Dynamics of biosynthesis of thyroglobulin subunits and their polymerization in rabbit thyroid slices in vitro

The dynamics of biosynthesis and aggregation of subunits into thyroglobulin (TG) was studied in vitro in rabbit thyroid slices incubated from 5 to 300 min in the presence of I-14C leucine. The incorporation of labelled amino acid was followed in total soluble and microsome-bound proteins, as well as its distribution in soluble protein fractions. The incorporation of lebelled amino acid into soluble and microsome-bound proteins increased with time of incubation. The analysis of individual soluble proteins indicated that the label was incorporated very early, not only into 3--8S, but also into a protein which corresponds to the 12S fraction. Maximal incorporation into 12S protein was achieved after 60 min of incubation and then the intensity of incorporation decreased, followed by an increase in the relative and absolute amount of TG. The appearance of 14C-leucine in the TG region was not observed before 30 min of incubation. The dynamics of incorporation of 14C-leucine into thyroid proteins indicates very rapid transformation of newly synthesized 12S subunits into TG. After two hours of incubation the newly synthesized TG already showed the same sedimentation properties as pre-formed rabbit TG or native rat 19S TG-125I thus demonstrating that maturation, similar to polymerization, is a very rapid process, i. e. the newly formed TG is quickly transformed into its mature form. Our previous studies and the results presented in this work suggest that 12S protein may also have a precursor character. The presence of 12S protein in thyroid extract is not always the result of dissociation of TG, but also a consequence of polymerization of the basic subunits of TG.