Localization of the incorporation of 3H-galactose and 3H-sialic acid into thyroglobulin in relation to the block of intracellular transport induced by monensin. Studies with isolated porcine thyroid follicles.

The Na+/K+ ionophore monensin is known to arrest the intracellular transport of newly synthesized proteins in the Golgi complex. In the present investigation the effect of monensin on the secretion of 3H-galactose-labeled and 3H-sialic acid-labeled thyroglobulin was studied in open thyroid follicles isolated from porcine thyroid tissue. Follicles were incubated with 3H-galactose at 20 degrees C for 1 h; at this temperature the labeled thyroglobulin remains in the labeling compartment (Ring et al. 1987a). The follicles were then chased at 37 degrees C for 1 h in the absence or presence of 1 microM monensin. Without monensin substantial amounts of labeled thyroglobulin were secreted into the medium, whereas in the presence of the ionophore secretion was inhibited by 80%. Since we have previously shown (Ring et al. 1987b) that monensin does not inhibit secretion of thyroglobulin present on the distal side of the monensin block we conclude that galactose is incorporated into thyroglobulin on the proximal side of this block. Secretion was also measured in follicles continuously incubated with 3H-galactose for 1 h at 37 degrees C in the absence or presence of monensin. In these experiments secretion of labeled thyroglobulin was inhibited by about 85% in the presence of monensin. Identically designed experiments with 3H-N-acetylmannosamine, a precursor of sialic acid, gave similar results, i.e., almost complete inhibition of secretion of labeled thyroglobulin in the presence of monensin. The agreement between the results of the galactose and sialic acid experiments indicates that sialic acid, like galactose, is incorporated into thyroglobulin on the proximal side of the monensin block.(ABSTRACT TRUNCATED AT 250 WORDS)