Presence of asparagine-linked N-acetylglucosamine and chitobiose in Pyrus pyrifolia S-RNases associated with gametophytic self-incompatibility

S‐RNases encoded by the S‐locus of rosaceous and solanaceous plants discriminate between the S‐alleles of pollen in gametophytic self‐incompatibility reactions, but it is not clear how. We report the structures of N‐glycans attached to each of the N‐glycosylation sites of seven S‐RNases in Pyrus pyrifolia of the Rosaceae. The structures were identified by chromatographic analysis of pyridylaminated sugar chains prepared from S4‐RNase and by liquid chromatography/electrospray ionization‐mass spectrometric analysis of the protease digests of reduced and S‐carboxymethylated S‐RNases. S4‐RNase carries various types of sugar chains, including plant‐specific ones with β1→2‐linked xylose and α1→3‐linked fucose residues. More than 70% of the total N‐glycans of S4‐RNase are, however, an N‐acetylglucosamine or a chitobiose (GlcNAcβ1→4GlcNAc), which has not been found naturally. The N‐acetylglucosamine and chitobiose are mainly present at the N‐glycosylation sites within the putative recognition sites of the S‐RNase, suggesting that these sugar chains may interact with pollen S‐product(s).