RNA sensing: the more RIG-I the merrier?

Helicases translocate along and unwind double-stranded nucleic acids coupled to ATP hydrolysis, which provides the required energy. They are essential to life and have important roles in, for example, DNA replication and repair, transcription and RNA interference. In 2004, an essential function of helicases was discovered in the innate immune response [1]. Retinoic acid inducible gene I (RIG-I) encodes a multidomain protein that contains a superfamily 2 DExD/H-Box RNA helicase domain (Fig 1A). It specifically recognizes viral RNAs in cells infected with many RNA viruses—such as influenza A— and initiates a host response that leads to the production of type I interferons (IFNs), a family of secreted cytokines with potent antiviral effects. Despite the progress in our understanding of RIG-I activity, as detailed below, the role of ATP hydrolysis by the heli case domain remains poorly understood, although it is crucial for downstream signalling [1]. Whether RIG-I forms oligomers on RNA is also unclear. Two studies in this issue of EMBO reports by the GarciaSastre and Pyle groups provide new insights into these issues [2,3]. The types of RNA that activate RIG-I, the signal transduction pathway and the viral antagonists that target it have been intensively studied [4]. Moreover, crystal structures of RIG-I, with and without RNA, have provided detailed molecular insights into its activation [5]. RIG-I surveys the cytosolic RNA content for molecules bearing a 5ʹ-triphosphate (5ʹPPP) group, which is present on the RNA genome of several RNA viruses but is not found on most cellular cytosolic RNAs [4]. RIG-I first recognizes the 5ʹPPP group on target RNAs with its carboxy-terminal domain (CTD) and then binds to the RNA—which must be base-paired to a complementary stretch of RNA—with the helicase domain. RNA binding induces major structural rearrangements that displace the two caspase recruitment domains (CARDs; Fig 1A) from their interaction with the helicase domain [5], enabling CARD interaction with the adaptor protein mitochondrial antiviral signalling protein (MAVS), which relays the signal to transcription factors that induce IFN. The Pyle group reports, in this issue, the crystal structures of RIG-I bound to a 10 base pair (bp) double-stranded RNA (dsRNA), with an empty ATP-binding pocket and with ADP-Mg2+, and compares these with a previously published structure containing sulphate [2]. Depending on the occupancy of the ATP-binding site, the helicase domain makes contact with various nucleotides of the bound RNA. This suggests that the helicase domain scans the RNA in a manner dependent on ATP binding and hydrolysis. In these and previous structures of RIG-I, the RNA sensing: the more RIG-I the merrier?