HALF-1, a bZIP-type protein, interacting with the wheat transcription factor HBP-1a contains a novel transcriptional activation domain

Background: Nuclear factors bind to cis-acting elements and mediate transcriptional regulation through protein-protein interactions with other factors. The bZIP-type wheat nuclear protein HBP-1a(17) is a putative transcriptional activator specifically binding to the Hex (ccACGTCA) and G-box (CCACGTGG) motifs, which are often found in the cis-acting elements critical for various responses in plants. Results: In order to investigate the mechanisms for gene expression mediated via the Hex and G-box motifs, we attempted to isolate proteins interacting with HBP-1a(17) based on protein-protein interactions. A cDNA expression library from wheat seedlings was screened with P-32-labelled HBP-1a(17), and a bZIP-type protein, termed HALF-1 (HBP-1-associated leucine-zipper factor-1), was isolated. GST-pulldown assay, yeast two-hybrid system and EMSA showed that HALF-1 and HBP-1a(17) interact with each other through their leucine-zipper regions. Dissection experiments showed that HALF-1 has at least one potential trans-activation domain which includes a nine amino acid motif conserved between several plant bZIP-type proteins. This motif, named GCB (GBF-conserved box; consensus, NLNIGMDXW), activated the expression of a reporter gene, when fused to the GAL4 DNA-binding,domain. The corresponding region of Arabidopsis GBF1 also stimulated transcription. However, the trans-activation domain of HALF-1 did not function in yeast. Conclusions: We identified a novel trans-activation domain which contains the GCB motif conserved among plant bZIP-type factors. The trans-activation appears to be mediated by interaction between the GCB motif and a factor conserved in plants.