What is back propegation?

Opsins contain seven transmembrane α-helical domains connected by three extra-cellular and tree cytoplasmic loops. Many amino acid residues, termed functionally conserved residues, are highly conserved between all opsin groups, indicative of important functional roles. All residue positions discussed henceforth are relative to the 348 amino acid bovine rhodopsin crystalized by Palczewski et al. Lys296 is conserved in all known opsins and serves as the site for the Schiff base linkage with the chromaphore. Cys138 and Cys110 form a highly conserved disulfide bridge. Glu113 serves as the counterion, stabalizing the Lys296/chromaphore interaction by neutralizing the positive charge that builds up when the Schiff linkage occurs, The Glu134-Arg135-Tyr136 is another highly conserved motif, involved in the propegation of the transduction signal once a photon has been absorbed.