The tandem zinc-finger region of human ZHX adopts a novel C2H2 zinc finger structure with a C-terminal extension.

Binding of the nuclear factor-Y complex (NF-Y) to the inverted CCAAT-box interferes with transcription activation through nucleosome reorganization. The three homologous proteins forming the zinc-fingers and homeoboxes (ZHX) family interact with the activation domain of NF-Ya to repress transcription. Each ZHX-protein contains two generic C2H2 zinc-fingers (ZNF1 and ZNF2) followed by five homeodomains. Although the proteins have been related to the occurrence of certain cancers, the function and structure of the individual ZHX domains are still unknown. Here, we determined the structure of the tandem zinc-finger region of human ZHX1. Folding and secondary structure predictions combined with expression screening revealed that the C-terminal extension (E) to ZNF2 could form a single domain with the two hZHX1 zinc-fingers. We therefore decided to determine the solution structure of the zinc-fingers followed by this extension. We show that both zinc-fingers adopt canonical betabetaalpha-folds in which a zinc ion is coordinated by two cysteine and two histidine residues. The C-terminal extension to ZNF2 forms two beta-strands to make a beta-sheet with the beta-strands of this zinc-finger. The ZNF1 and ZNF2-E domains do not show evident contacts and their mutual orientation seems variable. The high degree of sequence conservation among ZHX family members permitted us to prepare homology models for ZHX2 and ZHX3, revealing distinct surface characteristics for each family member. Implications of these structural features for ZHX-functioning in transcription regulation are discussed.