A computer model shows side and top views of two different proteins produced by anthrax bacteria. The green molecule is "protective antigen" (PA), which spontaneously forms pores that penetrate organic membranes such as cell walls. The yellow molecule is "lethal factor (LF)." When a voltage is applied across a membrane studded with PA pores, both positive and negative ions flow through. Once LF binds to the pore, however, current only flows in one direction.
Credit: T. Nguyen, National Cancer Institute
Related reading: http://www.nist.gov/public_affairs/techbeat/tb2005_0826.htm#anthrax