Fucosylation of Glycoproteins Begins in the Rough Endoplasmic Reticulum of Mouse Active Thyrotrophs

Our aim was to determine whether fucosylation of glycoproteins begins in the rough endoplasmic reticulum (RER) of active thyrotrophs. This would contrast with most cells studied, in which fucosylation generally is associated with the Golgi apparatus. Mouse thyrotropic tumor tissue was incubated with [³⁵S]methionine for 2, 5, 7, 10, 30, and 90 minutes. TSH and free α-subunits were immunoprecipitated from cell lysates, and they displayed a time-dependent increase in affinity for lentil lectin (which binds oligosaccharides having core fucose), even at short times. Since no 20-30 minute lag in onset of TSH- and free α-subunit-lentil binding was appreciated, as might have been expected had fucosylation begun only in the Golgi, it appeared that fucosylation was beginning in the RER of thyrotrophs. Pituitary tissue from euthyroid and hypothyroid mice was incubated with [³H]fucose, then subjected to electron microscopic autoradiography. The pituitaries of hypothyroid mice had numerous "thyroidectomy cells," which had 40% of silver grains over dilated cisternae of RER. "Nonthyroidectomy" cells had few silver grains over RER; most were over secretory granules and Golgi areas. Thus, active mouse thyrotrophs appear to shift the subcellular site of fucosylation partially from Golgi to RER, and this phenomenon may represent one cellular mechanism whereby the endocrine regulation of the structure of TSH oligosaccharides is accomplished.