Proteolytic activation of receptor-bound anthrax protective antigen on macrophages promotes its internalization

Immunofluorescence and other methods have been used to probe the self-assembly and internalization of the binary toxin, anthrax lethal toxin (LeTx), in primary murine macrophages. Proteolytic activation of protective antigen (PA; 83 kDa, the B moiety of the toxin) by furin was the rate-limiting step in internalization of LeTx and promoted clearance of PA from the cell surface. A furin-resistant form of PA remained at the cell surface for at least 90 min. Oligomerization of receptor-bound PA63, the 63 kDa active fragment of PA, was manifested by its conversion to a pronase-resistant state, characteristic of the heptameric prepore form in solution. That oligomerization of PA63 triggers toxin internalization is supported by the observation that PA20, the complementary 20 kDa fragment of PA, inhibited clearance of nicked PA. The PA63 prepore, with or without lethal factor (LF), cleared slowly from the cell surface. These studies show that proteolytic cleavage of PA, in addition to permitting oligomerization and LF binding, also promotes internalization of the protein. The relatively long period of activation and internalization of PA at the cell surface may reflect adaptation of this binary toxin that maximizes self-assembly.