A reinvestigation provides no evidence for sugar residues on structural proteins of poleroviruses and argues against a role for glycosylation of virus structural proteins in aphid transmission.

Poleroviruses are strictly transmitted by aphids. Glycosylation of Turnip yellows virus (TuYV) was previously reported and this modification was supposed to be required for aphid transmission. Using different approaches based on (i) a lectin-binding assay, (ii) use of specific complex glycan antibodies and (iii) mass spectrometry, we found no evidence that the structural proteins of TuYV and Cucurbit aphid-borne yellow virus (CABYV) carry glycan residues. Moreover, mutation of each of the four potential N-glycosylation sites of the structural protein sequences of CABYV indicated that, unless more than one site on the structural protein is glycosylated, N-glycosylation is not involved in aphid transmission. These results did not corroborate the previous hypothesis for the role of glycosylation in aphid transmission. They, however, revealed the presence of a glycosylated plant protein in purified polerovirus suspensions, whose function in aphid transmission should be further investigated.

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.