Molecular characterization of the DNA-binding and dimerization domains of the bZIP transcription factor, EmBP-1

The wheat basic-leucine zipper (bZIP) DNA-binding protein EmBP-1 has been implicated in the mechanisms of abscisic acid (ABA) mediated gene regulation. Sequence and structural homology to the yeast bZIP protein GCN4 has been used to predict the location of the functional domains of EmBP-1. In order to test these predictions, the presumptive DNA-binding and dimerization domains of EmBP-1 were mapped by producing a series of truncated protein fragments and functionally testing them in vitro. Deletion of 5 amino acids of the predicted basic domain resulted in a loss of all DNA-binding activity. A fragment containing all six leucine repeat elements showed strong DNA-binding activity. Sequential deletion of the leucine repeat elements resulted in first an increase in DNA-binding activity (-L6 and -L5) followed by a reduction in binding activity (-L4) and eventually complete elimination of all detectable DNA-binding activity (-L3 and -L2). This demonstrates the importance of an intact leucine zipper domain of at least 4 repeat elements for efficient DNA-binding. The smallest polypeptide that retained DNA-binding activity is a fragment spanning amino acid residues 248-308 (ca. 8.4 kDa) consisting of minimal basic and leucine zipper domains. Dimerization of EmBP-1 was demonstrated by co-translation of fragments of differing molecular weights and identification of a DNA-protein complex with intermediate mobility to that produced by each fragment alone. A unique leucine-proline repeat element found N-terminal to the DNA-binding domain of EmBP-1 does not appear to play a role in DNA-binding or dimerization. These results confirm the locations of the functional domains of EmBP-1 predicted by similarity to GCN4. The high degree of functional conservation of the bZIP proteins spanning organisms from plants to fungi highlights the ancient origin of this class of transcription factors and of the mechanisms of gene regulation in which they participate.