Recognition of viruses in the cytoplasm by RLRs and other helicases-how conformational changes, mitochondrial dynamics and ubiquitination control innate immune responses

Mammalian cells possess multiple sensors for recognition of invasion by a broad range of microbes. This recognition occurs through specific molecular signatures found across various pathogens. Toll-like receptors (TLRs), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) and C-type lectin receptors (CLRs) are the major cellular pathogen-recognition receptors (PRRs) responsible for this recognition. TLRs are transmembrane sensors, whereas other PRRs mainly localize in the cytoplasm for the activation of type I interferons and pro-inflammatory cytokines. Among these PRRs, RLRs are well known for their indispensable role in sensing the invasion of RNA viruses. This review summarizes recent advances in knowledge about viral recognition by RLRs and their signalling pathways, and introduces newly emerging RNA helicases involved in innate immune responses.