Structure of Cry3A delta-endotoxin within phospholipid membranes

Interaction of delta -endotoxin and its proteolytic fragments with phosphol ipid vesicles was studied using electron microscopy, scanning microcalorime try, and limited proteolysis. It was shown that native protein destroys lip osomes. The removal of 4 N-terminal alpha -helices and the extreme 56 C-ter minal amino acid residues did not affect this ability. The results obtained by limited proteolysis of delta -endotoxin bound to lipid vesicles show es sential conformational changes in three or four N-terminal helices and in t he C-terminal region. The calorimetric method used in this study provides a unique possibility for the validation of existing models of protein bindin g and for a more accurate determination of the regions where conformational changes take place. It was found that the binding of the protein to model liposomes does not alter its structure in the regions starting with the fou rth alpha -helix of domain I. This can be concluded from the fact that the activation energy of denaturation of the protein remains unchanged upon its binding to the phospholipid membranes. A new structural model has been pro posed which agrees with the data obtained.