Comparison of human RNase 3 and RNase 7 bactericidal action at the Gram-negative and Gram-positive bacterial cell wall

The eosinophil cationic protein/RNase 3 and the skin-derived RNase 7 are two human antimicrobial RNases involved in host innate immunity. Both belong to the RNase A superfamily and share a high cationicity and a common structural architecture. However, they present significant divergence at their primary structures, displaying either a high number of Arg or Lys residues, respectively. Previous comparative studies with a membrane model revealed two distinct mechanisms of action for lipid bilayer disruption. We have now compared their bactericidal activity, identifying some features that confer specificity at the bacterial cell wall level. RNase 3 displays a specific Escherichia coli cell agglutination activity, which is not shared by RNase 7. The RNase 3 agglutination process precedes the bacterial death and lysis event. In turn, RNase 7 can trigger the release of bacterial cell content without inducing any cell aggregation process. We hypothesize that the RNase 3 agglutination activity may depend on its high affinity for lipopolysaccharides and the presence of an N-terminal hydrophobic patch, and thus could facilitate host clearance activity at the infection focus by phagocytic cells. The present study suggests that the membrane disruption abilities do not solely explain the protein bacterial target preferences and highlights the key role of antimicrobial action at the bacterial cell wall level. An understanding of the interaction between antimicrobial proteins and their target at the bacterial envelope should aid in the design of alternative peptide-derived antibiotics.