Abstract
Trip230 is a novel coactivator of the thyroid hormone receptor that is negatively regulated by the retinoblastoma tumor-suppressor protein. In an examination of its subcellular distribution, Trip230 localized predominantly to the vicinity of the Golgi instead of the nucleus, as other nuclear hormone receptor coactivators. Using a series of deletion mutants, a critical region identified for Golgi area targeting coincided with a previously defined thyroid hormone receptor-binding domain of Trip230. During cell cycle progression, the expression level of Trip230 is constant and a significant portion is imported into the nucleus at S phase. Within an hour of treating cells with T3, Trip230 immunofluorescence transiently colocalized with TR in prominent subnuclear structures. T3-dependent nuclear import of Trip230 does not require new protein synthesis. Coincident with T3 treatment and nuclear import, newly phosphorylated residue(s) appeared in Trip230, suggesting that phosphorylation may be involved in its nuclear import. These findings provided a novel mechanism for the regulation of nuclear hormone transcription factors by hormone-responsive phosphorylation and nuclear import of cytoplasmically located coactivators.