Participation of residue F-552 in domain III of the protective antigen in the biological activity of anthrax lethal toxin

The protective antigen (PA) component of anthrax toxin translocates the cat alytic moieties lethal factor (LF) and edema factor (EF) into the cytosol. The proteolytically activated 63 kDa form of PA (PA63) has the ability to o ligomerize and bind LF/EF. PA has four distinct domains performing speciali zed functions; whereas the function of domains I, II and IV has been well c haracterized, domain III has no known role in the biological activity of PA . Here we report the role of amino acid residues lining an exposed hydropho bic patch of domain III in the biological activity of PA. The residues Phe( 552), Phe(554), Ile(562), Leu(566) and Ile(574) were individually substitut ed with alanine and the effect was studied. All mutant PA proteins except P he552Ala were equally active as wild-type PA in exhibiting a toxic phenotyp e to J774A.1 cells in the presence of LF. Substitution of Ala for Phe(552) reduced the ability of PA to intoxicate cells by more than 250-fold. Howeve r, Phe552Ala was equally active in receptor binding and susceptibility to t rypsin and chymotrypsin as wild-type PA, the activities that have been show n to be essential for the biological activity of PA. This mutated PA protei n had a decreased ability to bind LF, oligomerize on cells and to induce re lease of Rb-86(+) from Chinese hamster ovary cells. These results suggest t hat the residue Phe(552) in PA plays an important role in LF binding and ol igomerization. Our study provides a basis for further exploration of the bi ological significance of domain III of PA.