6-bromoacetyl-2-dimethylaminonaphthalene and Hemolysis

Escherichia coli hemolysin (HlyA) is a membrane-permeabilizing protein belonging to the family of RTX-toxins. Lytic activity depends on binding of Ca2(+) to the C-terminus of the molecule. The N-terminus of HlyA harbors hydrophobic sequences that are believed to constitute the membrane-inserting domain. In this study, 13 HlyA cysteine-replacement mutants were constructed and labeled with the polarity-sensitive fluorescent probe 6-bromoacetyl-2-dimethylaminonaphthalene (badan). The fluorescence emission of the label was examined in soluble and membrane-bound toxin. Binding effected a major blue shift in the emission of six residues within the N-terminal hydrophobic domain, indicating insertion of this domain into the lipid bilayer. The emission shifts occurred both in the presence and absence of Ca2(+), suggesting that Ca2(+) is not required for the toxin to enter membranes. However, binding of Ca2(+) to HlyA in solution effected conformational changes in both the C-terminal and N-terminal domain that paralleled activation. Our data indicate that binding of Ca2(+) to the toxin in solution effects a conformational change that is relayed to the N-terminal domain, rendering it capable of adopting the structure of a functional pore upon membrane binding.

Topics: 2-Naphthylamine; Bacterial Proteins; Calcium; Cell Membrane; Chromatography, Gel; Cloning, Molecular; Cysteine; Electrophoresis, Polyacrylamide Gel; Erythrocyte Membrane; Escherichia coli; Escherichia coli Proteins; Fluorescent Dyes; Hemolysin Proteins; Hemolysis; Liposomes; Mutagenesis; Protein Binding; Protein Conformation; Protein Structure, Tertiary; Spectrometry, Fluorescence