Identification and characterization of a nuclear localization signal of TRIM28 that overlaps with the HP1 box.

Tripartite motif-containing 28 (TRIM28) is a transcription regulator, which forms a repressor complex containing heterochromatin protein 1 (HP1). Here, we report identification of a nuclear localization signal (NLS) within the 462-494 amino acid region of TRIM28 that overlaps with its HP1 binding site, HP1 box. GST-pulldown experiments revealed the interaction of the arginine-rich TRIM28 NLS with various importin α subtypes (α1, α2 and α4). In vitro transport assay demonstrated that nuclear localization of GFP-TRIM28 NLS is mediated by importin αs, in conjunction with importin β1 and Ran. Further, we demonstrated that HP1 and importin αs compete for binding to TRIM28. Together, our findings suggest that importin α has an essential role in the nuclear delivery and preferential HP1 interaction of TRIM28.