Residues 1-254 of anthrax toxin lethal factor are sufficient to cause cellular uptake of fused polypeptides.

Anthrax lethal toxin is a complex of protective antigen (PA, 735 amino acids) and lethal factor (LF, 776 amino acids) that lyses certain eukaryotic cells. LF interacts with PA to gain access to the cytosol to assert its toxicity. The internalization of LF requires that PA bind to a specific membrane receptor and be cleaved by a cell-surface protease (probably furin), so as to expose a site on PA to which LF binds with high affinity. To localize LF functional domains, amino, carboxyl, and internal deletions of LF were made. Toxicity was eliminated by deletion of 40 and 47 residues from the amino and carboxyl termini, respectively. Similarly, deleting the first of the four imperfect repeats of 19 amino acids located at residues 308-383 made LF non-toxic, showing that this region is also essential for activity. To identify the minimum region of LF which is required for binding to PA, varying amino-terminal portions of LF were fused to the ADP-ribosylation domain of Pseudomonas exotoxin A. Fusion proteins containing residues 1-254 of LF were toxic when administered with PA, while those having only residues 1-198 of LF were inactive, showing that the PA-binding domain of LF lies within residues 1-254.