title: Precursors of ricin and Ricinus communis agglutinin. Glycosylation and processing during synthesis and intracellular transport.
abstract: During synthesis in vivo the castor bean lectin precursors initially appear in the endoplasmic reticulum as a group of core glycosylated polypeptides of relative molecular mass 64 000-68 000. Pretreatment of intact castor bean endosperm tissue with tunicamycin partially inhibits the cotranslational core glycosylation step and results in the accumulation of a single sized unglycosylated precursor polypeptide of relative molecular mass 59 000. The glycosylated precursors in the endoplasmic reticulum were enzymically converted to the 59 000-Mr form by incubation with endoglucosaminidase H. Intracellular transport of the glycosylated lectin precursors from the endoplasmic reticulum to a denser vesicle fraction was accompanied by modifications to the oligosaccharide moieties which conferred resistance to the action of endoglucosaminidase H. The post-translational addition of fucose to the carbohydrate chain was identified as one of the oligosaccharide modification steps. Fucose addition was catalysed by a glycosyltransferase associated with a smooth-surfaced membrane fraction which was distinct from the endoplasmic reticulum and which was tentatively identified as the Golgi apparatus. Glycosylation was not essential for intracellular transport of the lectin precursors: unglycosylated precursor synthesized in the presence of tunicamycin gave rise to unglycosylated lectin subunits in the protein bodies.
mesh_terms: D000372:Agglutination Tests; D001692:Biological Transport; D002367:Ricinus communis; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D010455:Peptides / Q000378:metabolism; D037121:Plant Lectins; D010947:Plants, Toxic; D011498:Protein Precursors / Q000378:metabolism*; D012276:Ricin / Q000302:isolation & purification*; D014158:Transcription, Genetic / Q000187:drug effects; D014415:Tunicamycin / Q000494:pharmacology
chemical_list: D010455:Peptides; D037121:Plant Lectins; D011498:Protein Precursors; D014415:Tunicamycin; D012276:Ricin