Biosynthesis and immunolocalization of Lewis a-containing N-glycans in theplant cell

We recently demonstrated the presence of a new asparagine-linked complex gl ycan on plant glycoproteins that harbors the Lewis a (Le(a)), or Gal beta(1 -3)[Fuc alpha(1-4)]GlcNAc, epitope, which in mammalian cells plays an impor tant role in cell-to-cell recognition. Here we show that the monoclonal ant ibody 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 plan ts, but is less apparent in the Cruciferae, the family that includes Arabid opsis. Le(a)-containing oligosaccharides are found in the Golgi apparatus, and our immunocytochemical experiments suggest that it is synthesized in th e trans-most part of the Golgi apparatus. Le(a) epitopes are abundantly pre sent on extracellular glycoproteins, either soluble or membrane bound, but are never observed on vacuolar glycoproteins. Double-labeling experiments s uggest that vacuolar glycoproteins do not bypass the late Golgi compartment s where Le(a) is built, and that the absence of the Lea epitope from vacuol ar glycoproteins is probably the result of its degradation by glycosidases en route to or after arrival in the vacuole.