Quantitation of endogenous thyroid hormone receptors α and β during embryogenesis and metamorphosis in Xenopus laevis (original) (raw)

Greater than 90% of the endogenous thyroid hormone receptor proteins TRa and TRP in tissues of Xenopus Zaevis comigrate with their respective in uitro synthesized counterparts, and these major components are not phosphorylated detectably. Maternally inherited TRa protein is stable through early embryogenesis during a time in which there is no detectable TRa mRNA synthesis. At stage 35 when TRa mRNA is first detectable, the inherited T R a protein is present at about 100 molecules/ cell. TRa protein subsequently increases to levels of about 1500 and 6000 moleculedcell in tail and head regions, respectively, in stage 52 tadpoles. Even though TRa mRNA gradually increases during metamorphosis (from stage 52 to 62), TRa protein remains constant, suggesting strongly that post-transcriptional events control the ultimate levels of TRa protein. In contrast, there is no detectable TRP protein (less than 100 moleculedcell) throughout embryogenesis until stage 52. Both TRP mRNA and protein rise along with the increase in endogenous thyroid hormone, reaching a maximum at the climax of metamorphosis, when TRP protein exceeds TRa protein in concentration. As with TRa protein, TR/3 protein in tail is consistently about one-fourth that of T R p protein in the head region. The number of T R a protein molecules in extracts of premetamorphic tadpoles and cultured cells grown in the absence of thyroid hormone fully accounts for all of the sites to which "'I-T, bind. W e interpret this to mean that TRa protein must be a necessary, if not sufficient, component in the pathway toward metamorphosis triggered by thyroid hormone and required for the phenomenon of competence in tissues and cells.