Wiley, European Journal of Biochemistry, 3(268), p. 800-808, 2001
DOI: 10.1046/j.1432-1327.2001.01937.x
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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.