Mechanisms Enhancing Antiviral Immune Responses Uncovered

A new study details how LGP2 and MDA5 proteins interact in the initial detection of viral infections. The innate immune system provides the body's frontline protection against viruses, which generate double-stranded RNA (dsRNA) during replication inside cells. Cytoplasmic receptors like melanoma differentiation-associated protein 5 (MDA5) scan for this dsRNA, forming filaments along it that, when sufficiently assembled, launch a signaling pathway producing antiviral agents. 

Laboratory of genetics and physiology 2 (LGP2) also binds viral RNA, translocating along it using ATP energy, though it lacks direct signaling ability. Long recognized for partnering with MDA5, LGP2's exact supportive role was unclear until now. Led by Kazuki Kato from the Institute of Science Tokyo, the team used biochemical tests, cryo-electron microscopy, and high-speed atomic force microscopy to map their interactions. Their findings were published in Molecular Cell.

The researchers first showed that LGP2 is especially important when viral RNA molecules are relatively short. On its own, MDA5 responds best to long dsRNA strands. However, when LGP2 was present, MDA5 could efficiently form short filaments even on shorter RNA molecules, activating downstream antiviral signaling.

To understand how this happens at the molecular level, the team used CEM to probe the proteins’ structure and high-speed atomic force microscopy to visualize them in action. They discovered that LGP2 first binds to the ends of a dsRNA molecule. Then, it uses energy from ATP to move along the RNA strand. As it advances, LGP2 functions as a scaffold, recruiting MDA5 molecules behind it and helping them assemble into stable filament structures. The researchers drew an analogy to threading beads on a string: the string represents dsRNA, while LGP2 acts as the leading bead of a group of MDA5 beads.

The team also found that LGP2 ultimately promotes the formation of small clusters of MDA5 filaments. These clusters enhance the activation of mitochondrial antiviral signaling (MAVS), a key signaling protein that amplifies antiviral responses inside cells. “Our study clarifies the precise mechanisms by which LGP2 recognizes viral RNA and cooperates with MDA5 to activate immune responses,” says Kato.