In vitro cytotoxic effect of alpha-hemolytic and nonhemolytic Escherichia coli on human blood granulocytes and monocytes

Calcium is required for binding of Escherichia coli hemolysin (HlyA) to erythrocyte membranes

The calcium requirement for hemolytic activity of Escherichia coli hemolysin was investigated by using hemolytic assays and immunoblotting of toxin-treated erythrocytes. The hemolytic activity of cell culture supernatants obtained during growth of E. coli in Luria-Bertani (LB) broth or calcium-free LB broth was calcium dependent. The hemolytic activity of culture supernatants obtained during growth in LB broth supplemented with calcium was calcium independent. Osmotic protection experiments using Dextran 4 to prevent cell lysis indicated that calcium was required for the binding of hemolysin to erythrocytes at both 4 and 37 degrees C. The binding efficiency at 4 degrees C was 50% of that occurring at 37 degrees C. The calcium-dependent binding was confirmed by immunoblotting saline-washed, toxin-treated erythrocytes with a monoclonal antibody after sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation of membrane proteins. Bound hemolysin increased the calcium permeability of the cell membranes as evidenced by calcium-induced membrane protein alterations. The alterations in membrane proteins did not directly cause lysis of the cells. The results were consistent with a mechanism of lysis involving the formation of cation-selective pores in the membranes of target cells.

Domains of Escherichia coli hemolysin (HlyA) involved in binding of calcium and erythrocyte membranes

The primary structure of Escherichia coli hemolysin (HlyA) contains a 9-amino-acid sequence which is tandemly repeated 13 times near the C terminus and which is essential for hemolytic activity. Hemolysin also requires an unknown modification by an accessory protein, HlyC, for hemolytic activity. The role of calcium in the interaction of HlyA with erythrocytes was investigated by using recombinant strains which produced inactive hemolysins unmodified by HlyC or deleted of the repeat sequences. 45Ca2+ autoradiography of the recombinant hemolysins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose showed that full-length, active hemolysin bound calcium. The domain involved in binding calcium was identified as the tandemly repeated sequences, since the deletion derivative missing 11 of the 13 repeats did not bind calcium. Inactive hemolysin, unmodified by HlyC, contained the repeated sequences and bound calcium as efficiently as the active, full-length toxin. The binding of the inactive toxins to erythrocytes was investigated by immunoblotting saline-washed, toxin-treated cells with monoclonal antibodies after sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation of membrane proteins. The binding of full-length, active hemolysin to erythrocytes was calcium dependent. Inactive hemolysin deleted of the repeat units did not bind to cells. The inactive hemolysin, unmodified by HlyC, bound calcium but did not bind to erythrocytes. These results highlight the importance of calcium in the binding of hemolysin to erythrocytes and suggest that the binding of hemolysin to cells requires an interaction between the calcium-binding repeat domain and the modification produced by the HlyC protein.