N-glycosylation in Haloferax volcanii: adjusting the sweetness

Abstract

Long believed to be restricted to Eukarya, it is now known that cells of all three domains of life perform N-glycosylation, the covalent attachment of glycans to select target protein asparagine residues. Still, it is only in the last decade that pathways of N-glycosylation in Archaea have been delineated. In the haloarchaeon Haloferax volcanii, a series of Agl (archaeal glycosylation) proteins is responsible for the addition of an N-linked pentasaccharide to modified proteins, including the surface (S)-layer glycoprotein, the sole component of the surface layer surrounding the cell. The S-layer glycoprotein N-linked glycosylation profile changes, however, as a function of surrounding salinity. Upon growth at different salt concentrations, the S-layer glycoprotein is either decorated by the N-linked pentasaccharide introduced above or by both this pentasaccharide as well as a tetrasaccharide of distinct composition. Recent efforts have identified Agl5-Agl15 as components of a second Hfx. volcanii N-glycosylation pathway responsible for generating the tetrasaccharide attached to S-layer glycoprotein when growth occurs in 1.75 M but not 3.4 M NaCl-containing medium.

Keywords: Archaea; Haloferax volcanii; N-glycosylation; S-layer glycoprotein; post-translational modification; protein glycosylation.

FIGURE 1

Schematic depiction of the Hfx. volcanii Agl pathway used for the assembly and attachment of the pentasaccharide N-linked to the S-layer glycoprotein Asn-13 and Asn-83 positions. See the text for details. In the figure, dolichol phosphate is in purple, hexose is in yellow, hexuronic acid is in green, mannose is in blue and OMe corresponds to a methyl ester group. The bottom half of the figure corresponds to the cell interior.

FIGURE 2

Schematic depiction of the Hfx. volcanii pathway used for the assembly of the tetrasaccharide N-linked to the S-layer glycoprotein Asn-498 position when the cells are grown in low salt (1.75 M NaCl)-containing medium. See the text for details. In the figure, dolichol phosphate is in purple, hexose is in yellow and rhamnose is in pink. The bottom half of the figure corresponds to the cell interior.