Dithiothreitol treatment induces heterotypic aggregation of newly synthesized secretory proteins in HepG2 cells.

To analyze the importance of the endoplasmic reticulum oxidizing state for the folding and aggregation of newly made polypeptides, we have incubated intact HepG2 hepatoma cells in the presence of dithiothreitol. When dithiothreitol-treated cells were extracted under nondenaturing conditions immunoprecipitates of newly synthesized albumin showed a complex polypeptide profile. Using direct and sequential immunoprecipitation protocols we identified eight other polypeptide chains, transferrin, plasminogen, ceruloplasmin, alpha 2-macroglobulin, the three fibrinogen chains, and haptoglobin, that were among the proteins co-immunoprecipitated with albumin. The heterotypic aggregates are larger than several hundred kilodaltons and are stabilized by noncovalent interactions. Of the 10 polypeptide chains we examined, only one, alpha 1-antitrypsin, failed to aggregate. This protein is distinguished from the others by the absence of disulfide bonds. We propose a model in which the function of the oxidizing conditions of the endoplasmic reticulum is to promote the rapid formation of disulfide bonds that stabilize adhesive domains in a buried state, thus preventing "global" heterotypic aggregation of newly synthesized chains.