Ricin A chain without its partner B chain is degraded after retrotranslocation from the endoplasmic reticulum to the cytosol in plant cells

When expressed in tobacco cells, the catalytic subunit of the dimeric ribos ome inactivating protein, ricin, is first inserted into the endoplasmic ret iculum (ER) and then degraded in a manner that can be partially inhibited b y the proteasome inhibitor clasto-lactacystin beta -lactone. Consistent wit h the implication of cytosolic proteasomes, degradation of ricin A chain is brefeldin A-insensitive and the polypeptides that accumulate in the presen ce of the proteasome inhibitor are not processed in a vacuole-specific fash ion. Rather, these stabilized polypeptides are in part deglycosylated by a peptide:N-glycanase-like activity. Taken together, these results indicate t hat ricin A chain, albeit a structurally native protein, can behave as a su bstrate for ER to cytosol export, deglycosylation in the cytosol, and prote asomal degradation. Furthermore, retrotranslocation of this protein is not tightly coupled to proteasomal activity. These data are consistent with the hypothesis that ricin A chain can exploit the ER-associated protein degrad ation pathway to reach the cytosol. Although well characterized in mammalia n and yeast cells, the operation of a similar pathway to the cytosol of pla nt cells has not previously been demonstrated.