When a virus invades, our cells don't just surrender. They sound an alarm, unleashing a powerful defense program orchestrated by molecules called interferons. At the heart of this rapid response is a small but mighty protein, Interferon-Stimulated Gene 15, or ISG15. First spotted in the late 1970s as one of the most dramatically upregulated proteins during an interferon alert, ISG15 was initially typecast as a straightforward antiviral soldier [1]. But as scientists dug deeper, they uncovered a story of remarkable complexity. This 165-amino-acid protein is not just a soldier; it’s a master regulator, a signaling molecule, and sometimes, a player with a mysterious dual allegiance in the cellular theatre of health and disease.
So, how does this tiny protein pack such a punch? ISG15 (UniProt ID: P05161) operates with the versatility of a Swiss Army knife, employing two distinct strategies: one where it attaches to other proteins, and another where it acts alone.
The primary mechanism is a process called ISGylation. Think of it as a specialized molecular tagging system. While its more famous cousin, ubiquitination, often tags proteins for destruction, ISGylation is more like attaching a mission-critical directive. This process unfolds in a precise three-step enzymatic cascade: an E1 enzyme (UBE1L) first activates ISG15, an E2 enzyme (UBCH8) then carries it, and finally, an E3 ligase like HERC5 attaches it to a target protein [2]. This "tag" can alter the target's function, stability, or location, with hundreds of cellular and viral proteins identified as substrates. The system is also dynamic; enzymes like USP18 can swiftly remove the ISG15 tag, allowing the cell to fine-tune its response in real-time [2].
But ISG15 doesn't always need to be attached to make a difference. In its free, unconjugated form, it can be secreted from the cell and act as a cytokine—a signaling molecule that rallies the troops. By binding to the LFA-1 receptor on immune cells like Natural Killer (NK) cells, free ISG15 directly boosts their activation, adding another layer to its role as an immune modulator [3]. This dual functionality as both a protein modifier and a signaling molecule makes ISG15 a uniquely powerful player in cellular communication.
On a grander scale, ISG15’s actions have profound consequences for the entire organism. Its role as a security detail is most evident in our defense against viruses. Studies using knockout mice lacking ISG15 revealed a dramatic increase in susceptibility to a wide range of viruses, from influenza to herpesviruses, cementing its status as a critical component of our innate immunity [6, 8]. By ISGylating viral proteins, it can directly inhibit their function or mark them for degradation. Simultaneously, it modifies host proteins to amplify the antiviral state, effectively turning the cell into a fortress [9].
However, the plot thickens when we look at cancer. Here, ISG15's role is paradoxically complex. In some contexts, it acts as a tumor suppressor, helping the immune system spot and eliminate malignant cells. In others, its expression is co-opted by cancer cells to promote their own survival and progression [10]. This duality highlights the delicate balance of immune regulation and underscores the "double agent" nature of this fascinating protein.
Perhaps its most crucial role is as an immune thermostat. While a strong interferon response is vital for fighting infection, an unchecked one can lead to chronic inflammation and autoimmunity. Groundbreaking research has shown that humans with a genetic deficiency in ISG15 suffer from severe autoinflammatory conditions known as type I interferonopathies [10]. This proves that ISG15 is not just an activator but also a critical negative regulator, essential for dialing down the immune response and maintaining homeostasis.
The deep understanding of ISG15 biology has paved the way for exciting therapeutic applications. Its broad-spectrum antiviral activity makes it a prime target for developing host-directed therapies. Instead of targeting a single viral protein that can easily mutate, boosting the ISG15 pathway could offer protection against a wide array of existing and emerging viruses—a vital strategy for pandemic preparedness [12].
In oncology, ISG15 is emerging as a powerful ally for immunotherapy. By modulating the tumor microenvironment and enhancing T-cell activity, manipulating the ISG15 pathway could significantly boost the effectiveness of treatments like immune checkpoint inhibitors [13]. Beyond therapy, ISG15's expression levels are also being explored as a valuable diagnostic and prognostic biomarker in cancer and infectious diseases, offering clinicians a window into the body's immune status [15].
Despite decades of research, many of ISG15's secrets remain locked away. A key challenge is understanding its context-dependent nature: why does it protect us from a virus but potentially help a tumor grow [17]? Answering this requires untangling its complex interactions with other cellular pathways, a task that demands more advanced tools and models.
Developing specific drugs to precisely activate or inhibit the ISGylation pathway is another major frontier. The hunt for such molecules is a high-stakes game of molecular hide-and-seek, requiring the screening of immense chemical libraries to find the perfect key for the lock [18]. This is where new approaches are changing the game. For instance, platforms like Ailurus vec® enable the high-throughput screening of tens of thousands of genetic designs in a single experiment, rapidly identifying optimal expression constructs that could accelerate the discovery and production of ISG15-modulating therapeutics.
Ultimately, cracking the ISG15 code will likely depend on the synergy between biology and artificial intelligence. By generating massive, high-quality datasets on how genetic variations affect ISG15 function, we can train predictive AI models. This AI+Bio flywheel, moving from slow trial-and-error to rapid, data-driven design, promises to illuminate the remaining mysteries of ISG15 and unlock its full therapeutic potential for a new generation of medicine.
Ailurus Bio is a pioneering company building bioprograms, which are genetic codes that act as living software to instruct biology. We develop foundational DNAs and libraries to turn lab-grown cells into living instruments that streamline complex procedures in biological research and production. We offer these bioprograms to scientists and developers worldwide, empowering a diverse spectrum of scientific discovery and applications. Our mission is to make biology a general-purpose technology, as easy to use and accessible as modern computers, by constructing a biocomputer architecture for all.
