N-Linked glycans play important roles in the determination of glycoprotein fates in cells through interactions with a variety of intracellular lectins. Most of the intracellular lectins possess carbohydrate recognition domain, which is homologous to legume lectins or mannose 6-phosphate receptors (MPRs). These lectins are categorized as L-type or P-type lectins. Besides L-type lectins, recently accumulated frontal affinity chromatography and glycan microarray data have demonstrated that P-type lectins and the MPR homology (MRH) domain-containing proteins have distinct sugar-binding specificity profiles. Furthermore, newly emerged three-dimensional structural data have revealed sugar recognition mechanisms at an atomic level. This review summarizes the current state of knowledge of molecular and structural basis for sugar recognition by P-type lectins and MRH domain-containing proteins that control folding, transport, and degradation of N-linked glycoproteins in the secretory pathway.
Glycosyl sulfonium ions are positively charged organosulfur compounds having the positive charge on a sulfur atom that is covalently attached to the anomeric carbon. These species have received attention because of their potential as glycosylation intermediates. Recently, novel methods for the preparation of glycosyl sulfonium ions have been reported. The reactivity of glycosyl sulfonium ions can be controlled by changing the substituents on the sulfur atom and the protecting groups of the hydroxyl groups. Recent studies have also shown that glycosyl sulfonium ions are unique glycosylation intermediates that have modifiable reactivity.
β-L-Arabinofuranosides are hydroxyproline (Hyp)-linked sugar chains of extensin observed in plant cell wall fractions. Despite the broad distribution of β-L-arabinofuranosyl residues in plants, degradative enzymes have not yet been identified. In 2011, we cloned and characterized the first degradative enzymes for Hyp-linked β-L-arabinofuranosides from Bifidobacterium longum. These enzymes were composed of a glycoside hydrolase (GH) family 43 α-L-arabinofuranosidase (HypAA) releasing L-arabinose from Arafα1-3Arafβ1-2Arafβ1-2Arafβ-Hyp, a GH121 β-L-arabinobiosidase (HypBA2) releasing Arafβ1-2Araf (β-Ara2) from Arafβ1-2Arafβ1-2Arafβ-Hyp, and a GH127 β-L-arabinofuranosidase (HypBA1) degrading β-Ara2 to L-arabinose. These enzymes are encoded in a conserved gene cluster on several B. longum genomes, but not in genome sequences of other intestinal bacteria. Hyp-linked β-L-arabinofuranosides were utilized as carbohydrate sources by B. longum. This review presents the functional features of enzymes for Hyp-linked β-L-arabinofuranosides and the predicted metabolic pathway in B. longum.