THYROGLOBULIN SYNTHESIS IN A THYROID POLYRIBOSOMAL CELL-FREE SYSTEM

Thyroglobulin (Tg), the major protein of the thyroid gland, has a molecular weight of 660.000 and a sedimentation coefficient close to 19 S (Edelhoch 1965). It contains 8,5% carbohydrate, under the form of 23 oligosaccharides of 2 types: a small unit containing only mannose and N-acetylglucosamine, and a larger unit, with a complex structure, containing in addition sialic acid, fucose and galactose (Cheftel et al. 1964, Spiro 1965). Thyroid polyribosomal cell-free systems have been used by several authors in an attempt to dissociate the steps leading to the completion of the Tg molecule (Cartouzou et al., 1967; Nunez et al., 1967; Morais and Goldberg, 1967; Soffer 1967; Kondo et al., 1967, 1968a, 1968b). Conflicting results were reported. Although such systems can synthesize protein fragments immunochemically related to thyroglobulin, all but one group came to the conclusion that additional factors, related to the membrane components of the cell, are required for the completion of the molecule (Nunez et al., 1967). This report presents evidence for the synthesis of 19 S Tg in a polyribosomal cell free system, and suggests as explanation a mechanism based on an exchange between newly synthesized subunits and subunits of the native Tg molecule. © 1969.