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

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 a 1.72 A resolution, being the first report for the protein three-dimensional 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 sulphate anions bound to RNase 6 were found, interacting to 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 to 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 59 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.