A protein’s intracellular location is an integral part of its function. Mislocalization caused by mutations within a protein can lead to protein dysfunction and leave an individual susceptible to diseases such as type II diabetes, certain cancers, and Resistance to Thyroid Hormone Syndrome (RTH). Such is the case for the thyroid hormone receptor α1 (TRα1), as certain mutations in TRα1’s ligand binding domain can cause RTH. TRα1 binds to thyroid hormone (T3) and acts as a transcription factor in the nucleus. As such, maintenance of TRα1’s nucleocytoplasmic shuttling and intranuclear dynamics is essential to ensure proper protein function. Prior studies have demonstrated that certain RTH TRα1 mutants have an increased affinity to one of TRα1’s coregulators, nuclear corepressor 1 (NCoR1), and form a more stable complex compared to wild type TRα1. This finding suggests that the intranuclear mobility of RTH TRα1 mutants would decrease relative to their wild-type counterpart, and that nucleocytoplasmic shuttling would be impacted due to having one of several nuclear export motifs (NES-H12) compromised. The major aim of this thesis was to evaluate the intranuclear dynamics of RTH TRα1 mutants E403X, Ala382ProfsX7, and F397fs406X in response to increased levels of NCoR1. The intranuclear dynamics of each mutant RTH TRα1 was evaluated in human cells using fluorescence recovery after photobleaching (FRAP), and statistically analyzed for a significant difference compared to wild type TRα1. A mutant NCoR1 that lacks TR interaction domains, NCoRΔID, was used to control for wild type NCoR1’s increased binding affinity to RTH TRα1 mutants. While overexpressing NCoR1 did not significantly impact RTH TRα1 mutant intranuclear mobility, NCoRΔID overexpression significantly decreased RTH-F397fs406X intranuclear mobility, with RTH-E403X showing a similar trend. Interestingly, RTH-A383PfsX7 did not appear to be affected by NCoRΔID. As a second measure of intracellular dynamics, the nucleocytoplasmic shuttling ability of RTH-A383PfsX7 and wild type TRα1 was evaluated using heterokaryon assays. Despite having a compromised or missing NES-H12, RTH-A383PfsX7 maintained shuttling capabilities qualitatively similar to those of wild type TRα1. These findings offer new insight into RTH TRα1’s molecular phenotype and warrant further investigation into the interaction between NCoR1/ NCoRΔID with TRα1 and RTH-E403X, Ala382ProfsX7, and F397fs406X, as well as evaluation of RTH-E403Xand F397fs406X’s nucleocytoplasmic shuttling.

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