Effect of pH on the catalytic function and zinc content of native and immobilized anthrax lethal factor

Translocation of the zinc-dependent metalloendopeptidase anthrax lethal factor (LF) from the endosome to the cytosol requires an acidic endosomal milieu. In the current study, we utilized immobilized (to prevent protein aggregation below pH 5.5) and native LF to assess the effect of pH on the function and metal content of LF. Our results reveal the diminution of LF's catalytic competence under moderately acidic conditions (pH ∼6) to be uncorrelated to the metal content of the protein. However, a significant degree of demetallation of LF (∼30%) was observed at pH values close to those found in late endosomes (pH ∼5), thus raising the possibility that a substantial proportion of LF molecules may not be in their zinc-bound state prior to translocation.