Interferon-gamma inducible GTPases disrupt viral replication complex upon their targeting by the LC3 conjugation system of autophagy

Abstract

Abstract Viruses with a single strand (+) sense RNA genome replicate within vacuole-like structures in cytoplasm, called replication complex (RC). RC is made through reorganization of cellular organelle membranes, and it provides a favorable microenvironment for the viruses to replicate. How the host immune system counteracts viral RCs has been obscure. Recently, we found that interferon-gamma (IFNγ) inhibits the replication of murine norovirus (MNV) at the stage of RC formation. Intriguingly, this anti-viral activity of IFNγ depends on a protein complex involved in cellular autophagy. Autophagy is an evolutionarily conserved pathway that sequesters cytoplasmic materials in membrane-bound autophagosomes and delivers them to lysosome for degradation. To form an autophagosome, the microtubule-associated-protein-1-light-chain-3 (LC3) conjugation system (Atg7, Atg3, Atg12-Atg5-Atg16L1) is essential. Surprisingly, only the LC3 conjugation system of autophagy, but not lysosomal degradation through autophagy, was required for IFNγ to inhibit MNV RC formation. Through a comparative mechanism study, we found that the LC3 conjugation system was required to target IFNγ inducible GTPases - immunity related GTPases (IRGs) and guanylate binding proteins (GBPs) - to the RC. Both IRGs and GBPs are known to be targeted to the membrane of vacuoles containing bacteria, protozoa, or fungus. The targeted membranes are vesiculated and vacuoles get ruptured, leading to the death of exposed pathogens. Similarly, the GTPases disrupt the membrane structures of the RC and thus the replication of MNV. These data suggest that IFNγ utilizes the effectors against vacuolar pathogens for the control of viruses replicating in cytosolic membranous structures.