The subunit structure of thyroglobulin.

Human and rat thyroglobulin were reduced and alkylated in aqueous alkaline conditions in the absence of denaturants; the product of reduction in both cases has been found to have mol.wt. about 165000, or one-quarter that of the native molecule.     

Properties of thyroglobulin. XII. Comparison of the configurational states of reduced and unreduced thyroglobulin

The relaxation time of thyroglobulin has been determined in water at. neutral pH, in concentrated urea and guanidine solutions, at alkaline pH, both before and after reduction with β-mercaptoethanol. The structure of thyroglobulin in concentrated urea solutions is markedly affected by the pH, Time-dependent changes occur in thyroglobulin in concentrated urea or guanidine solutions which arc observable by polarization of fluorescence but not by optical rotation or viscosity. The reduction of the disulfide crosslinks of thyroglobulin in urea at high pH or in guanidine produces linear polypeptide chains with few if any permanent contacts between segments.     

Reverse transcription of thyroglobulin 33-S mRNA.

Bovine thyroglobulin 33-S mRNA has been used as a template for the synthesis of a complementary DNA, using RNA-directed DNA polymerase from the avian myeloblastosis virus. The yield of the reaction was relatively poor and the size of the cDNA did not exceed 10 S. Nevertheless, a copy of high specific radioactivity (approximately 10(7) counts. min-1 microgram-1) could be obtained which hybridized specifically back to its template with an rot1/2 value about 5 times higher than that observed in hybridizations between hemoglobin mRNA (alpha + beta chain) and hemoglobin cDNA. This suggests that thyroglobulin mRNA does not contain extensive internal repetitive sequences. Quantification of thyroglobulin mRNA sequences among various RNA preparations from the beef thyroid was performed using cDNA/RNA hybridizations in RNA excess. The results confirmed that thyroglobulin mRNA represents the large majority of mRNA in membrane-bound polysomes and indicated the virtual absence of thyroglobulin sequences on free polyosomes. The cDNA transcribed from mRNA of bovine origin hybridized efficiently with thyroid RNA from goats, dogs and humans. Although the heterologuous hybrids exhibited the expected decrease in thermal stability, the bovine cDNA provides an appropriate probe for studies dealing with the expression of the thyroglobulin gene in various mammals including man.     

Restricted antigenicity of thyroxyls in human thyroglobulin.

Interactions between two human iodothyronine-binding autoantisera and three preparations of human thyroglobulin (Tg) were not proportional to the latter's thyroxyl residue content. Probably only one of the several thyroxyl-containing sites in Tg reacted with the immunoglobulins from both antisera. In the case of one of the antisera, which was thyroxine (T4)-specific, the thyroxyl residue was the immunodominant feature of the antigenic site. The other antiserum, which had a specificity for 3,5,3'-tri-iodothyronine (T3), recognized different determinants around the same thyroxyl residue, but this residue was not itself an important element of the binding site. Thus, despite the specificity for T3 free in solution, the presence of T4 in the complete antigenic site was tolerated, since other structures supplied the bulk of the binding energy. 'Specificity' of this antiserum for T3 in solution is therefore coincidental and need not be ascribed to the presence of T3 in the original immunogen. Some results obtained in these studies may be interpreted as supporting the possibility that a modified Tg was the immunogen for the generation of these naturally occurring human antisera.     

Electron microscopy of low iodinated thyroglobulin molecules.

Thyroglobulin molecules were studied in the electron microscope with negative staining technique. In a first series of experiments samples of thyroglobulin varying in iodine content from 0.5 to 0.03% were prepared from the thyroids of mice and rats kept on iodine-poor diets. All samples contained thyroglobulin molecules of the normal ovoid shape, not deviating in size or shape from molecules obtained from normal thyroids. However, in addition, another type of molecule having a cylindrical shape was observed in all samples. The proportion of these cylindrical molecules increased from a few per cent in the moderately iodine-poor thyroglobulin samples to more than 80% in the highly iodine-deficient thyroglobulin (0.03%). In a second series of experiments extremely iodine-poor thyroglobulin (smaller than 0.005%) was obtained from propylthiouracil-treated rats. In these preparations practically all molecules had a cylindrical shape. These samples also contained smaller particles interpreted to be dissociation products. The cylindrical molecules were of two types, one appearing compact and measuring 250 times 135 A (length times diameter) and the other appearing porous and having a length of 145 and a diameter of 205 A. It is concluded that the cylindrical molecules represent non- or low-iodinated thyroglobulin and it is suggested that the porous cylindrical molecule is an unfolded form of the compact cylinder.     

