The first crystal structure of human RNase 6 reveals a novel substrate-binding and cleavage site arrangement

©2016 The Author(s). Published by Portland Press Limited on behalf of the Biochemical Society. Human RNase 6 is a cationic secreted protein that belongs to the RNase A superfamily. Its expression is induced in neutrophils and monocytes upon bacterial infection, suggesting a role in host defence. We present here the crystal structure of RNase 6 obtained at 1.72 Å (1 Å = 0.1 nm) resolution, which is the first report for the protein 3D structure and thereby setting the basis for functional studies. The structure shows an overall kidney-shaped globular fold shared with the other known family members. Three sulfate anions bound to RNase 6 were found, interacting with residues at the main active site (His15 , His122 and Gln14 ) and cationic surface-exposed residues (His36 , His39 , Arg66 and His67 ). Kinetic characterization, together with prediction of protein-nucleotide complexes by molecular dynamics, was applied to analyse the RNase 6 substrate nitrogenous base and phosphate selectivity. Our results reveal that, although RNase 6 is a moderate catalyst in comparison with the pancreatic RNase type, its structure includes lineage-specific features that facilitate its activity towards polymeric nucleotide substrates. In particular, enzyme interactions at the substrate 5? end can provide an endonuclease-type cleavage pattern. Interestingly, the RNase 6 crystal structure revealed a novel secondary active site conformed by the His36 -His39 dyad that facilitates the polynucleotide substrate catalysis.