ENHANCED BINDING TO THE MOLECULAR CHAPERONE BIP SLOWS THYROGLOBULIN EXPORT FROM THE ENDOPLASMIC-RETICULUM

To examine how binding of BiP (a molecular chaperone of the hsp70 fami ly that resides in the endoplasmic reticulum) influences the conformat ional maturation of thyroglobulin (Tg, the precursor for thyroid hormo ne synthesis), we have developed a system of recombinant Tg stably exp ressed in wild-type Chinese hamster ovary (CHO) cells and CHO-B cells genetically manipulated for selectively increased BiP expression. The elevation of immunoreactive BiP in CHO-B cells is comparable to that s een during the unfolded protein response in the thyrocytes of certain human patients and animals suffering from congenital hypothyroid goite r with defective Tg. However, in CHO-B cells, we expressed Tg containi ng no mutations that induce misfolding (i.e. no unfolded protein respo nse), so that levels of all other endoplasmic reticulum chaperones wer e normal. Increased availability of BiP did not accelerate Tg secretio n; rather, the export of newly synthesized Tg was delayed. Tg detained intracellularly was concentrated in the endoplasmic reticulum. By coi mmunoprecipitation, BiP exhibited enhanced binding to Tg in CHO-B cell s. Moreover, two-dimensional gel analysis showed that BiP associated e specially well with intracellular Tg containing mispaired disulfide bo nds, thought to represent early Tg folding intermediates. An endoplasm ic reticulum chaperone of the hsp90 family, GRP94, was also associated in Tg-chaperone complexes. The results suggest that increased binding of BiP to Tg leads to its delayed conformational maturation in the en doplasmic reticulum.