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 (Lea), or Galbeta(1-3)[Fucalpha(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 Lea 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. Lea-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. Lea 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 Lea is built, and that the absence of the Lea epitope from vacuolar glycoproteins is probably the result of its degradation by glycosidases en route to or after arrival in the vacuole.