Misfolding and aggregation of vacuolar glycoproteins in plant cells

Phaseolin and lectin-related polypeptides, the abundant oligomeric glycopro teins of bean seeds, are synthesized on the endoplasmic reticulum (ER) and then transported to the storage vacuole via the Golgi apparatus. Glycosylat ion and folding are among the major modifications these proteins undergo in the ER. Although a recurrent role of N-glycosylation is on protein folding , in previous studies on common bean (Phaseolus vulgaris) seeds we demonstr ated that the oligosaccharide side-chains are not required for folding, int racellular transport and activity of storage glycoproteins. We show here th at in lima bean (Phaseolus lunatus), incubation of the developing cotyledon with tunicamycin to prevent glycosylation has a dramatic effect on the int racellular transport of the storage glycoproteins. When lacking their glyca ns, phaseolin and lectin-related polypeptides misfold and are retained in t he ER as mixed aggregates to which the chaperone BiP irreversibly associate s. The lumen of the ER becomes enlarged to accommodate the aggregated polyp eptides. Intracellular transport of legumin, a naturally unglycosylated sto rage protein, is mostly unaffected by the inhibitor, indicating that the ob served phenomenon specifically occurs on glycoproteins. Furthermore, recomb inant lima bean phaseolin synthesized in tobacco protoplasts is also correc tly folded and matured in the presence of tunicamycin. To our knowledge, th is is the first report that describes in detail the block of intracellular transport of vacuolar glycoproteins in plant cells due to aggregation follo wing glycosylation inhibition.