Nuclear localization domains in human thyroid transcription factor 2.

Abstract Thyroid transcription factor-2 (TTF2) is a nuclear protein involved in morphogenesis and gene expression in thyroid gland, belonging to the family of the forkhead /winged-helix transcription factors. In the present study we have investigated the sequence determinants for transport and accumulation into the nucleus of the TTF2 protein. By transient expression of fusion proteins constructed by joining different parts of TTF2 to the reporter gene of the jellyfish green fluorescent protein (GFP) and, in a separate set of deleted constructs, the glutathione S -transferase (GST) coding sequence, we have demonstrated that a basic amino acid stretch present at both ends of the DNA-binding domain is a bona fide nuclear localization signal (NLS). We have analyzed the subcellular localization of deleted GFP-GST-TTF2 fusion proteins and have shown that residues inside the forkhead domain (FHD) contributed to the complete nuclear TTF2 protein accumulation. Furthermore, by means of GST binding assays we have shown that distinct TTF2 fragments, containing the NLS, were able to bind the nuclear import receptor importin α. Taken together, our results provide the first documentation about nuclear targeting of a forkhead protein containing two identical NLS signal flanking the DNA-binding domain.