Structural analysis of the N-linked glycan chains from a stylar glycoprotein associated with expression of self-incompatibility in Nicotiana alata.

Self-incompatibility in flowering plants of the family Solanaceae is mediated by the product of the S-allele. The allelic products of the S-gene in the female sexual tissues of the pistil are glycoproteins in the mol. wt range 28-32 kDa. These S-glycoproteins have been isolated from styles of Nicotiana alata, homozygous for the S1- and S2-alleles. Earlier studies have indicated that the single potential N-glycosylation site on the S1-glycoprotein bears a glycan chain, whereas of the four potential N-glycosylation sites on the S2-glycoprotein, three are glycosylated. This paper describes the purification and characterization of the N-linked glycan chains from these two glycoproteins. Oligosaccharides were cleaved off the glycoproteins using peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F (N-glycanase F) and separated by anion-exchange HPLC. Four types of hybrid structure were defined by chemical techniques, fast atom bombardment-mass spectrometry (FAB-MS) and 1H-NMR. Although the relative amounts differed, all four structures were found on both the S1- and S2-glycoproteins, and are heterogeneous at some N-glycosylation sites. No O-linked glycans were detected on the S2-glycoprotein. These results are discussed in relation to the potential of the structural diversity residing in this array of glycoforms to play a rôle in allelic specificity.