Free N-glycans already occur at an early stage of seed development

As a part of our studies to elucidate the physiological significance of free N-glycans in differentiating or growing plant cells, we first demonstrate that two kinds of free N-glycans already occur at an early stage of seed development. In this report, we used the developing Ginkgo biloba seeds as a model plant, since we have already revealed a functional feature of the Ginkgo endo-β-N-acetylglucosaminidase and structural features of N- glycans linked to storage glycoproteins in the developing seeds [Kimura, Y. et al. (1998) Biosci. Biotechnol. Biochem. 62, 253-261; Kimura, Y. and Matsuo, S. (2000) Biosci. Biotechnol. Biochem. 64, 562-568]. The structures of free N-glycans, which were determined by a combination of ESI-MS, sequential α-mannosidase digestions, partial acetolysis, and two dimensional sugar chain map, fell into two categories. One dominant species is a high- mannose type structure having one GlcNAc residue at the reducing end (Man9·5GlcNAc₁). The concentration of this type of free glycan (as the pyridylaminated derivatives) is about 2.2 nmol in 1 g fresh weight. The detailed structural analysis revealed that the high-mannose type structures have a common core unit; Manα16(Man1-3)Manα1-6(Manα1-3)Manβ1-4GlcNAc. The other minor species of free N-glycans is the plant complex type structure having an N-acetylchitobiose unit at the reducing end (Man₃Xyl₁Fuc₁GlcNAc₂). The concentration of this type of free glycan (as the pyridylaminated derivative) was about 75 pmol in 1 g fresh weight.