Protein disulfide isomerase acts as a molecular chaperone in the intracellular retention of mouse mutant thyroglobulin

Abstract

Relatively few point mutations in the thyroglobulin (Tg) gene that cause thyroid diseases have been identified in man. Here, we have examined the intracellular fate of a mouse full-length missense Tg mutant (R39K) that is equivalent to the corresponding human Tg mutant recently reported in a Brazilian kindred with congenital goiter and hypothyroidism. When expressed in COS-7 cells, markedly reduced export of the R39K Tg was associated with increased stable association with the endoplasmic reticulum (ER) chaperone BiP/GRP78, pointing to a folding defect. More prolonged association with calnexin was also observed, suggesting an important role for the lectin pathway of ER quality control in the processing of the mutant Tg that had not been previously described. Moreover, the most stable chaperone-Tg association was observed for protein disulfide isomerase (PDI), which normally functions as a redox foldase, providing new evidence that PDI may also be a molecular chaperone in the intracellular processing of the mutant Tg. Eventually, R39K Tg was degraded by the 26S proteasome in the cytosol. It was concluded that these three ER chaperones, BiP/GRP78, calnexin, and PDI are part of a quality control machinery that associates with the mutant Tg, as it is targeted for ER-associated degradation.