Patrick: Anthrax Toxin: Biological Function

Anthrax Lethal Toxin, Rate My Science

Overview of MAPK cascades in mammals. Anthrax lethal factor cleaves and disables all MKK's but not MKK5. Image courtesy of http://en.wikipedia.org/wiki/File:MAPK-pathway-mammalian.png

Overview of the anthrax toxin mechanism and entry of lethal factor into cells. Am J Respir Crit Care Med. 2011 December 15; 184(12): 1333–1341.

Anthrax lethal factor is one of three distinct proteins secreted by Bacillus anthracis. This protein is responsible for the disruption of MAPKK proteins. Lethal factor disrupts and cleaves all mammalian MEK's and MKK's with exception to MKK5. Even with todays technology it is unclear the specifics behind this inactivating mechanism. Below is what we currently know about lethal factor's biological function and it's mechanism. 

Substrate Binding and Cleavage 

Lethal factor is a highly adapted protein that makes specific contacts with MAPKK's which is important in stabilization of the substrate. The binding grove is overall acidic in nature, containing glutamic and aspartic acids that interacts with slightly basic MAPKK's N-terminus. Interactions at Glu333, Glu334, and Asp328 on the 3rd domain are thought to be the primary anchor site for MAPKK's.

Furthermore, compared to a clothespin, the substrate binding groove is open on both ends to complement the tails of longer substrates. Once substrates dock, the LF complex undergoes a slight conformational change prior to substrate cleavage. Therefore, substrate binding is not only specific to corresponding MAPKK lysine or arginine side chains, it is structurally based. The MAPKK's must fit snug into the catalytic grove in order for conformational change and cleavage to occur. 

Once docked, lethal factor cleaves MAPK's near their n-terminal domain effectively inactivating their docking sequences and consequently disrupting downstream signaling. Since MAPKK's are key factors in cell proliferation and regulation through transcription factor activation, their destruction by lethal factor is detrimental to the host.

Immune Response

Once the spores germinate inside their respective hosts and are taken up by macrophages, the bacteria generate a polypeptide coat of poly-D-glutamate, which is nearly undetectable by future macrophages. The bacteria also have evolved to cleave p38 mitogen activated protein kinases. By disrupting p38 MAPKK, lethal factor successfully induces apoptosis of macrophages and other cells. Since p38 MAPK signaling plays a crucial role in cell apoptosis, growth/differentiation, inflammation and cell cycle arrest, it is extremely detrimental to host cells when destroyed by lethal factor. Thus, the bacteria has evolved to not only seek transport to lymph glands by macrophages, but also hide from immune attack (at least for a brief period of time) once in their desired location. 

Project Pages

1. Patrick: Anthrax Toxin: Introduction 
2. Patrick: Anthrax Toxin: Biological Function
3. Patrick: Anthrax Toxin: Biosynthesis
4. Patrick: Anthrax Toxin: Gene Sequence
5. Patrick: Anthrax Toxin: Amino Acid Sequence and Composition
6. Patrick: Anthrax Toxin: Secondary and Tertiary Structure
7. Patrick: Anthrax Toxin: Domains and Structural Motifs
8. Patrick: Anthrax Toxin: Interactions with macromolecules and small molecules
9. Patrick: Anthrax Toxin: Molecular biodiversity and evolution
10. Patrick: Anthrax Toxin: PyMOL Images
11. Patrick: Anthrax Toxin: Literature Review
12. Patrick: Anthrax Toxin: Useful online resources

References 

Dalkas, G., Papakyriakou, A., Vlamis-Gardikas, A., Spyroulias, G. 2009. Insights into the anthrax lethal factor–substrate interaction and selectivity using docking and molecular dynamics simulations. Protein Science. 18 (8):1774-1785 

Pannifer AD et al. 2001. Crystal Structure of the Anthrax Lethal Factor .Nature. 414 (6860): 229-33. 

Raza, S., Timmer, Anjuli., Bilgrami, S., Park, E., Eckmann, L., Nizet, V., Karin, M. 2011. Anthrax toxin induced macrophage death by p38 MAPK inhibition but leads to inflammasome activation via ATP leakage.Immuity. 35(1): 34-44. 

Vitale, G., Bernardi, L., Napolitani, G., Mock, M., Montecucco, C. 2000. Susceptibility of mitogen-activated protein kinase kinase family members to proteolysis by anthrax lethal factor. Biochem. 352: 739-745.