Identification of oleic acid binding sites in cytolysin A-III from the heteronemertine Cerebratulus lacteus

We previously demonstrated that binding of oleic acid to Cerebratulus lacteus cytolysin A-III results in aggregation of this monomeric protein to a tetramer, concomitant with an increase in hemolytic activity. In the present study, incubation of cytolysin A-III with [¹⁴C]-oleic acid in the presence of a water-soluble carbodiimide results in covalent incorporation of a maximum of two molecules of the fatty acid into the protein. Labeling is restricted to two large tryptic peptides. Sequence analysis of peptide mixtures derived from the labeled protein reveals that the predominant sites of labeling are Lys-31 and Lys-71; the latter site is part of the C-terminal amphipathic helix previously shown to be important for hemolytic activity while the former lies in the other significant hydrophobic region of this largely hydrophilic protein.