Cells infected with viruses can mount an innate immune response that involves detection of viral DNA or RNA by cytoplasmic pattern-recognition receptors (PRRs). MDA5, LGP2, and RIG-I are all related PRRs that bind to double-stranded RNA (dsRNA) and possess helicase activity. LGP2 lacks the CARD domains present in the other PRRs, which is necessary for the formation of a signaling complex at the mitochondrial membrane that stimulates antiviral responses. LGP2 has been reported to have both positive and negative effects on the antiviral response. By exposing cells to increasing amounts of LGP2-expressing plasmid with the same amount of MDA5-expressing plasmid, Bruns et al. found that low amounts of LGP2 enhanced the MDA5-dependent activation of an antiviral reporter gene, whereas high amounts of LGP2 inhibited the response. Expressing LGP2 with mutations that disrupted RNA binding, ATP hydrolysis activity, or the helicase domain did not enhance MDA5-mediated antiviral signaling at any dose, but still inhibited MDA5-mediated signaling at high doses. Coexpression of LGP2 in cells expressing mutants of MDA5 that could not oligomerize did not affect the expression of the antiviral reporter gene. In vitro biochemical assays with purified, tagged MDA5 lacking the CARD domains and tagged LGP2 in the presence of dsRNA of various lengths showed that LGP2 enhanced the rate of association of MDA5 with dsRNA: Association in the absence of LGP2 was linear with an initial lag period, whereas association in the presence of LGP2 was exponential without a lag. MDA5 can form long filaments on dsRNA, which are visible by electron microscopy. Electron microscopy analysis and in vitro biochemical assays revealed that the amount of wild-type MDA5 associated with dsRNA was increased in the presence of LGP2, but that the length of the MDA5-dsRNA filaments was shorter than those formed without LGP2 and was similar to the length of the filaments formed by a signaling-competent and oligomerization-defective mutant MDA5. Introduction of dsRNA-MDA5-LGP2 complexes into cells activated the antiviral reporter gene to a greater extent than did complexes containing the dsRNA and MDA5 only or complexes containing dsRNA, signaling-competent and oligomerization-defective mutant MDA5, and LGP2. Thus, LGP2 appears to promote the nucleation of MDA5 onto dsRNA, thereby producing shorter filaments that optimally stimulate the antiviral response.
A. M. Bruns, G. P. Leser, R. A. Lamb, C. M. Horvath, The innate immune sensor LGP2 activates antiviral signaling by regulating MDA5-RNA interaction and filament assembly. Mol. Cell 55, 771–781 (2014). [PubMed]