The adenylyl cyclase activity of anthrax edema factor

Abstract

Bacillus anthracis, the etiologic agent for anthrax, secretes edema factor (EF) to disrupt intracellular signaling pathways. Upon translocation into host cells and association with a calcium sensor, calmodulin (CaM), EF becomes a highly active adenylyl cyclase (AC) that raises the intracellular concentration of cyclic AMP (cAMP). Growing evidence shows that EF plays a key role in anthrax pathogenesis by affecting cellular functions vital for host defense. This strategy is also used by Bordetella pertussis, a bacterium that causes whooping cough. Pertussis bacteria secrete the bifunctional toxin CyaA which raises the intracellular cAMP. Here, we discuss recent advances from structural analyses that reveal the molecular basis of the conserved mechanism of activation and catalysis of EF and CyaA by CaM even though these two toxins use the completely different sequences to bind CaM. Comparison of the biochemical and structural characteristics of these two AC toxins with host ACs reveal that they have diverse strategies of catalytic activation, yet use the same two-metal-ion catalytic mechanism. Keywordsanthrax edema factor; pertussis CyaA; class III adenylyl cyclase Edema factor (EF) is an 89-kDa protein secreted by Bacillus anthracis, the Gram-positive bacterium that cause anthrax. EF belongs to a family of bacterial toxins that can specifically elevate the intracellular cyclic AMP (cAMP) level, which is the prototypic second messenger that regulates diverse cellular responses. The biological effects of cAMP are mediated by the binding of cAMP to three families of signal transducers: cAMP-dependent protein kinases, cyclic nucleotide gated channels (Biel and Michalakis 2009), and EPAC, the guanine nucleotide exchange factor for Ras GTPase homologs Rap1 and Rap2 (Borland, Smith, and Yarwood 2009). There are two major mechanisms by which bacterial toxins can raise the intracellular cAMP level. The first mechanism is by the action of the bacterial adenylyl cyclase (AC) toxins, which possess AC activity, the enzymatic activity that converts ATP to cAMP. Their AC activities are activated only upon their entrance into host cells and association with the specific cellular proteins that serve as the activator. In addition to the aforementioned EF from Bacillus anthracis, two other examples of AC toxins include CyaA from Bordetella pertussis and ExoY from Pseudomonas aeruginosa (Leppla 1984;Wolff et al. 1980;Yahr et al. 1998). EF and CyaA share CaM as a common activator while the activator of ExoY is still unknown. The second mechanism by which intracellular cAMP is increased by bacteria is the ADP-ribosylation of heterotrimeric G proteins by bacterial toxins, resulting in increased catalytic activity of host membrane-bound AC (mAC). Cholera toxin from Vibrio cholera and labile toxin from Escherichia coli can ADP-ribosylate Gsα, rendering it constitutively active to stimulate host mAC. History of EF discovery and recent advancesEF was identified based on its ability to induce cutaneous edema and mortality of experi...

Differential Contribution of Bacillus anthracis Toxins to Pathogenicity in Two Animal Models

“…LF, a zinc metalloprotease, cleaves most isoforms of mitogen-activated protein kinase kinases, disrupting normal cellular signaling pathways in immune cells (39). EF, a calmodulin-dependent adenylate cyclase, elevates the intracellular cyclic AMP (cAMP) concentration, interfering with cellular signaling and membrane permeability regulation (38). B. anthracis binary toxins are thus termed lethal toxin (LT) (LF plus PA) and edema toxin (ET) (EF plus PA).…”

A non-glycosylated, plant-produced human monoclonal antibody against anthrax protective antigen protects mice and non-human primates fromB. anthracisspore challenge

“…1 Pathogenesis and molecular biology of B. anthracis have been recently reviewed in several articles. [2][3][4][5][6] In brief, the disease process is initiated in the host when the invading spores are phagocytosed by macrophages, wherein the spores germinate into vegetative bacteria and secrete a tripartite toxin. This exotoxin consists of protective antigen (PA) and two enzymatically active proteins: lethal factor (LF) and edema factor (EF).…”

Inhibition of the adenylyl cyclase toxin, edema factor, from Bacillus anthracis by a series of 18 mono- and bis-(M)ANT-substituted nucleoside 5′-triphosphates

“…As an AC, EF raises the concentration of the second messenger cyclic AMP (cAMP) inside host cells to supraphysiological levels (Tang and Guo 2009). EF is a key virulence factor for anthrax pathogenesis.…”

“…An inactivating mutation in EF results in reduced survival of germinated anthrax spores in macrophages (Guidi-Rontani et al 2001), and a strain of anthrax with a defective EF gene has 100-fold reduced lethality in mice (Brossier et al 2000). EF enters host cells via a complex with protective antigen, which is a pH-dependent protein transporter (Tang and Guo 2009). The combination of toxaemia caused by anthrax toxins and bacteremia due to the rapid growth of anthrax bacteria in vital organs can result in sepsis, pulmonary oedema and/or meningitis within few days.…”