Amino acid sequence surrounding the retinal‐binding site in retinochrome of the squid, Todarodes pacificus

Squid (Todarodes pacificus) retinochrome was reduced to N‐retinyl protein with borane dimethylamine and cleaved by CNBr. The retinyl peptide was then isolated by chromatography while being monitored for absorbances at 215 and 330 nm, and the N‐terminal amino acid sequence was determined to be Ser‐Lys‐Thr‐Gly‐X‐Ala‐Leu‐Phe‐Pro. This sequence was the same that we had observed at the 7th transmembrane domain of retinochrome whose structure was reported previously. During Edman degradation of the retinyl peptide, the yield of the PTH‐lysine at the second cycle was lower than those of the other PTH‐amino acids, proving that the lysine residue forms a Schiff's base with retinal (Lys‐275 in retinochrome). The amino acid sequence surrounding the retinal‐binding lysine in retinochrome greatly differed from those in a variety of known visual pigments. This fact would be associated with the difference in the photoisomerization of chromophore between retinochrome and rhodopsin. The protein structure of retinochrome is also compared with that of rhodopsin in Todarodes.