Edema factor (EF), a key anthrax exotoxin, has an anthrax protective antigen-binding domain (PABD) and a calmodulin (CaM)-activated adenylyl cyclase domain. Here, we report the crystal structures of CaM-bound EF, revealing the architecture of EF PABD. CaM has N- and C-terminal domains and each domain can bind two calcium ions. Calcium binding induces the conformational change of CaM from closed to open. Structures of the EF-CaM complex show how EF locks the N-terminal domain of CaM into a closed conformation regardless of its calcium-loading state. This represents a mechanism of how CaM effector alters the calcium affinity of CaM and uncouples the conformational change of CaM from calcium loading. Furthermore, structures of EF-CaM complexed with nucleotides show that EF uses two-metal-ion catalysis, a prevalent mechanism in DNA and RNA polymerases. A histidine (H351) further facilitates the catalysis of EF by activating a water to deprotonate 3'OH of ATP. Mammalian adenylyl cyclases share no structural similarity with EF and they also use two-metal-ion catalysis, suggesting the catalytic mechanism-driven convergent evolution of two structurally diverse adenylyl cyclases.

Edema factor (EF), a key virulence factor in anthrax pathogenesis, has calmodulin (CaM)-activated adenylyl cyclase activity. We have found that adefovir dipivoxil, a drug approved to treat chronic infection of hepatitis B virus, effectively inhibits EF-induced cAMP accumulation and changes in cytokine production in mouse primary macrophages. Adefovir diphosphate (PMEApp), the active cellular metabolite of adefovir dipivoxil, inhibits the adenylyl cyclase activity of EF in vitro with high affinity (Ki ‫؍‬ 27 nM). A crystal structure of EF-CaM-PMEApp reveals that the catalytic site of EF forms better van der Waals contacts and more hydrogen bonds with PMEApp than with its endogenous substrate, ATP, providing an explanation for the Ϸ10,000-fold higher affinity EF-CaM has for PMEApp versus ATP. Adefovir dipivoxil is a clinically approved drug that can block the action of an anthrax toxin. It can be used to address the role of EF in anthrax pathogenesis.