On the disulphide bonds of rhodopsins

Carboxymethylation using 14C- or 3H-labelled iodoacetic acid has been used to identify the cysteine residues in bovine rhodopsin involved in the formation of the two intramolecular disulphide bridges. Iodo[2-14C]acetic acid was used to modify 5.8-5.9 residues of cysteine under non-reducing conditions. After dialysis and reduction of disulphide bridges by 2-mercaptoethanol, iodo[2-3H]acetic acid was employed to covalently modify 3.3-3.6 residues of cysteine. Peptide purification and sequencing has unambiguously shown that cysteine residues 322 and 323 are only carboxymethylated after reduction of disulphide bridges. Indirect evidence presented, now coupled with the earlier finding [Findlay & Pappin (1986) Biochem. J. 238, 625-642] suggests that the other disulphide bridge is formed between cysteine residues 110 and 187. A comparison is made of all the sequences of mammalian rhodopsins and colour pigments and attention is drawn to the fact that whereas Cys-322 and Cys-323 are conserved only in three rhodopsins (bovine, ovine and human), the residues corresponding to Cys-110 and Cys-187 are found in all the visual proteins (from rods as well as human cones).