Using ISG15 to Study the Immune System

Researchers have developed a method to identify and study proteins tagged with the chemical tag ISG15. The new method will allow scientists to unravel ISG15’s many functions in fighting disease, potentially leading to novel antimicrobial drugs. The work appeared yesterday in Nature Communications.

In order to regulate protein activity, the cell can attach chemical tags to proteins. For example, one well-known chemical tag is a small protein called ubiquitin. First discovered as a label to tag a protein for degradation, ubiquitin is now known to have various functions. Another chemical tag called ISG15 can also be used to modify target proteins; however, the molecular function of ISG15 is elusive since the identity of the modified proteins and their exact sites of modification are still unknown.

“ISG15 and ubiquitin share the same amino acid sequence at their end, exactly where these modifiers are attached to target proteins,” says corresponding author Francis Impens of VIB. “As a result, the peptides derived from the proteins modified by ISG15 display the same tag as peptides derived from proteins modified by ubiquitin. So, we took advantage of the technology developed to identify ubiquitin modification sites for the identification of ISG15 modification sites.”

Unlike ubiquitin, ISG15 is absent under normal conditions. It’s only expressed upon stresses such as a viral or bacterial infection. Thus, the team had to supplement their existing approach with an infection model. The researchers infected mice with Listeria and then analyzed their livers for ISG15 with the tools developed to study ubiquitin modification sites. They discovered nearly a thousand ISG15 sites on more than four hundred protein targets during bacterial infection.

“We found that ISG15 targets numerous enzymes involved in metabolic processes, but also that it targets key regulators of autophagy, a process in response to a lack of nutrients inside a cell,” says senior author Lilliana Radoshevich of the University of Iowa. “It leads to the destruction of cellular components to generate new sources of energy and promote cell survival. Alternatively, autophagy can be used as an antibacterial strategy. Our finding that ISG15 modulates this process is most exciting.”

This work revealed a new link between ISG15, cellular metabolism, and autophagy. The authors have already started to use their approach to investigate ISG15 targets during infection with other pathogens such as Influenza virus or Coxsackie virus. Together, these studies may reveal antimicrobial pathways of our immune system that can be exploited to design new drugs.