“…1e ). Functional and structural studies of the Na + translocating rhodopsins [ 16 , 38 , 40 , 41 , 51 , 59 ] indicate that photoisomerization of the retinal, by twisting the side chain of Lys255, is bound to unplug the Na + -binding site of KR2, cause deprotonation of the Schiff base (as it has been shown in bacteriorhodopsin [ 60 ]), and, concurrently, induce an outward movement of the 6 th helix (F-helix), opening a cleft that is needed for ion translocation across the hydrophobic part of the protein, as in other microbial rhodopsins [ 21 , 61 – 65 ]. The nucleophilic nitrogen atom of the deprotonated Schiff base would thereby provide one more ligand for the Na + ion on its way through KR2.…”