THYROGLOBULIN TRANSPORT ALONG THE SECRETORY PATHWAY - INVESTIGATION OF THE ROLE OF MOLECULAR CHAPERONE, GRP94, IN PROTEIN EXPORT FROM THE ENDOPLASMIC-RETICULUM

GRP94 serves as a molecular chaperone in the endoplasmic reticulum (ER ), In normal thyrocytes, GRP94 interacts transiently with thyroglobuli n (Tg), and in thyrocytes of animals suffering from congenital hypothy roid goiter with defective thyroglobulin, GRP94 and thyroglobulin asso ciate in a protracted fashion. In order explore possible consequences of GRP94 binding, we have studied recombinant nonmutant thyroglobulin expressed in control Chinese hamster ovary (CHO) cells in comparison t o that produced in CHO cells genetically manipulated for selectively i ncreased GRP94 expression. Levels of ER chaperones other than GRP94 di d not detectably differ, and thyroglobulin achieved transport competen ce in both kinds of CHO cells. However, increased availability of GRP9 4 caused the residence time of Tg in the ER to be remarkably prolonged , This was accompanied by a major increase in Tg directly associated w ith GRP94 and an increase in the ER pool size of Tg. Importantly, co-i mmunoprecipitation analysis revealed disulfide-linked Tg complexes (pr eviously reported as an early Tg-folding intermediate) especially asso ciated with GRP94. Indeed, non-native Tg, GRP94, and a 78-kDa protein likely to be BiP, appeared in ternary complexes. Under these condition s, GRP94 association appears directly involved in prolongation of Tg f olding and export, consistent with a role in quality control in the ER .