The GCN4 bZIP targets noncognate gene regulatory sequences: quantitative investigation of binding at full and half sites

We previously reported that a basic region/leucine zipper (bZIP) protein, a hybrid of the GCN4 basic region and C/EBP leucine zipper, not only recognizes cognate target sites AP-1 (5'-TGACTCA-3') and cAMP-response element (CRE) (5'-TGACGTCA-3') but also binds selectively to noncognate DNA sites: C/EBP (CCAAT/enhancer binding protein, 5'-TTGCGCAA), XRE1 (xenobiotic response element, 5'-TTGCGTGA), HRE (HIF response element, 5'-GCACGTAG), and E-box (5'-CACGTG). In this work, we used electrophoretic mobility shift assay (EMSA) and circular dichroism (CD) for more extensive characterization of the binding of wt bZIP dimer to noncognate sites as well as full- and half-site derivatives, and we examined changes in flanking sequences. Quantitative EMSA titrations were used to measure dissociation constants of this hybrid, wt bZIP, to DNA duplexes: Full-site binding affinities gradually decrease from cognate sites AP-1 and CRE with Kd values of 13 and 12 nM, respectively, to noncognate sites with Kd values of 120 nM to low microM. DNA-binding selectivity at half sites is maintained; however, half-site binding affinities sharply decrease from the cognate half site (Kd = 84 nM) to noncognate half sites (all Kd values > 2 microM). CD shows that comparable levels of alpha-helical structure are induced in wt bZIP upon binding to cognate AP-1 or noncognate sites. Thus, noncognate sites may contribute to preorganization of stable protein structure before binding target DNA sites. This work demonstrates that the bZIP scaffold may be a powerful tool in the design of small, alpha-helical proteins with desired DNA recognition properties.     

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.     

Identification of C/EBP basic region residues involved in DNA sequence recognition and half-site spacing preference.

C/EBP and GCN4 are basic region-leucine zipper (bZIP) DNA-binding proteins that recognize the dyad-symmetric sequences ATTGCGCAAT and ATGAGTCAT, respectively. The sequence specificities of these and other bZIP proteins are determined by their alpha-helical basic regions, which are related at the primary sequence level. To identify amino acids that are responsible for the different DNA sequence specificities of C/EBP and GCN4, two kinds of hybrid proteins were constructed: GCN4-C/EBP chimeras fused at various positions in the basic region and substitution mutants in which GCN4 basic region amino acids were replaced by the corresponding residues from C/EBP. On the basis of the DNA-binding characteristics of these hybrid proteins, three residues that contribute significantly to the differences in C/EBP and GCN4 binding specificity were defined. These residues are clustered along one face of the basic region alpha helix. Two of these specificity residues were not identified as DNA-contacting amino acids in a recently reported crystal structure of a GCN4-DNA complex, suggesting that the residues used by C/EBP and GCN4 to make base contacts are not identical. A random binding site selection procedure also was used to define the optimal recognition sequences for three of the GCN4-C/EBP fusion proteins. These experiments identify an element spanning the hinge region between the basic region and leucine zipper domains that dictates optimal half-site spacing (either directly abutted for C/EBP or overlapping by one base pair for GCN4) in high-affinity binding sites for these two proteins.     

A comparison of the different DNA binding specificities of the bZip proteins C/EBP and GCN4.

The bZip proteins GCN4 and C/EBP differ in their DNA binding specificities: GCN4 binds well to the pseudopalindromic AP1 site 5'-A4T3G2A1C0T1C2'A3'T4'-3' and to the palindromic ATF/CREB sequence 5'-A4T3G2A1-C0*G0'T1'C2'A3'T4'-3'; C/EBP preferentially recognizes the palindromic sequence 5'-A4T3T2G1C0*G0'C1'A2'-A3'T4'-3'. According to the X-ray structures of GCN4-DNA complexes, five residues of the basic region of GCN4 are involved in specific base contacts: asparagine -18, alanine -15, alanine -14, serine -11 and arginine -10 (numbered relative to the start point of the leucine zipper, which we define as +1). In the basic region of C/EBP position -14 is occupied by valine instead of alanine, the other four residues being identical. Here we analyse the role of valine -14 in C/EBP-DNA complex formation. Starting from a C/EBP-GCN4 chimeric bZip peptide which displays C/EBP specificity, we systematically mutated position -14 of its basic region and characterized the DNA binding specificities of the 20 possible different peptides by gel mobility shift assays with various target sites. We present evidence that valine -14 of C/EBP interacts more strongly with thymine 2 than with cytosine 1' of the C/EBP binding site, unlike the corresponding alanine -14 of GCN4, which exclusively contacts thymine 1' of the GCN4 binding sites.     

