Role of catalytic and non-catalytic subsite residues in ribonuclease activity of human eosinophil-derived neurotoxin

Human eosinophil-derived neurotoxin (EDN), a secretory protein from eosinophils, is a member of the RNase A superfamily. The ribonucleolytic activity of EDN is central to its biological activities. EDN binds RNA in a cationic cleft, and the interaction between EDN and RNA substrate extends beyond the scissile bond. Based on its homology with RNase A, putative substrate binding subsites have been identified in EDN. The B1 and B2 subsites interact specifically with bases, whereas P0, P1, and P2 subsites interact with phosphoryl groups. In this study, we evaluated the role of putative residues of these subsites in the ribonucleolytic activity of EDN. We demonstrate that of the two base binding subsites, B1 is critical for the catalytic activity of EDN, as the substrate cleavage was dramatically reduced upon substitution of B1 subsite residues. Among the phosphate-binding subsites, P1 is the most crucial as mutations of its constituting residues totally abolished the catalytic activity of EDN. Mutation of P0 and P2 subsite residues only affected the catalytic activity on the homopolymer Poly(U). Our study demonstrates that P1 and B1 subsites of EDN are critical for its catalytic activity and that the other phosphate-binding subsites are involved in the activity on long homopolymeric substrates.