Elementary chain composition of guinea pig thyroglobulin.

Thyroglobulin obtained from guinea pigs was examined by Na dodecyl-SO4-polyacrylamide gel electrophoresis after reduction and alkylation. In contrast to thyroglobulin from other mammalian sources, only three groups of polypeptide chains accounted for 95% or more of the protein. Determinations of the molecular weights of these purified proteins by equilibrium centrifugation in 6 M guanidine HCl gave values of 295,000 (species A), 210,000 (species B), and 110,000 (species C). Molecular weights determined by gel filtration in 6 M guanidine HCl gave similar results. Due to the large size of the polypeptides, satisfactory molecular weights could not be obtained from Na dodecyl-SO4-polyacrylamide gel electrophoresis. Amino acid analysis of the three species was similar to that of whole thyroglobulin. Only slightly higher level of lysine and histidine and a lower level of glutamic acid were seen in species C. The iodine contents were found to range from 0.07 to 0.12 to 0.20% for species A, B, and C, respectively.     

Structure-function relationship in thyroglobulin: amino acid sequence of two different thyroxine-containing peptides from porcine thyroglobulin.

From the cyanogen bromide (CNBr) treatment of porcine thyroglobulin a peptide of mol. wt. 15 000, CNBr-b1, was purified by gel filtration and ion-exchange chromatography. CNBr-b1 contained 50% of the thyroxine (T4) content of the protein. After digestion with trypsin and protease from Staphylococcus aureus V-8, thyroxine-containing peptides were purified and analyzed by microsequence analysis using the colored Edman's reagent dimethylaminoazobenzeneisothiocyanate . Two different sequences harboring T4 were identified: sequence 1, His-Asp-Asp-Asp-T4-Ala-Thr-(Glx,Gly)-Leu-Tyr-Phe-Ser-Ser-Arg, which contains 1 mol T4/mol peptide and sequence 2, Asp-(Tyr/MIT/DIT/T4)-Phe-Ile-Leu-X-Pro-Val-, which is a mixture of the same peptide at different levels of iodination and coupling. These sequences are likely to be representative of distinct hormonogenic sites, the former giving evidence of early iodinated tyrosine residues where preferential coupling into hormonal residues occurs especially at low iodine levels and the latter representing less reactive site(s) operative at higher iodine levels.     

Process of iodination of thyroglobulin and its maturation. I. Properties and distribution of thyroglobulin labeled with radioiodine in pig thyroid slices.

Pig thyroid slices were incubated with Na131I and the 17--19S 131I-labeled thyroglobulin isolated was subjected to dissociation with 0.3 mM sodium dodecyl sulphate SDS) on sucrose density gradient centrifugation and to iodoamino acid analysis. During the incubation, initially dissociable thyroglobulin was gradually altered to 0.3 mM SDS-resistant species with increasing incorporation of iodine. Microsome-bound, poorly iodinated thyroglobulin and preformed thyroglobulin were chemically iodinated and then subjected to analysis of dissociability and iodoamino acid contents with newly incorporated iodine. The results indicated that the behavior of the former thyroglobulin resembled that of 131I-thyroglobulin obtained from the slices. Then, thyroid slices were incubated for 3 min with Na131I and 3H-leucine with or without 10-min chase incubation. The sucrose density gradient centrifugation patterns of 131I and 3H-radioactivity of cytoplasmic extracts indicated that 131I-thyroglobulin is contained in particulates, especially in vesicles with low density(d=1.12) and that some of them are released into the soluble fraction within 10 min. The vesicles contained peroxidase and NADH-cytochrome c reductase, and are probably exocytotic vesicles in the apical area of cytoplasm of follicular cells. No positive evidence was obtained that plasma membranes participate in the iodination of thyroglobulin under the present experimental conditions. These results suggest that, in the incubation of thyroid slices, iodine atoms are preferentially incorporated into newly synthesized, less iodinated thyroglobulin, rather than preformed thyroglobulin, and that the iodination occurs, at least to a certain degree, in apical vesicles before the thyroglobulin is secreted into the colloid lumen.