The resulting RNA-protein complexes could then be isolated from the cells and, after detachment of the proteins, the nucleotide sequences of the RNAs could be determined. "This allowed us to determine how RLRs recognize foreign RNAs and how the latter differ from endogenous cellular RNAs," says Hopfner.
The researchers found that the RLR proteins RIG-I and MDA5 indeed recognize defined elements within viral RNAs in living cells that have been infected by measles virus. Like many other viruses, including the one that causes rabies, the measles virus possesses a single-stranded RNA genome. Unlike DNA viruses, it therefore delivers an RNA template directly into the host cell. However, this molecule must then be transcribed by its associated viral RNA polymerase to generate the mRNAs required for synthesis of viral proteins and propagation of the infection.
Sensors bind to specific regions
"And while RIG-I preferentially binds to certain sequence patterns found at the exposed ends of different viral RNAs both in vitro and in vivo, MDA5 rather surprisingly recognizes not the viral genome itself, but apparently certain regions located within viral mRNAs," Hopfner explains. Moreover, these regions differ in their base composition from sequences found in other viral RNAs, suggesting that MDA5 relies on these structural differences to discriminate between viral and endogenous RNAs.
Hopfner and his team now plan to investigate the interaction of RLRs with other viral nucleic acids, in order to obtain a clearer picture of the molecular mechanisms that enable these proteins to detect foreign RNAs. This should
|Contact: Luise Dirscherl|