Structure-function studies of helicases
DNA helicases play an essential role in facilitating cellular DNA metabolisms including genome replication, DNA repair, recombination, transcription as well as telomere maintenance. In general, a helicase functions as a molecular motor that uses the energy of nucleotide triphosphate (NTP) hydrolysis to translocate along a strand of nucleic acids and to separate the complementary strand of a nucleic acid duplex. NTPase activity and the conversion of the resulting energy into work to drive double-stranded nucleic acid unwinding, is coordinated by a series of seven sequence motifs that are hallmarks of superfamily-1 and -2 helicases.
Actually, we are interested in the RIG-I family RNA helicases. About ten years ago, a gene regulated by retinoic acid has been cloned from an acute promyelocytic leukaemia cell line (NB4) in Drs. Chen Zhu and Chen Saijuan's laboratory. The sequence analysis of the gene revealed that it encodes a RNA helicase termed RIG-I (from retinoic acid induced gene). RIG-I consists of an N-terminal caspase recruitment domain (CARD), a domain with the signature of DExD/H box helicase (helicase domain), and a C-terminal repression domain (RD). While Dr. Chen's laboratory studied how RIG-I was implicated in t(15;17) acute promyelocytic leukaemia, Japanese researchers have found that this helicase plays an essential role in innate immune response. Today, it is well established that Rig-I helicase is also induced by IFN?, TNF-a and IFNa. Rig-I belongs to a family of RNA helicases that also contain MDA-5 (melanoma differentiation associated gene-5) and LGP2. In resting cells, the close contact of the CARD domain and the RD domain of RIG-I constrain RIG-I helicase in an autoinhibited state. After viral infection, viral RNA binding to RD induces a conformational change that promotes exposure of CARD domain. The CARD domain of RIG-I binds to the adaptor molecule termed IPS that provides a link between RIG-I sensing of incoming viral RNA and downstream activation events. More interestingly, IPS localizes to the mitochondrial membrane, suggesting a link between viral infection, mitochondrial function and development of innate immunity. We are currently studying the molecular mechanisms of RIG-I functions in innate immunity and in acute promyelocytic leukaemia.
The picture illustrates how RIG-I helicase interacts with the mitochondrial protein MAVS to induce the expression of IFN
