Transient Covalent Interactions of Newly-Synthesized Thyroglobulin with Oxidoreductases of the Endoplasmic Reticulum.

Newly-synthesized thyroglobulin (Tg), the thyroid prohormone, forms detectable high molecular weight mixed disulfide adducts: until now only Tg adduct-B was identified as primarily engaging the endoplasmic reticulum oxidoreductases ERp57 and Protein Disulfide Isomerase. Here we demonstrate that the faster-migrating Tg adduct-C primarily engages the CaBP1/P5 oxidoreductase while the slower-migrating Tg adduct-A primarily engages ERp72. Upon siRNA-mediated knockdown of CaBP1/P5 or ERp72, adducts C or A, respectively, are decreased. Within the three Tg adduct bands that do not exhibit a precursor-product relationship, Tg exhibits distinct oxidation patterns. We present evidence suggesting that disulfide maturation occurs within Tg monomers engaged in each of the adduct bands. Moreover, the same Tg substrate molecules can form simultaneous mixed disulfides with both CaBP1/P5 and PDI although these are generally viewed as components of distinct oxidoreductasechaperone protein complexes. Such substrate-oxidoreductase combinations offer Tg the potential for simultaneous oxidative maturation along different parallel tracks leading to the native state.