Accumulation and degradation in the endoplasmic reticulum of a truncated ER-60 devoid of C-terminal amino acid residues

The accumulation and degradation in the endoplasmic reticulum (ER) of a tru ncated ER-60 protease, from which the C-terminal 89 amino acid residues hav e been deleted (K 417 ochre), was examined. K 417 ochre overexpressed in CO S-l cells is not secreted into the medium, but accumulates as insoluble agg regates in non-ionic detergent without degradation in unusual clump membran e structures. K 417 ochre, stably expressed, forms soluble aggregates in no n-ionic detergent and is distributed in the reticular structures of ER, Und er these conditions, K 417 ochre is not secreted into the medium but is deg raded with a half-life time of more than 8 h, Since K 417 ochre/C all S, in which all the Cys residues of K 417 ochre are replaced by Ser, also forms aggregates, an inter-disulfide bond appears unnecessary for aggregation. In both types of aggregates, Ig heavy chain binding protein, calnexin, glucos e regulated protein 94, calreticulin, ERp72, and protein disulfide isomeras e are scarcely found. Since degradation of the stably expressed K 417 ochre was not inhibited by lactacystin, leupeptin, NH4Cl, or cytocharasin B, but was inhibited by N-acetyl-leucyl-leucyl-norleucinal, the self-aggregated a bnormal protein in the lumen of ER is assumed to be degraded by an unknown protease system other than proteasome, lysosome or autophagy.