Anthrax lethal toxin is a typical A-B type
protein toxin secreted by Bacillus anthracis. Lethal factor (LF) is the catalytic A-subunit, a
metalloprotease having
MEKs as targets. LF relies on the cell-binding B-subunit, protective
antigen (PA), to gain entry into the cytosol of target cells. PA binds to cell surface toxin receptors and is activated by
furin protease to form an LF-binding-competent oligomer-PA pre-pore, which converts to a functional
protein-conductive pore in the acidic endocytic vesicles, allowing translocation of LF into the cytosol. During PA pre-pore-to-pore conversion, the intermolecular
salt bridge interactions between Lys397 and Asp426 on adjacent PA
protomers play a critical role in positioning neighboring
luminal Phe427 residues to form the Phe-clamp, an essential
element of the PA functional pore. This essential intermolecular interaction affords the opportunity to create pairs of PA variants that depend on intermolecular complementation to form a functional pore. We have previously generated PA variants with
furin-cleavage site replaced by substrate sequences of
tumor-associated
proteases, such as
urokinase or
MMPs. Here we show that PA-U2-K397Q, a
urokinase-activated PA variant with Lys397 residue replaced by
glutamine, and PA-L1-D426K, a
MMP-activated PA variant with Asp426 changed to
lysine, do not form functional pores both in vitro or in vivo unless they are used together. Further, the mixture of PA-U2-K397Q and PA-L1-D426K displayed potent anti-
tumor activity in the presence of LF. Thus, PA-U2-K397Q and PA-L1-D426K form a novel intermolecular complementation system with toxin activation relying on the presence of two distinct
tumor-associated
proteases, i.e.,
urokinase and
MMPs.