Quality control mechanism for secretory proteins in intracellular protein sorting pathway

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In the endoplasmic reticulum (ER) lumen, many of secretory proteins acquire N-linked oligosaccharide side chains which may help folding assembly and sorting of the proteins. In the present study, we examined roles of the N-glycosylation on cathepsin E (CE) in its folding, stability and localization by expressing N-glycosylation-deficient mutant CE in cells or by treatment of the cells with a N-glycosylation inhibitor, and tried to know how these glycosylation-deficient CEs suffered the quality control in the cells. When normal rat kidney(NRK) cells expressing exogenous rat CE gene were treated with tunicamycin, rapid degradation of CE within the cells was observed. The degradation was not inhibited by brefeldin A,bafilolmycin Al and NH4Cl, suggesting that the degradation occurs in pre-Golgi compartments. Moreover, the degradation was not affected by inhibitors for proteasome, such as lactacystin or ALLN.The de-stabilization of CE was not solely due to the elimination of N-glycosylation of the enzyme, because non-glycosylation mutant of CE was found to be stably retained in the NRK cells. In the same type of tunicamycin treatment, no change was observed in the intracellular stability of BiP (GRP78), an ER resident molecular chaperone. These results suggest that tunicamycin induces some unknown proteasome-independent degradation system within the ER to eliminate proteins defect in properN-glycosylation.