Purification and specificity of beta1,2-xylosyltransferase, an enzyme that contributes to the allergenicity of some plant proteins.

The enzyme that transfers D-xylose from UDP-xylose to the beta-linked mannose of plant N-linked oligosaccharides was purified about 51,000-fold to apparent homogeneity from soybean microsomes. On SDS gels, two proteins of 56 and 59 kDa were detected and both were labeled to the same extent by the photoaffinity label, 5-N3-UDP-[32P]xylose. Labeling of both proteins was inhibited by cold UDP-xylose, but not by UDP-glucose. The amount of 5-N3-UDP-[32P]xylose that bound to the two protein bands was greatly increased in the presence of oligosaccharide acceptors. The best acceptor for xylose transfer and for stimulation of UDP-xylose binding was GlcNAc2Man3GlcNAc2-T, but GlcNAc1Man3GlcNAc2, with the GlcNAc on the 3-branch, was also a good acceptor and a good stimulator. A number of other N-linked oligosaccharides were poor acceptors, especially those with galactose units at the nonreducing termini. Many of the properties of this enzyme have been described, and the product of the reaction of UDP-xylose and GlcNAc2Man3(GlcNAc)2 was characterized as GlcNAcbeta1, 2Manalpha1, 6(GlcNAcbeta1,2Manalpha1,3)(Xylbeta1,2)Manbeta1, 4GlcNA c2-T by chemical and NMR methods.