Biosynthesis and Immunolocalization of Lewis a-Containing N-Glycans in the Plant Cell

We recently demonstrated the presence of a new asparagine-linked complex glycan on plant glycoproteins that harbors the Lewis a (Le a ), or Galβ(1-3)[Fucα(1-4)]GlcNAc, epitope, which in mammalian cells plays an important role in cell-to-cell recognition. Here we show that the monoclonal antibody JIM 84, which is widely used as a Golgi marker in light and electron microscopy of plant cells, is specific for the Le a antigen. This antigen is present on glycoproteins of a number of flowering and non-flowering plants, but is less apparent in the Cruciferae, the family that includes Arabidopsis. Le a -containing oligosaccharides are found in the Golgi apparatus, and our immunocytochemical experiments suggest that it is synthesized in the trans-most part of the Golgi apparatus. Le a epitopes are abundantly present on extracellular glycoproteins, either soluble or membrane bound, but are never observed on vacuolar glycoproteins. Double-labeling experiments suggest that vacuolar glycoproteins do not bypass the late Golgi compartments where Le a is built, and that the absence of the Le a epitope from vacuolar glycoproteins is probably the result of its degradation by glycosidases en route to or after arrival in the vacuole.