Identification of RNA species with messenger activity in the thyroid gland

Iodination of thyroglobulin. An intracellular or extracellular process?

The location in the thyroid follicle of the iodination of thyroglobulin has been a matter of debate for several decades. This problem is not a question of mere academic interest. Knowledge of the locus--or loci--of iodination is necessary for a full understanding of the mechanisms involved in thyroid-hormone synthesis and release. In the discussion about this problem 3 fundamentally different views have been--and still are--advocated. The first view implies that the site of iodination is the follicle lumen, the second that iodination is an intracellular process restricted to some organelle(s) in the follicle cells and the third that iodination occurs at the interface between the follicle cells and the follicle lumen. Below I will survey the major observations on which these different opinions are based and discuss the validity of the interpretations.

Effect of thyrotropin on the properties of messenger RNA from cultured human thyroid cells

When added to the culture medium of thyroid cells isolated from diffuse nontoxic goiter, thyrotropin increased the poly(adenylic acid) content and the template activity of the unfractionated RNA. This increase was correlated with higher thyroglobulin messenger activity, as demonstrated by specific immunoprecipitation of the labeled peptides synthesized in two heterologous cell-free systems. When RNAs were separated in a sucrose gradient, thyrotropin was shown to enhance the poly(adenylic acid) content and template activity of fractions with sedimentation coefficients of 34, 23 and 15 S. Specific immunoprecipitation showed that a thyroglobulin messenger activity was present in these three fractions. Another way by which thyrotropin regulates the thyroid protein synthesis is suggested by the shift of poly(adenylic acid)-containing RNA to large polysomes when thyroid cells were cultured in the presence of the hormone.

Synthesis of high molecular weight thyroglobulin peptides by thyroid polysome in vitro

Bovine thyroid polysomes were isolated under conditions which had yielded large polysomes in other systems. Between 25 and 40% of the protein synthesized by these polysomes could be precipitated by thyroglobulin antibody. When these immunoprecipitates were separated by dodecyl sulfate-polyacrylamide gel electrophoresis (4% running gel), over 50% of the radioactivity was located in the regions of polypeptides greater than 100,000 daltons. Between 11 and 13% of the total radioactivity was found as a single peak co-migrating with the main band of bovine thyroglobulin (Mr = 330,000). Peaks of radioactivity were also found in regions of molecular weights between 130,000 and 200,000. When the immunoprecipitates were separated in a 10% running gel, about 50% of the radioactivity was located in the top 8 mm of the gel. Most of the remaining radioactivity was distributed in regions corresponding to molecular weights greater than 68,000. No peak of radioactivity was seen corresponding to peptides of 15,000 daltons.