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Ribbon diagram of a heptameric, 63 kDa cleavage fragment (PA₆₃) of the protective antigen (PA) protein, produced by Bacillus anthracis as part of the anthrax toxin. The quaternary structure is that of a pre-pore, which later forms a channel that allows other toxins into the cytosol of the target cell, so the pathogen can cause damage to the host.

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