Molecular organization of 19 S calf thyroglobulin

Controlled freezing and thawing of 19 S calf thyroglobulin resulted in a specific and reproducible breakdown of the protein. Beside the elementary chain (300,000 Da), new, discrete bands are revealed by gel electrophoresis in sodium dodecyl sulfate under reducing conditions. These new species consist of a major peptide of 100,000 Da and several faster-migrating bands. Most of these polypeptides were purified by preparative gel electrophoresis and individually digested in formic acid with CNBr. A comparative gel electrophoresis under denaturating and reducing conditions of (i) the fragments obtained from the native protein, (ii) the electrophoretically purified elementary chain, (iii) the 100,000-Da peptide, and (iv) a smaller fragment (of about 50,000) was performed. It revealed a very close homology among the peptide maps of the intact 19 S, the elementary chain, and the 100,000-Da peptide. Furthermore, it was shown that the digestion products of the smaller fragment, present in the peptide map of the native protein, were absent in both the elementary chain and the 100,000-Da species. These results support the idea that calf thyroglobulin, even though it has an apparently complex molecular organization, contains structural motifs which are repeated in the elementary chain.