Molecular weight and structural studies on cephalopod rhodopsin.

The protein moiety of squid (Watasenia scintillans) rhodopsin has been shown to have a molecular weight of 46 800 by means of amino acid analysis. This value was comparable to the value (51 000) obtained from SDS-polyacrylamide gel electrophoresis. After the squid eyes were incubated at 10 degrees C for 8 days, the rhodopsin showed a molecular weight of 39 000 on electrophoresis. The smaller molecular weight was ascertained by amino acid analysis of the rhodopsin; and may result from autolysis by the lysosomal enzyme. The rhodopsin in rhabdomeric membranes and in detergent solution was treated with chymotrypsin, papain or subtilisin. These enzymes first produced the 39 000 dalton rhodopsin and then cleaved this into the 25 000 and 14 000 dalton peptides without bleaching. The rhodopsin was attacked by proteases and readily lost an approx. 12 000 dalton peptide portion. This portion included the COOH-terminal and was rich in glutamic acid, proline, glycine, alanine and tyrosine residues.