Purification and Characterization of β-xylosidase That is Active for Plant Complex Type N-glycans from Tomato (Solanum Lycopersicum): Removal of Core α1-3 Mannosyl Residue is Prerequisite for Hydrolysis of β1-2 Xylosyl Residue

In this study, we purified and characterized the β-xylosidase involved in the turnover of plant complex type N-glycans to homogeneity from mature red tomatoes. Purified β-xylosidase (β-Xyl'ase Le-1) gave a single band with molecular masses of 67 kDa on SDS-PAGE under a reducing condition and 60 kDa on gelfiltration, indicating that β-Xyl'ase Le-1 has a monomeric structure in plant cells. The N-terminal amino acid could not be identified owing to a chemical modification. When pyridylaminated (PA-) N-glycans were used as substrates, β-Xyl'ase Le-1 showed optimum activity at about pH 5 at 40 °C, suggesting that the enzyme functions in a rather acidic circumstance such as in the vacuole or cell wall. β-Xyl'ase Le-1 hydrolyzed the β1-2 xylosyl residue from Man₁Xyl₁GlcNAc₂-PA, Man₁Xyl₁Fuc₁GlcNAc₂-PA, and Man₂Xyl₁Fuc₁GlcNAc₂-PA, but not that from Man₃Xyl₁GlcNAc₂-PA or Man₃Xyl₁Fuc₁GlcNAc₂-PA, indicating that the α1-3 arm mannosyl residue exerts significant steric hindrance for the access of β-Xyl'ase Le-1 to the xylosyl residue, whereas the α1-3 fucosyl residue exerts little effect. These results suggest that the release of the β1-2 xylosyl residue by β-Xyl'ase Le-1 occurs at least after the removal the α-1,3-mannosyl residue in the core trimannosyl unit.