Distribution of protein disulfide isomerase in rat hepatocytes.

We investigated quantitatively the distribution of protein disulfide isomerase (PDI) in rat hepatocytes by immunocytochemistry using a post-embedding protein A-gold technique. In hepatocytes, gold particles were mainly localized in the intracisternal space of the rough and smooth endoplasmic reticulum (ER) and nuclear envelopes. Autolysosomes engulfing ER were occasionally densely labeled, especially in rat hepatocytes previously treated with leupeptin in vivo, suggesting that the autophagosome-autolysosome system may be an important route for degradation of PDI. A few gold particles were also found on the plasma membranes. Localization of gold particles on the other subcellular organelles, such as Golgi apparatus, peroxisomes, and nuclear matrix, was sparse and at the control level. The predominant localization of PDI on the intracisternal surface of the ER and nuclear envelope supports a potential role of PDI in the formation of disulfide bonds of nascent polypeptides, thus accelerating formation of the higher-order structure of secretory and membrane proteins and rendering the translocation process irreversible.