Figure 1.

Alignment of the third transmembrane regions of vertebrate and invertebrate visual pigments. The position of the counterion to the protonated Schiff base of vertebrate visual pigments is denoted by ∗, where a glutamic acid residue is conserved in all known vertebrate visual pigments. This residue is replaced by a tyrosine or phenylalanine in invertebrate visual pigments. Rhod, rhodopsin; Bov, bovine; Hum, human; Lmp, lamprey; Chk, chicken; Gck, Gecko gecko; Oct, octopus; Lim, Limulus; Dro, Drosophila melanogaster.