Structural and biochemical studies on the mechanism and the role of HCF-1 self-association and nuclear localization signal in VP16-induced complex formation

Abstract

Host cell factor 1 (HCF-1) is a highly conserved transcriptional regulator that participates in formation of various transcription regulatory complexes. HCF-1 undergoes an unusual proteolytic maturation process to generate $HCF-1_N$ and $HCF-1_C$ subunits that remain noncovalently associated via self-association sequence elements. Here, I present the crystal structure of the self-association sequence 1 (SAS1) and the C-terminal nuclear localization signal (NLS) showing that the SAS1 elements from each of the $HCF-1_N$ and $HCF-1_C$ promotes the association by forming an interdigitated fibronectin type 3 (Fn3) tandem repeat structure. I also show that the C-terminal NLS is required for effective formation of a transcriptional regulatory complex: the herpes simplex virus VP16-induced complex formation. Interestingly, the basic residues of NLS play critical role in both VIC formation and nuclear localization. Thus, this study reveals that the role of self-association via an interdigitated Fn3 structure is to hold the $HCF-1_N$ and $HCF-1_C$ subunits together to facilitate a transcriptional regulatory complex formation by recruiting the NLS. In addition, these results present a novel function of NLS to promote the VIC formation suggesting that the NLS of HCF-1 has dual function in localizing HCF-1 to the nucleus and promoting VIC formation.