Cross-linking of bacteriorhodopsin using specific carboxyl modifications and proteolytic cleavage

Specific carboxyl modification of purple membrane using a water-soluble carbodiimide yielded a mixture of oligomers, revealed by gel electrophoresis. Purple membrane pre-treated with papain or trypsin, cleaving the C-terminal tail, showed the same pattern of cross-linked products. Chymotryptic cleavage released amino acids 1-72 (7kD fragment) from the cross-linked products, as it did with native membrane. The tail and helices A and B are not, therefore, involved in carbodiimide-promoted cross-linking. Similar cleavage of a hydrophobic dihydroquinoline-modified sample showed that mainly intra-molecular cross-linking occurs, with little cross-linking between the large and small chymotryptic fragments.