Structural and functional homologies of BR and ChR. (A) Sequence homology between the algal ChRs and BR within the third transmembrane helix. Identical and conserved residues are highlighted in yellow and blue, respectively; amino acids interacting with the chromophore are shown in red. In BR, R82 functions as a connector between counterion and the PRC; D85 is the counterion to which the RSB proton is transferred; W86 provides hydrophobicity for retinal binding; and the hydrogen bond between T90 on helix 3 and D115 on helix 4 (not shown) stabilizes the structure of the binding pocket and enables both residues to control rapid reprotonation and reisomerization of retinal to the all-trans state following a light pulse. (NpHR) Natronomonas pharaonis HR. (B) Anticipated homology in the retinal binding pocket between BR and ChR2. The BR pocket is shown based on structure 1KGB (Facciotti et al. 2001) with key amino acids that are involved in the proton transfer reaction. The ChR2 pocket was modeled after the BR structure, based on sequence homology in the relevant positions. (C) Model for the photocycle of ChR2. The RSB configuration and conductive state of each photocycle component are shown in red and black, respectively. The D470 dark state is converted by a light-induced isomerization of retinal via the early intermediate P500 and the transient intermediate P390 to the conducting state P520. The recovery of the D470 dark state proceeds either thermally via the nonconducting P480 intermediate or photochemically via possible short-lived intermediates (green arrow). The late or desensitized P480 state can also be activated (blue arrow) to yield the early intermediate P500.