Dictyosomes participate in the intracellular pathway of storage proteins in developing Vicia faba cotyledons

The main storage globulins of Vicia faba seeds, vicilin and legumin, were localized in ultrathin sections of developing cotyledons employing the protein A/colloidal gold-technique. Double labelling experiments with monospecific immunoglobulins directed against vicilin or legumin using different size classes of protein A-colloidal gold showed that both globulins are associated with protein lumps in the vacuoles of early developmental stages of the seeds and with the rough endoplasmic reticulum, the site of storage protein biosynthesis. Furthermore, vicilin and legumin have been detected in protein-containing vesicles in the cytoplasm as well as in dictyosomes and the dictyosome-derived vesicles. The association of storage globulins with the dictyosomes was also indicated by cell fractionation and immunoblotting of organelle contents derived from microsome, enriched dictyosome and protein body preparations. Protein blotting analyses employing a Concanavalin A/peroxidase technique showed that certain vicilin polypeptides were already glycosylated in the rough endoplasmic reticulum. Another part of the vicilin fraction as well as legumin polypeptides did not show any affinity to Concanavalin A. These data provide a direct evidence for a transient involvement of dictyosomes in the intracellular pathway of legume storage globulins from the rough endoplasmic reticulum to their accumulation site in the protein bodies.