Arenaviridae exoribonuclease presents genomic RNA edition capacity

Elsie Yekwa; Chutima Aphibanthammakit; Xavier Carnec; Bruno Coutard; Caroline Picard; Bruno Canard; Sylvain Baize; François Ferron

Our functional study demonstrates that the ExoN structure, substrate specificity, and mechanism is indeed conserved across the family. It is also clear from the structure-ion analysis [43] , that toying with the catalytic ion leads to slight structural changes which impact dramatically [61] , correlated innate immunity suppression to ExoN mutants, postulating a direct involvement of the ExoN activity. We observed that mutant D382A completely loses ExoN activity consistently with results from reverse genetic studies [61] . A noticeable difference concern the mutant E384A, that was shown to have no effect and be dispensable for ExoN activity [61] , is rather shown critical in our in vitro study and consistent with the structural data as E384 (equivalent to MOPV E392) is involved in binding one of the catalytic ion (S4 Fig). Under our conditions, D459A and H517A still retains their ability to cleave two nucleotides meanwhile D522A shows a significant activity leading to the removal of two to three nucleotides. Theses latter three mutants were not reported before but the analysis of the structure of NP-exo MOPV confirms that major features such as fold, and the two ion binding sites (catalytic and structural) are conserved within the Arenaviridae [25, 32, 35, 36, 63] . Residues D390, E392, D534 of NP-exo MOPV directly coordinate the catalytic ion. Mutation of these residues logically alter ion binding and thus leads to complete loss of catalytic activity. The residual activity observed for D522A of NP-exo LCMV is rather difficult to explain as the two structures present no clear differences in the ion binding mode. ExoNs [36, 43] . Therefore MOPV ExoN activity alone is not per se responsible for the differences in innate immunity suppression between MOPV and LASV. Rather, the presence of the ExoN activity may serve other purposes in the viral life cycle, which might be connected directly or indirectly to innate immunity.