularly within the loop regions and the large cavity in the cytoplasmic part of heliorhodpsin. Heliorhodpsins are present in Archaea, Bacteria, Eukarya, and viruses. |
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