Agl22 and Agl23 are involved in the synthesis and utilization of the lipid-linked intermediates in the glycosylation pathways of the halophilic archaeaon Haloarcula hispanica

Like both eukaryotes and bacteria, archaea can decorate proteins with N- and O-linked glycans. Whereas pathways and roles of N-glycosylation have been studied in several model archaeal organisms, little is known of O-glycosylation. To explore commonalities and variations of these two versions of glycosylation, we used Haloarcula hispanica as a model. Our previous work showed that H. hispanica S-layer glycoproteins are modified by an N-linked glucose-α-(1, 2)-[sulfoquinovosamine-β-(1, 6)-]galactose trisaccharide and an O-linked glucose-α-(1, 4)-galactose disaccharide. Here, we found that H. hispanica membrane contains C60 dolichol phosphate (DolP) as a lipid carrier for glycosylation. As revealed by bioinformatics, gene deletion and phenotype analysis, gene HAH_1571, renamed agl22, encodes a predicted glucosyltransferase that transfers glucose from glucose-DolP onto galactose-DolP to form the glucose-α-(1, 4)-galactose-DolP precursor of the N-glycosylation. Gene HAH_2016, renamed agl23, encodes a putative flippase-associated protein responsible for flipping of hexose-DolPs across the membrane to face the exterior. Our results also suggested that the synthesis of the N- and O-linked glycans onto target protein occurs on the outer surface of the cell using hexose-DolPs as sugar donors. Deletion mutant showed that N- and O-glycosylation are required for growth in the defined medium mimicking the natural habitat of H. hispanica.