The N-glycan Cluster from Xanthomonas campestris pv. campestris: a Toolbox for Sequential Plant N-glycan Processing

Abstract N-glycans are widely distributed in living organisms but represent only a small fraction of carbohydrates found in plants. This probably explains why they have not previously been considered as substrates exploited by phytopathogenic bacteria during plant infection. Xanthomonas campestris pv. campestris (Xcc), the causal agent of black rot disease of Brassica plants, possesses a specific system for N-acetylglucosamine (GlcNAc) utilization expressed during host plant infection. This system encompasses a cluster of 8 genes (nixE to nixL) encoding glycoside hydrolases (GHs). In this paper, we have characterized the enzymatic activities of these GHs and demonstrated their involvement in sequential degradation of a plant N-glycan using a N-glycopeptide containing 2 GlcNAc, 3 mannose, 1 fucose and 1 xylose (N2M3FX) as a substrate. The removal of the alpha-1,3-mannose by the alpha-mannosidase NixK (GH92) is a prerequisite for the subsequent action of the beta-xylosidase NixI (GH3) which is involved in the cleavage of the beta-1,2-xylose, followed by the alpha-mannosidase NixJ (GH125) which removes the alpha-1,6-mannose. These data, combined to the subcellular localization of the enzymes, allowed us to propose a model of N-glycopeptide processing by Xcc. This study constitutes the first evidence suggesting N-glycan degradation by a plant pathogen, a feature shared with human pathogenic bacteria. Plant N-glycans should therefore be included in the repertoire of molecules putatively metabolized by phytopathogenic bacteria during their life cycle.