The influence of pH and chemical denaturants on metal binding and structure of anthrax lethal factor

Abstract

Bacillus anthracis is the causative agent of anthrax, and secretes a three-component toxin consisting of protective antigen (PA), edema factor (EF), and lethal factor (LF). LF is a zinc-dependent metallopeptidase responsible for cleaving mitogen-activated protein kinase kinases in the cytosol. To reach the cytosol, LF must be unfolded to pass through the narrow lumen of the PA channel embedded in the endosomal membrane. Whether the metal-binding motif in LF remains intact, and the Zn2+ ion is co-translocated along with the protein is currently unknown. Using a combination of intrinsic tryptophan fluorescence spectroscopy and chelator studies to probe the unfolding and metal status of LF, this study shows that acidification from pH 7 to pH 5 leads a marked destabilization of LF’s native fold, and a significantly increased degree of Zn2+ accessibility (to chelation) and release. Furthermore, red-edge excitation shift studies show that LF still retains a partial fold even when exposed to low pH and high concentrations of denaturants. Taken together, these results provide insights into the structure and metal status of LF under conditions similar to those encountered during translocation in vivo, and they suggest that LF’s Zn2+ ion is likely lost during PA-mediated translocation.