The interaction of δ-hemolysin with calmodulin

Abstract

δ-Hemolysin forms a 1 : 1 complex with Ca 2+-liganded calmodulin. Probably because of the pronounced tendency of δ-hemolysin to self-associate, the apparent binding affinity is much less than that for melittin. Complex formation is reflected by an increase in quantum yield of Trp-15 of δ-hemolysin and by increased shielding from acrylamide quenching. There is, however, no indication of a change in peptide molecular ellipticity. The binding of 2-toluidinyl-naphthalene-6-sulfonate is reduced by complex formation, suggesting the involvement of a hydrophobic region. Complex formation also blocks the proteolysis by trypsin of the bond between residues 77 and 78. The time decays of fluorescence intensity and anisotropy for tryptophan are multiexponential for both free and complexed δ-hemolysin; the average decay time for intensity is substantially increased for the complex. The localized mobility of tryptophan is greatly reduced in the complex. Complex formation appears to involve both the C-terminlobe and the connecting strand of calmodulin.