The bZIP mutant CEBPB (V285A) has sequence specific DNA binding propensities similar to CREB1

The bZIP homodimers CEBPB and CREB1 bind DNA containing methylated cytosines differently. CREB1 binds stronger to the C/EBP half-site GCAA when the cytosine is methylated. For CEBPB, methylation of the same cytosine does not affect DNA binding. The X-ray structure of CREB1 binding the half site GTCA identifies an alanine in the DNA binding region interacting with the methyl group of T, structurally analogous to the methyl group of methylated C. This alanine is replaced with a valine in CEBPB. To explore the contribution of this amino acid to binding with methylated cytosine of the GCAA half-site, we made the reciprocal mutants CEBPB(V285A) and CREB1(A297V) and used protein binding microarrays (PBM) to examine binding to four types of double-stranded DNA (dsDNA): 1) DNA with cytosine in both strands (DNA(C|C)), 2) DNA with 5-methylcytosine (M) in one strand and cytosine in the second strand (DNA(M|C)), 3) DNA with 5-hydroxymethylcytosine (H) in one strand and cytosine in the second strand (DNA(H|C)), and 4) DNA with both cytosines in all CG dinucleotides containing 5-methylcytosine (DNA(5mCG)). When binding to DNA(C|C), CEBPB (V285A) preferentially binds the CRE consensus motif (TGACGTCA), similar to CREB1. The reciprocal mutant, CREB1(A297V) binds DNA with some similarity to CEBPB, with strongest binding to the methylated PAR site 8-mer TTACGTAA. These data demonstrate that V285 residue inhibits CEBPB binding to methylated cytosine of the GCAA half-site.     

The effect of C-terminal helix stabilization on specific DNA binding by monomeric GCN4 peptides.

DNA binding by a 29-residue, monomeric, GCN4 basic region peptide, GCN4br, as well as by peptide br-C, a monomeric basic-region analogue that is helix stabilized at its C-terminal end by a Lys25. Asp29 side-chain lactam-bridged alanine-rich sequence, was studied at 25 C in an aqueous buffer containing 100 mm NaCl. Mixing of both peptides with duplex DNA containing the cAMP-responsive element (CRE) was accompanied by significant helix stabilization in the peptides, whereas mixing of the peptides with duplex DNA containing a scrambled CRE site was not. Peptide NBD-br-C was synthesized as a fluorescent probe to evaluate these peptide-DNA interactions further. Quantitative analysis of the fluorescence quenching of peptide NBD-br-C by CRE half-site DNA indicated the formation of a 1:1 complex with a dissociation constant of 1.41 +/- 0.22 microm. Competitive displacement fluorescence assays of CRE half-site binding gave dissociation constants of 0.65 +/- 0.09 microm for peptide br-C and 3.9 +/- 0.5 microM for GCN4br, which corresponds to a free energy difference of 1.1 kcal/mol that is attributed to the helix stabilization achieved in peptide br-C. This result indicates that helix initiation by the alpha-helical leucine zipper dimerization motif in native bzip proteins, such as GCN4, contributes significantly to the affinity of basic region peptides for their recognition sites on DNA. Our fluorescence assay should also prove useful for determining dissociation constants for CRE binding by other GCN4 basic region analogues under equilibrium conditions and physiological salt concentrations.     

The contribution of the methyl groups on thymine bases to binding specificity and affinity by alanine-rich mutants of the bZIP motif.

We have used fluorescence anisotropy to measure in situ the thermodynamics of binding of alanine-rich mutants of the GCN4 basic region/leucine zipper (bZIP) to short DNA duplexes, in which thymines were replaced with uracils, in order to quantify the contributions of the C5 methyl group on thymines with alanine methyl side chains. We simplified the alpha-helical GCN4 bZIP by alanine substitution: 4A, 11A, and 18A contain four, 11, and 18 alanine mutations in their DNA-binding basic regions, respectively. Titration of fluorescein-labeled duplexes with increasing amounts of protein yielded dissociation constants in the low-to-mid nanomolar range for all bZIP mutants in complex with the AP-1 target site (5'-TGACTCA-3'); binding to the nonspecific control duplex was >1000-fold weaker. Small changes of <1 kcal/mol in binding free energies were observed for wild-type bZIP and 4A mutant to uracil-containing AP-1, whereas 11A and 18A bound almost equally well to native AP-1 and uracil-containing AP-1. These modest changes in binding affinities may reflect the multivalent nature of protein-DNA interactions, as our highly mutated proteins still exhibit native-like behavior. These protein mutations may compensate for changes in enthalpic and entropic contributions toward DNA-binding in order to maintain binding free energies similar to that of the native protein-DNA complex.     

Synapsis, strand scission, and strand exchange induced by the FLP recombinase: analysis with half-FRT sites.

We have used a previously described cross-linking assay and half-FRT site substrates to examine the requirements for synapsis, strand exchange, and strand scission. The cross-linking assay showed that the minimum functional FRT site needed for synapsis contains two inverted FLP-binding elements surrounding an 8-bp core. This indicates that four FLP molecules interact with four binding elements in a synaptic complex. The analysis using half-sites showed that the enzyme can catalyze efficient strand exchange between a half-site and the intact FRT site. The reaction occurred only if the half-site had at least 2 bp but no more than 4 bp of the adjoining core sequence. The exchange occurred exclusively at the regions of limited core homology between the respective half-site and the FRT site. The absence of strand exchange between an intact site and a half-site bearing regions of core nonhomology indicates that 1 bp of homology is not sufficient for the formation of stable recombinant structures. Qian et al. (X.-H. Qian, R. B. Inman, and M. M. Cox, J. Biol. Chem. 265:21779-21788, 1990) have recently shown that the FLP protein can catalyze the formation of dimeric, trimeric, and tetrameric complexes with half-FRT sites. We show that only half-sites that contained at least 2 bp of adjacent core could form stable dimer products and be cleaved by the enzyme. Stable dimers were formed between a noncleavable half-site and a cleavable half-site, suggesting that only a single cleavage event is needed for the formation of the dimer.