Compartment-specific phosphorylation of rat thyroid hormone receptor α1 regulates nuclear localization and retention

The thyroid hormone receptor α1 (TRα1) is a transcription factor, which can activate or repress gene expression in response to thyroid hormone. In addition, some of its actions, including DNA binding and transcriptional activation, are thought to be regulated by phosphorylation. Results presented here, using Xenopus oocyte microinjection assays, demonstrate that a phosphorylated form of rat TRα1 is present in the nucleus, whereas unphosphorylated TRα1 remains cytoplasmic. Changes in the phosphorylation state of TRα1 occur rapidly and point to the possibility that phosphorylation occurs in the nucleus. Furthermore, increasing the overall phosphorylation state of the cell leads to enhanced nuclear retention of TRα1, suggesting that compartment-specific phosphorylation regulates nuclear localization of TRα1. Enhanced nuclear retention of TRα1 is not dependent on phosphorylation of serine 12, a well-characterized casein kinase II site, nor is phosphorylation of this site necessary for import of TRα1 into the Xenopus oocyte nucleus. Similarly, mutational analysis in mammalian cells shows that nuclear localization and partitioning of TRα1 to the nuclear matrix are independent of serine 12 phosphorylation. Taken together, these studies suggest that phosphorylation of one or more sites in TRα1, excluding serine 12, enhances nuclear retention and/or inhibits nuclear export but is not directly involved in nuclear import.