The Structure of Host-Pathogen Interactions

Upon infection, cellsof higher vertebratesproduce interferons (IFNs),whichactivate variousantiviral programs in the infectedcell andsurroundingcells, thus limiting the spreadof infection.Oneof these antiviral mediators is RNase L, an endoribonuclease that cleaves viral and cellular RNA on demand. RNase L is activated by 2’,5’-linked oligoadenylates (2-5A), products of IFN-inducible 2-5A synthetases,which in turn are sensors for double-strandedRNA (dsRNA) resulting fromcellular damageor viral infection. RNase L thus integrates infection signals from IFNs and dsRNA. Two studies now provide a precise molecular view of RNase L and shed light into the recently emerged functions of the related receptor IRE1, which mitigates stress caused by unfolded protein overload of the endoplasmic reticulum (ER) by cleaving transcripts encoding membrane and secreted proteins. Huang et al. (2014) show, by structural analysis of porcine RNase L in complex with 2-5A and a nonhydrolysable ATPmimetic (AMP-PNP), that RNase L forms a specific dimer configuration that juxtaposes the two RNase domains and creates the enzyme’s catalytic site. The authors propose that dimerization of inactive RNase Lmonomers occurs upon 2-5Abinding and that the binding of abundant cellular nucleotides ATP/ADP further adds rigidity to the structure that is required for catalytic function. In a related study, Han et al. (2014) solve the crystal structure of human RNase L in complex with an RNA substrate. As with the Huang et al. study, the homodimer reveals a surprising third binding site for 2-5A in a composite pocket created by the ankyrin and kinase homology domains. Thiswork suggests that RNaseL uses its dimeric active site for asymmetric RNA recognition. Structural and biochemical analyses of the catalytic site, comprisedwithin the interface of KEN/KEN (kinase extension nuclease) dimers, explain the cleavage site selection inhostandviralRNA. IRE1shares theKENmoduleofRNaseL,andanalysesof IRE1 target transcripts revealssimilarities insequence determinants suggesting commonality at some level in the substrate recognition and cleavagemechanisms used by both enzymes. These findings also open opportunities for the design of RNase L agonists or antagonists aimed at modulating the IFN response. Huang, H., et al. (2014). Mol Cell. 53, 221–234. Han, Y., et al. (2014). Science. 343, 1244–1248.