Oligomeric assembly of thrombospondin in the endoplasmic reticulum of thyroid epithelial cells.

The thyroid endoplasmic reticulum (ER) provides an environment in which conformational maturation of thyroglobulin monomers occurs with progressive dissociation from BiP (a molecular chaperone), prior to thyroglobulin dimerization. This pattern of folding is thought to represent a pathway common to many exportable polypeptides. Thyrocytes also synthesize and secrete thrombospondin, an extracellular matrix glycoprotein that forms disulfide-linked trimers. Using a monoclonal antibody recognizing the N-terminal heparin-binding domain of thrombospondin, pulse-chase/immunoprecipitation experiments indicate that this epitope forms essentially cotranslationally. Dependent upon structural information contained within the N-terminal region, thrombospondin trimers also form and are rapidly stabilized by interchain disulfide bonds in the peritranslational period. Within 30 to 60 sec, a new epitope in the mid-molecule is detected. Additional approaches (including thrombospondin dissociation from BiP-an indirect measure of conformational maturation; t1/2 approximately 20 min) independently suggest that significant folding of monomers occurs within the trimer, i.e., well after oligomerization. These later events appear rate limiting for thrombospondin export from the ER (t1/2 approximately 30 min). The results highlight plasticity in the relationship between oligomerization and specific folding events for different proteins exported from the thyroid ER.