Differential role of mannose and glucose trimming in the ER degradation of asialoglycoprotein receptor subunits.

To gain insight into how sugar chain processing events modulate endoplasmic reticulum (ER)/proteasomal degradation we looked at human asialoglycoprotein receptor polypeptides H2a and H2b, variants which differ only by an extra pentapeptide (EGHRG) present in H2a. Membrane-bound H2a is a precursor of a soluble secreted form while H2b reaches the plasma membrane. Uncleaved precursor H2a molecules are completely retained in the ER and degraded as well as a portion of H2b. Inhibition of N-linked sugar chain mannose trimming stabilized both variants. In contrast, inhibition of glucose trimming with castanospermine greatly enhanced the degradation rate of H2a but not that of H2b. We studied a possible involvement of the ER chaperone calnexin, as inhibitors of glucose trimming are known to prevent calnexin binding. Incubation of cells with low concentrations of castanospermine (30 microg/ml) did not interfere with calnexin binding to H2a while causing the same accelerated degradation as high concentrations (>100 microg/ml) which did inhibit the association. Castanospermine treatment after calnexin binding blocked the dissociation of the chaperone but still caused accelerated degradation. The increased degradation could be blocked by a specific proteasome inhibitor, ZL(3)VS. Our results suggest that extensive mannose trimming or retention of glucose residues due to lack of glucose trimming are signals for ER/proteasomal degradation independent of interaction with calnexin.