Haloferax volcanii AglB and AglD are Involved in N-glycosylation of the S-layer Glycoprotein and Proper Assembly of the Surface Layer

In this study, the effects of deleting two genes previously implicated in Haloferax volcanii N-glycosylation on the assembly and attachment of a novel Asn-linked pentasaccharide decorating the H. volcanii S-layer glycoprotein were considered. Mass spectrometry revealed the pentasaccharide to comprise two hexoses, two hexuronic acids and an additional 190 Da saccharide. The absence of AglD prevented addition of the final hexose to the pentasaccharide, while cells lacking AglB were unable to N-glycosylate the S-layer glycoprotein. In AglD-lacking cells, the S-layer glycoprotein-based surface layer presented both an architecture and protease susceptibility different from the background strain. By contrast, the absence of AglB resulted in enhanced release of the S-layer glycoprotein. H. volcanii cells lacking these N-glycosylation genes, moreover, grew significantly less well at elevated salt levels than did cells of the background strain. Thus, these results offer experimental evidence showing that N-glycosylation endows H. volcanii with an ability to maintain an intact and stable cell envelope in hypersaline surroundings, ensuring survival in this extreme environment.