This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Bacillus anthracis, the cause of anthrax, is one of the most lethal bacteria (www.ks.uiuc.edu/Research/anthrax/). The bacterium attacks the cells of the host s immune system, the so-called macrophages, as well as many tissue cells. For this purpose the anthrax bacterium releases three types of proteins, or toxins, into the blood stream of the host: protective antigen (PA), lethal factor (LF), and edema factor (EF) [56, 57]. To invade a host cell, the toxins take an intricate entry route that involves binding to cellular receptors and inducing the cell to internalize the toxins in a bubble like endosome. The bubble membrane wall is then punctured by PAs forming a pore upon an acidifying trigger from the host, and finally the LFs and EFs slip into the cell through the pore. Although the general picture of the anthrax intoxication pathway has been established, many details still remain a mystery. One key question is how the PA-receptor complex initiates formation of the pore for ferrying the toxic cargo. Recently, a high resolution crystal structure of PA in complex with its cellular receptor capillary morphogenesis gene 2 (CMG2) resolved has become available [58], providing an opportunity to study the pore-formation mechanism with MD simulations. The project required extremely extensive (136ns from 92,000 atom system) MD simulations that became feasible only through optimizing the Resource program NAMD for multi-processor (128) compuation. We have performed simulations of the PA-CMG2 complex with NAMD to study how acidic conditions in the endosome trigger conformational changes of the PA complex necessary for pore formation. We also carried out SMD simulations to understand the role of interactions between PA and CMG2 [59]. Simulations revealed that under neutral pH conditions PA orients one of its loops so that a positively charge amino acid (Arg344PA) interacts optimally with a negatively charged amino acid (Glu122CMG2) on the receptor. The interaction between the pair, a so-called salt bridge, prevents the toxin from dissociation. Low pH conditions induce the protonation of two amino acids involving or neighboring the salt bridge (His121CMG2 and Glu122CMG2), which triggers rapid dissociation of PA from the receptor for the pore formation purpose. The amino acids, His121 and Glu122 of CMG2, and Arg344 of PA, appear to serve as a pH-sensitive switch that controls the pore formation.
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