A Receptor-based Switch that Regulates Anthrax Toxin Pore Formation

Abstract

Cellular receptors can act as molecular switches, regulating the sensitivity of microbial proteins to conformational changes that promote cellular entry. The activities of these receptor-based switches are only partially understood. In this paper, we sought to understand the mechanism that underlies the activity of the ANTXR2 anthrax toxin receptor-based switch that binds to domains 2 and 4 of the protective antigen (PA) toxin subunit. Receptor-binding restricts structural changes within the heptameric PA prepore that are required for pore conversion to an acidic endosomal compartment. The transfer cross-saturation (TCS) NMR approach was used to monitor changes in the heptameric PA-receptor contacts at different steps during prepore-to-pore conversion. These studies demonstrated that receptor contact with PA domain 2 is weakened prior to pore conversion, defining a novel intermediate in this pathway. Importantly, ANTXR2 remained bound to PA domain 4 following pore conversion, suggesting that the bound receptor might influence the structure and/or function of the newly formed pore. These studies provide new insights into the function of a receptor-based molecular switch that controls anthrax toxin entry into cells.

Anthrax toxin protective antigen integrates poly-γ- d -glutamate and pH signals to sense the optimal environment for channel formation

“…The PA prechannel-to-channel transition (5) is cooperative, occurring at pH 7; however, when PA prechannels are bound to ANTXR2, the required pH threshold for channel formation is shifted to pH 5-6 (4, 15). Recent studies suggest PA-ANTXR2 interactions begin to dissociate at pH 6 (18). Histidine protonation within PA (15) and at H121 in the ANTXR2 domain (4) may trigger channel formation, but 2-fluorohistidine (19,20) and mutagenesis (21) studies suggest histidine protonation may be important for dissociation from ANTXR2 but not for channel formation in solution.…”

Interaction of the Clostridium difficile Binary Toxin CDT and Its Host Cell Receptor, Lipolysis-stimulated Lipoprotein Receptor (LSR)

“…However, it has been shown that the Ig-like domain of CMG2 is essential for proper functioning of the receptor-bound protective antigen pore (40). Furthermore, the crystal structure of the protective antigen-CMG2 complex (41), as well as a study from Pilpa et al (42), confirmed the involvement of the CMG2 receptor in protective antigen pore formation. Thus, it remains to be studied whether the Ig-like domain of LSR participates in the pore formation of the B component of CDT.…”

Inhibitors of pore-forming toxins

“…The formation of the prepore induces three [32] or four [31] binding sites for LF, EF, or both at the interface of two adjacent PA 63 molecules. The formation of the oligomeric prepores also generates a receptor-mediated signaling pathway that triggers endocytosis of the oligomeric anthrax toxin complexes [33]. After endocytosis of the complexes occur, the acidic environment of the endosomes causes substantial structural changes of the oligomeric PA 63 prepore.…”