Role of the α Clamp in the Protein Translocation Mechanism of Anthrax Toxin.
Membrane-embedded molecular machines are utilized to move water-soluble proteins across these barriers. Anthrax toxin forms one such machine through the self-assembly of its three component proteins--protective antigen (PA), lethal factor, and edema factor. Upon endocytosis into host cells, acidification of the endosome induces PA to form a membrane-inserted channel, which unfolds lethal factor and edema factor and translocates them into the host cytosol. Translocation is driven by the proton motive force, composed of the chemical potential, the proton gradient (ΔpH), and the membrane potential (Δψ). A crystal structure of the lethal toxin core complex revealed an "α clamp" structure that binds to substrate helices nonspecifically. Here, we test the hypothesis that, through the recognition of unfolding helical structure, the α clamp can accelerate the rate of translocation. We produced a synthetic PA mutant in which an α helix was crosslinked into the α clamp to block its function. This synthetic construct impairs translocation by raising a yet uncharacterized translocation barrier shown to be much less force dependent than the known unfolding barrier. We also report that the α clamp more stably binds substrates that can form helices than those, such as polyproline, that cannot. Hence, the α clamp recognizes substrates by a general shape-complementarity mechanism. Substrates that are incapable of forming compact secondary structure (due to the introduction of a polyproline track) are severely deficient for translocation. Therefore, the α clamp and its recognition of helical structure in the translocating substrate play key roles in the molecular mechanism of protein translocation.
Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
Michael J
MJ
Brown
Department of Chemistry, University of California, Berkeley, CA 94720, USA.
Katie L
KL
Thoren
Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA; Department of Chemistry, University of California, Berkeley, CA 94720, USA; Department of Microbial Pathogenesis, School of Dentistry, University of Maryland, Baltimore, 650 West Baltimore Street, Baltimore, MD 21201, USA. Electronic address: bkrantz@umaryland.edu.
Bryan A
BA
Krantz
D000942:Antigens, Bacterial / Q000378:metabolism* / Q000648:ultrastructure*; D001408:Bacillus anthracis / Q000378:metabolism; D001427:Bacterial Toxins / Q000378:metabolism*; D001665:Binding Sites; D002462:Cell Membrane / Q000378:metabolism; D020285:Cryoelectron Microscopy; D018360:Crystallography, X-Ray; D004705:Endocytosis; D008564:Membrane Potentials; D021381:Protein Transport / Q000502:physiology*; D018892:Proton-Motive Force
D000942:Antigens, Bacterial; D001427:Bacterial Toxins; C030325:anthrax toxin
Bacillus anthracis; anthrax toxin; electrophysiology; protective antigen; protein engineering