Ubiquitination of overexpressed protein and enclosure by inducible ER membrane

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It has been reported that overexpression in the culture cells of endoplasmic reticulum (ER) membrane proteins, such as microsomal aldehyde dehydrogenase (ALDH), causes the induction of a specific ER membrane structure (crystalloid ER, cER). We recently found for the first time that overexpression of an ER-unrelated, mitochondrial precursor for serine : pyruvate aminotransferase (SPT45K) induced the generation of cER, and overexpressed pOTC and pAd, other mitochondrial precursor proteins, also induced the ER aggregates. We also found that the chimeric fluorescent protein having the C terminal ER targeting sequence of ALDH at the C terminus had the remarkable ER aggregating activity by the overexpression. Because these cER (or ER aggregates) inducing activities were not observed in the cases of proteins (SPT43K, ALDHdC, EYFP) lacking organelle targeting signals, the activities is dependent on their structures. It was further indicated that the cER (ER aggregates) inducing activities was dependent on their expression levels in the cell. These data suggest that the cell has a novel system for the detection of the quality (structure) and the quantity of proteins expressed in the cytoplasm by inducing the new ER membrane structure.