Foot soldiers of the immune system
The discovery was made by teams led by Bhushan Nagar, a professor in the Department of Biochemistry at McGill’s Faculty of Medicine, and Dr. Giulio Superti-Furga at the CeMM. Building on the 2011 CeMM discovery by Dr. Andreas Pichlmair that IFIT proteins unexpectedly interact directly with the viral 㽶Ƶ to inhibit its replication, the group’s latest discovery reveals the molecular mechanism behind how IFIT proteins capture only the viral 㽶Ƶ and distinguishes it from normal molecules belonging to the host. Their research will be published on January 13 in the journal Nature.
“Infection by pathogens such as viruses and bacteria are caught by a layer of the immune system that consists of guard-like proteins constantly on the lookout for foreign molecules derived from the pathogen,” explains Prof. Nagar. “Once the pathogen is detected, a rapid response by the host cell is elicited, which includes the production of an array of defender molecules that work together to block and remove the infection. The IFIT proteins are key members of these defender molecules.”
When a virus enters a cell, it can generate foreign molecules such as 㽶Ƶ with three phosphate groups (triphosphate) exposed at one end, in order to replicate itself. Triphosphorylated 㽶Ƶ is what distinguishes viral 㽶Ƶ from the 㽶Ƶ found in the human host. During this time, the receptors of the innate immune system are usually able to detect the foreign molecules from the virus and turn on signaling cascades in the cell that leads to the switching on of an antiviral program, both within the infected and nearby uninfected cells. Hundreds of different proteins are produced as part of this anti-viral program, which work in concert to resist the viral infection.
In the Nagar lab, McGill graduate student Yazan Abbas used an arsenal of biophysical techniques, most notably X-ray crystallography, to capture the IFIT protein directly in the act of recognizing the foreign 㽶Ƶ. The work shed light on the interaction between IFITs and 㽶Ƶs. The researchers determined that IFIT proteins have evolved a specific binding pocket, chemically compatible and big enough to fit only the triphoshorylated end of the viral 㽶Ƶ. Human 㽶Ƶ is not able to tightly interact with this pocket, thereby circumventing autoimmune reactions.
“Once the IFIT protein clamps down on the viral 㽶Ƶ, the 㽶Ƶ is then presumably prevented from being used by the virus for its own replication,” says Superti-Furga, “Since many viruses, such as influenza and rabies, rely on triphosphate 㽶Ƶ for their lifecycle, these results have widespread implications in understanding how our cells interact with viruses and combat them.”
This work could help advance the development of new drugs for combating a wide array of immune system disorders. “Our findings will be useful for the development of novel drugs directed at IFIT proteins, particularly in cases where it is necessary to dampen the immune response, such as inflammation or cancer therapy,” says Nagar.