A Novel Secreted Endoglycosidase from Enterococcus Faecalis with Activity on Human Immunoglobulin G and Ribonuclease B

The human pathogen Enterococcus faecalis can degrade the N-linked glycans of human RNase B to acquire nutrients, but no gene or protein has been associated with this activity. We identified an 88-kDa secreted protein, endoglycosidase (Endo) E, which is most likely responsible for this activity. EndoE, encoded by ndoE, consists of an alpha-domain with a family 18 glycosyl hydrolase motif and a beta-domain similar to family 20 glycosyl hydrolases. Phylogenetic analysis of EndoE indicates that the alpha-domain is related to human chitobiases, and the beta-domain is related to bacterial and human hexosaminidases. Recombinant expression of full-length EndoE or EndoEalpha, site-directed mutagenesis of the catalytic residues, mass spectroscopy, and homology modeling shows that EndoEalpha hydrolyzes the glycan on human RNase B, whereas EndoEbeta hydrolyzes the conserved glycan on IgG. Denaturation experiments indicate that the chitinase activity on RNase B is not dependent on the tertiary structure, although it is on IgG. The ndoE gene and secreted EndoE are present in most E. faecalis but not in Enterococcus faecium isolates. Correspondingly, E. faecalis, but not E. faecium, degrades the glycan on RNase B during growth. Thus, we have identified a secreted enzyme from E. faecalis, EndoE, which by two distinct activities hydrolyzes the glycans on RNase B and IgG. Both activities could be important for the molecular pathogenesis and persistence of E. faecalis during human infections.