Comparative Structure and Function Analysis of the RIG-I-Like Receptors: RIG-I and MDA5

Brisse, Morgan; Ly, Hinh

The formation of longer filaments by RIG-I has been more controversial, giving rise to two alternate models of RIG-I activation: formation of individual single unit of RIG-I with short dsRNA monomers (leaving a free dsRNA end, such as a hairpin loop), which then oligomerizes via CARD tetramerization that is linked by their ubiquitin chains, or filamentation on longer dsRNA. Like MDA5, RIG-I can form filaments in-situ independent of ubiquitin (141, 142) and induces MAVS to also form filaments (142) , and MAVS is known to form filaments in-vitro (129, 130) mediated by its own CARD domains (131, 132) . However, RIG-I filamentation on an RNA template (forming "beads on a string") as opposed to smallerscale oligomerization hasn't yet been shown to occur in-vitro. Part of the reasons for the suggestion that RIG-I was strongly activated by shorter dsRNA was based the comparison on mass equivalents of RNA species as there were less 5 ′ triphosphorylated ends for longer dsRNAs with greater mass than shorter dsRNAs with more 5 ′ triphosphorylated ends (76) . However, when RNA species were normalized by molar equivalence, dsRNA length appeared to be positively correlated with RIG-I signaling (141) (142) (143) , which became insignificant at around 500 bp (141, 143) . It is significantly shorter than the length of dsRNA that activates MDA5, which forms filaments on 2,000 bp dsRNA (137). The kinetics of RIG-I and MDA5 interacting with dsRNA (which will be discussed in detail below) might possibly explain the decrease in dsRNA length efficiency to activate RIG-I as compared to MDA5, as RIG-I seems to first recognize the 5 ′ ppp end before sliding down the length of the dsRNA (144), whereas MDA5 dynamically associates and disassociates along the length of long dsRNA (137). Meanwhile, it is still unclear whether RIG-I can preferentially be activated by longer dsRNA independently of its unknown ability to form filaments in-vitro (145) .