Taking a Raft to the Cell Interior

Abstract

Abrami et al. investigated the mechanism that leads to internalization of anthrax toxin and discovered that oligomerization-driven anthrax toxin receptor (ATR) clustering leads to its association with lipid rafts, cholesterol- and sphingolipid-rich microdomains of the plasma membrane, and promotes toxin internalization. Anthrax toxin consists of three subunits: edema factor (EF), a calmodulin-dependent adenylate cyclase; lethal factor (LF), which targets mitogen-activated protein kinase kinases (MAPKKs); and protective antigen (PA), which enables EF and LF to gain access to the cell interior. PA is an 83-kD protein (PA83) that binds to the ATR and is then cleaved by cell surface proteases to a 63-kD form (PA63) that forms a ring-shaped heptamer. EF and LF associate with heptamerized PA63, and then the entire toxin-receptor complex is endocytosed. After endocytosis, the PA63 heptamer forms a channel, and EF and LF enter the cytoplasm. The authors used Western analysis to show that PA63, but not PA83 or an unbound epitope-tagged version of the ATR, was associated with a lipid raft fraction from the membranes of CHO cells. Clustering PA83-bound ATR through antibody cross-linking promoted its association with lipid rafts and rapid internalization of a mutant PA that cannot be cleaved and normally remains at the cell surface. Depleting the cells of cholesterol inhibited intracellular accumulation of P63, channel formation, and LF-dependent cleavage of a MAPKK. Internalization did not depend on caveolin, but occurred through a clathrin-mediated pathway. The authors describe how the physiology of the ATR--slow internalization of the unbound or monomerically bound form but rapid internalization of the clustered form--provides an ideal target for anthrax toxin. They suggest that lipid rafts may provide a therapeutic target against anthrax. L. Abrami, S. Liu, P. Cosson, S. H. Leppla, F. G. van der Goot, Anthrax toxin triggers endocytosis of its receptor via a lipid raft-mediated clathrin-dependent process. J. Cell Biol . 160 , 321-328 (2003). [Abstract] [Full Text] T. Kurzchalia, Anthrax toxin rafts into cells. J. Cell Biol. 160 , 295-296 (2003). [Abstract] [Full Text]