Abstract
Protective antigen (PA) from anthrax toxin assembles into a homoheptamer on cell surfaces and forms complexes with the enzymatic components: lethal factor (LF) and edema factor (EF). Endocytic vesicles containing these complexes are acidified, causing the heptamer to transform into a transmembrane pore that chaperones the passage of unfolded LF and EF into the cytosol. We show in planar lipid bilayers that a physiologically relevant proton gradient (ΔpH, where the endosome is acidified relative to the cytosol) is a potent driving force for translocation of LF, EF and the LF amino-terminal domain (LF N) through the PA63 pore. ΔpH-driven translocation occurs even under a negligible membrane potential. We found that acidic endosomal conditions known to destabilize LFN correlate with an increased translocation rate. The hydrophobic heptad of lumen-facing Phe427 residues in PA (or φ clamp) drives translocation synergistically under a ΔpH. We propose that a Brownian ratchet mechanism proposed earlier for the φ clamp is cooperatively linked to a protonation-state, ΔpH-driven ratchet acting trans to the φ-clamp site. In a sense, the channel functions as a proton/protein symporter.
Original language | English (US) |
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Pages (from-to) | 968-979 |
Number of pages | 12 |
Journal | Journal of Molecular Biology |
Volume | 355 |
Issue number | 5 |
DOIs | |
State | Published - Feb 3 2006 |
Keywords
- Planar bilayers
- Protein unfolding
- Proton gradient
- Translocase
- Translocation
ASJC Scopus subject areas
- Structural Biology
- Molecular Biology