Intracellular and intramembranous localization of a protein disulfide isomerase in rat liver.

The intracellular distribution of a membrane-bound protein disulfide isomerase (PDI) in rat liver was studied by quantitative immunoprecipitation, and its microsomal localization was confirmed. The content of the enzyme was 1 to 2% of total microsomal protein, and it was almost equally distributed between rough and smooth microsomes. The enzyme was not solubilized from microsomes by high concentrations of KCl, but was readily solubilized by detergents. Since PDI in microsomes was susceptible to digestion by trypsin, at least some parts of the enzyme molecule are exposed on the outside surface of microsomal vesicles. However, the binding of antibodies to microsomal PDI and the modification of the glutamine residues of PDI molecules by transglutaminase suggested that the molecules are not extensively exposed on the surface. Solubilized PDI was unable to rebind to microsomes or to become incorporated into reconstituted membrane of detergent-solubilized microsomes, showing that the association of this enzyme with the membrane is not simply mediated by hydrophobic interaction.