The GCN4 bZIP can bind to noncognate gene regulatory sequences

We show that a minimalist basic region/leucine zipper (bZIP) hybrid, comprising the yeast GCN4 basic region and C/EBP leucine zipper, can target mammalian and other gene regulatory sequences naturally targeted by other bZIP and basic/helix-loop-helix (bHLH) proteins. We previously reported that this hybrid, wt bZIP, is capable of sequence-specific, high-affinity binding of DNA comparable to that of native GCN4 to the cognate AP-1 and CRE DNA sites. In this work, we used DNase I footprinting and electrophoretic mobility shift assay to show that wt bZIP can also specifically target noncognate gene regulatory sequences: C/EBP (CCAAT/enhancer binding protein, 5'-TTGCGCAA), XRE1 (Xenobiotic response element, 5'-TTGCGTGA), HRE (HIF response element, 5'-GCACGTAG), and the E-box (Enhancer box, 5'-CACGTG). Although wt bZIP still targets AP-1 with strongest affinity, both DNA-binding specificity and affinity are maintained with wt bZIP binding to noncognate gene regulatory sequences: the dissociation constant for wt bZIP in complex with AP-1 is 13 nM, while that for C/EBP is 120 nM, XRE1 240 nM, and E-box and HRE are in the microM range. These results demonstrate that the bZIP possesses the versatility to bind various sequences with varying affinities, illustrating the potential to fine-tune a designed protein's affinity for its DNA target. Thus, the bZIP scaffold may be a powerful tool in design of small, alpha-helical proteins with desired DNA recognition properties.     

The bZIP targets overlapping DNA subsites within a half-site, resulting in increased binding affinities

We previously reported that the wt bZIP, a hybrid of the GCN4 basic region and C/EBP leucine zipper, not only recognizes GCN4 cognate site AP-1 (TGACTCA) but also selectively targets noncognate DNA sites, in particular the C/EBP site (TTGCGCAA). In this work, we used electrophoretic mobility shift assay and DNase I footprinting to investigate the factors driving the high affinity between the wt bZIP and the C/EBP site. We found that on each strand of the C/EBP site, the wt bZIP recognizes two 4 bp subsites, TTGC and TGCG, which overlap to form the effective 5 bp half-site (TTGCG). The affinity of the wt bZIP for the overall 5 bp half-site is >or=10-fold stronger than that for either 4 bp subsite. Our results suggest that interactions of the wt bZIP with both subsites contribute to the strong affinity at the overall 5 bp half-site and, consequently, the C/EBP site. Accordingly, we propose that the wt bZIP undergoes conformational changes to slide between the two overlapping subsites on the same DNA strand and establish sequence-selective contacts with the different subsites. The proposed binding mechanism expands our understanding of what constitutes an actual DNA target site in protein-DNA interactions.     

135 Arranging enzymes and receptors with nanometer-scale precision on molecular switchboards

DNA is a useful material for constructing nanoscale structures in nearly any three-dimensional (3D) shape desired. The DNA nanostructure can also be equipped with specific docking sites for proteins. Cellular processes and chemical transformations take place in several reaction steps. Multiple enzymes cooperate in specific fashion to catalyze the sequential chemical transformation steps. Such natural systems are effectively reconstructed in vitro if the individual enzymes locate in the correct relative orientations. DNA-origami structures can be used as “molecular switchboards” to arrange enzymes and other proteins with nanometer-scale precision. A new method was developed for locating the proteins by means of special “adapters” known as zinc-finger proteins based only on proteins. Zinc fingers are suitable site-selective adapters for targeting specific locations within DNA-origami structures. Several different adapters carrying different proteins can independently bind at defined locations on this type of nanostructure. A basic leucine zipper (bZIP) protein is also a candidate for the site-selective adaptor. A well-characterized bZIP protein GCN4 was chosen as an adaptor for specific addresses. Analyses by atomic force microscopy and gel electrophoreses demonstrate specific binding of GCN4 adaptor to the addresses containing the GCN4 binding sites on DNA origami. The adaptor derived from GCN4 and that form a zinc-finger protein zif268, for which we have reported previously, acted as orthogonal adaptors to the respective addresses on DNA origami. Therefore, these orthogonal adaptors would be useful to place multiple engineered proteins at different addresses on DNA origami. Especially, the homodimeric nature of GCN4 adaptor is indispensable for constructing the assembly of the naturally abundant dimeric proteins and/or enzymes to efficiently carry out chemical reactions and signal transductions in vitro on DNA origami.