Thyroglobulin is selected as luminal protein cargo for apical transport via detergent-resistant membranes in epithelial cells

Thyroid hormone synthesis by thyrocytes depends upon apical secretion of th yroglobulin (Tg), the glycoprotein prohormone, In stably transfected MDCK c ells, recombinant Tg is also secreted apically. All secreted Tg has undergo ne Golgi carbohydrate modification, whereas most intracellular Tg (which is slow to exit the endoplasmic reticulum) is sensitive to digestion with end oglycosidase H. However, in MDCK cells and PC C13 thyrocytes, a subpopulati on of newly synthesized recombinant and endogenous Tg, respectively, is rec overed in a Triton X-100 insoluble, glycosphingolipid/cholesterol-enriched (GEM/raft) fraction, and this small subpopulation is overwhelmingly endogly cosidase H resistant. Upon apical secretion, Tg solubility is restored. Api cal secretion of Tg is inhibited by cellular cholesterol depletion. In FRT cells, recombinant Tg becomes Triton X-100 insoluble within 60 min after sy nthesis and a portion is actually endoglycosidase H-sensitive, suggesting e arly Tg entry into GEMs/rafts, Interestingly in FRT cells,Tg remains associ ated with the apical plasma membrane upon exocytosis, and all surface Tg is GEM/raft-associated. Thus, Tg is the first secretory protein demonstrated to enter Triton X-100 insoluble membranes en route to the apical surface of epithelial cells. The data imply that Tg utilizes a cargo-selective mechan ism for apical sorting.