Evolution of antiviral activity in the ribonuclease A gene superfamily: evidence for a specific interaction between eosinophil-derived neurotoxin (EDN/RNase 2) and respiratory syncytial virus

We have demonstrated that the human eosinophil-derived neurotoxin (EDN, RNa se 2), a rapidly evolving secretory protein derived from eosinophilic leuko cytes, mediates the ribonucleolytic destruction of extracellular virions of the single-stranded RNA virus respiratory syncytial virus (RSV). While RNa se activity is crucial to antiviral activity, it is clearly not sufficient, as our results suggest that EDM has unique structural features apart from RNase activity that are necessary to promote antiviral activity. We demonst rate here that the interaction between EDN and extracellular virions of RSV is both saturatable and specific. Increasing concentrations of the antivir ally inactivated, ribonucleolytically inactivated point mutant form of reco mbinant human EDN, rhEDNdK(38), inhibits rhEDN's antiviral activity, while increasing concentrations of the related RNase, recombinant human RNase k6, have no effect whatsoever. Interestingly, acquisition of antiviral activit y parallels the evolutionary development of the primate EDN lineage, having emerged some time after the divergence of the Old World from the New World monkeys. Using this information, we created ribonucleotytically active chi meras of human and New World monkey orthologs of EDN and, by evaluating the ir antiviral activity, we have identified an N-terminal segment of human ED N that contains one or more of the sequence elements that mediate its speci fic interaction with RSV.