Pertussislike toxins in E. coli

September 11, 2017 § Leave a comment

Cladogram of AB5 toxin sequences identified in genome sequenced E. coli strains available on the NCBI database.

Pertussislike toxins are pathologic proteins released by various bacteria during infection.  (Pertussis toxin itself has a well-established role in whooping cough.) The unique structures shared by pertussislike toxins enable them to recognize cells by binding to long carbohydrate chains, known as glycans, on the cell surface and then enter the cytoplasm.

Pertussislike toxins are less studied than pertussis itself, and their cellular mechanisms remain relatively unknown, but a recent report in the Journal of Biological Chemistry elucidates the structures and functions of a subset of them.

“Bacteria that are able to cause disease are often only able to make us sick because they produce specialized proteins that misdirect or shut down parts of our immune system,” explained Dene R. Littler at Monash University, the lead author on the JBC report. Pertussis and pertussislike toxins are among these specialized proteins. They modify G proteins, critical signaling molecules that, as Littler put it, “transduce messages to immune cells, allowing them to respond to signs of infection.”

In the JBC report, Littler and colleagues describe using computational methods to zero in on pertussislike toxins in the genomes of various pathogenic E. coli strains.  They refer to the genes as E. coli pertussislike toxins, or EcPlt for short.

They next set out to determine the function of these genes. “While many of these toxins had been described as pertussislike, previously nobody really looked to see if they function in the same way” as pertussis itself, Littler said.  The researchers, he added, were “interested in determining what had been conserved amongst these related toxins and what had changed.”

They used cell-proliferation assays to determine the cytotoxicity of EcPlt. The researchers observed that EcPlt treatment of human embryonic kidney (HEK293T) or African green monkey kidney epithelial cells (Vero) halted cell proliferation.  EcPlt also bound to glycans on the cell surface.

They determined the protein targets of EcPlt by using an ADP-ribosylation assay. ADP-ribose is a large, bulky chemical group used by many bacterial toxins to modify and disrupt host signaling proteins, including G proteins. “This creates a large protrusion on the interacting interface and prevents receptor coupling,” Littler said. The researchers found that EcPlt targets the G proteins known as Gαi/o for ADP-ribosylation to interfere with their proper signaling. Interestingly, this modification occurs at lysine and asparagine residues in Gαi/o, as opposed to the typical cysteine residue targeted by other pertussislike toxins.

Finally, the researchers investigated the structure of EcPlt in fine detail using X-ray crystallography. They report that the toxin resembles a “blunted pyramid” in which a single A subunit sits atop five copies of a B subunit. This structure is typical of so-called AB5 toxins expressed by several bacterial pathogens. These toxins are oxidized and inactive outside the cell, but they become reduced and active upon entering the cytoplasm. According to Littler, “this form of the toxin is more dynamic and is optimized for rapid enzymatic modification of human proteins.”

Littler said he hopes the team’s “active toxin structures help identify residues that are essential for activity of pertussislike toxins and help define ways to produce inactive recombinant versions.” This would facilitate vaccine production.

Additionally, he said knowing the EcPlt structure “will help with the development of small-molecule inhibitors that may aid people whose immune systems struggle to curtail infection.”

This post was written by Stefan Lukianov, a Ph.D. candidate at Harvard Medical School and contributor to ASBMB Today.

 

 

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