Engrailed (Gln50-->Lys) homeodomain-DNA complex at 1.9 A resolution: structural basis for enhanced affinity and altered specificity

Dissecting the Roles of Electrostatic Interactions in Modulating the Folding Stability and Cooperativity of Engrailed Homeodomain

Engrailed homeodomain (EngHD), a highly charged transcription factor regulating over 200 genes, is a fast-folding protein. Recent studies have shown that the abundant charged residues in EngHD not only facilitate protein-DNA interactions but also influence the conformational disorder of its native structure. However, the mechanisms by which electrostatic interactions modulate the folding of EngHD remain unclear. Here, we employ a coarse-grained structure-based model that incorporates the salt-dependent Debye-Hückel model to investigate the (un)folding behavior of EngHD under various salt concentrations. Our findings demonstrate that increasing salt concentrations enhance both folding stability and cooperativity, while the folding barrier height remains relatively constant due to the distinct electrostatic effects on individual residues. By modulating the energetic balance between local and nonlocal interactions, we shift the folding of EngHD from a downhill process to a two-state process. Notably, we observe a nonmonotonic relationship between the strength of local interactions and residue-level coupling degree during (un)folding, likely attributed to the repulsive electrostatic interactions present in the native structure of EngHD. Additionally, we identify a critical turning point in the dependence of folding cooperativity on salt concentration, classified by the energetic balance of local and nonlocal interactions. Our results provide valuable insights into how electrostatic interactions influence the folding of EngHD, contributing to the theoretical framework for engineering highly charged proteins.

Cooperative Gsx2-DNA binding requires DNA bending and a novel Gsx2 homeodomain interface

The conserved Gsx homeodomain (HD) transcription factors specify neural cell fates in animals from flies to mammals. Like many HD proteins, Gsx factors bind A/T-rich DNA sequences prompting the following question: How do HD factors that bind similar DNA sequences in vitro regulate specific target genes in vivo? Prior studies revealed that Gsx factors bind DNA both as a monomer on individual A/T-rich sites and as a cooperative homodimer to two sites spaced precisely 7 bp apart. However, the mechanistic basis for Gsx-DNA binding and cooperativity is poorly understood. Here, we used biochemical, biophysical, structural and modeling approaches to (i) show that Gsx factors are monomers in solution and require DNA for cooperative complex formation, (ii) define the affinity and thermodynamic binding parameters of Gsx2/DNA interactions, (iii) solve a high-resolution monomer/DNA structure that reveals that Gsx2 induces a 20° bend in DNA, (iv) identify a Gsx2 protein-protein interface required for cooperative DNA binding and (v) determine that flexible spacer DNA sequences enhance Gsx2 cooperativity on dimer sites. Altogether, our results provide a mechanistic basis for understanding the protein and DNA structural determinants that underlie cooperative DNA binding by Gsx factors.

Double Stranded DNA Binding Stapled Peptides: An Emerging Tool for Transcriptional Regulation

Stapled peptides have rapidly established themselves as a powerful technique to mimic α-helical interactions with a short peptide sequence. There are many examples of stapled peptides that successfully disrupt α-helix-mediated protein-protein interactions, with an example currently in clinical trials. DNA-protein interactions are also often mediated by α-helices and are involved in all transcriptional regulation processes. Unlike DNA-binding small molecules, which typically lack DNA sequence selectivity, DNA-binding proteins bind with high affinity and high selectivity. These are ideal candidates for the design DNA-binding stapled peptides. Despite the parallel to protein-protein interaction disrupting stapled peptides and the need for sequence specific DNA binders, there are very few DNA-binding stapled peptides. In this review we examine all the known DNA-binding stapled peptides. Their design concepts are compared to stapled peptides that disrupt protein-protein interactions and based on the few examples in the literature, DNA-binding stapled peptide trends are discussed.

Cooperative Gsx2-DNA Binding Requires DNA Bending and a Novel Gsx2 Homeodomain Interface

The conserved Gsx homeodomain (HD) transcription factors specify neural cell fates in animals from flies to mammals. Like many HD proteins, Gsx factors bind A/T-rich DNA sequences prompting the question - how do HD factors that bind similar DNA sequences in vitro regulate specific target genes in vivo? Prior studies revealed that Gsx factors bind DNA both as a monomer on individual A/T-rich sites and as a cooperative homodimer to two sites spaced precisely seven base pairs apart. However, the mechanistic basis for Gsx DNA binding and cooperativity are poorly understood. Here, we used biochemical, biophysical, structural, and modeling approaches to (1) show that Gsx factors are monomers in solution and require DNA for cooperative complex formation; (2) define the affinity and thermodynamic binding parameters of Gsx2/DNA interactions; (3) solve a high-resolution monomer/DNA structure that reveals Gsx2 induces a 20° bend in DNA; (4) identify a Gsx2 protein-protein interface required for cooperative DNA binding; and (5) determine that flexible spacer DNA sequences enhance Gsx2 cooperativity on dimer sites. Altogether, our results provide a mechanistic basis for understanding the protein and DNA structural determinants that underlie cooperative DNA binding by Gsx factors, thereby providing a deeper understanding of HD specificity.

The Increased Expression of an Engrailed to Sustain Shell Formation in Response to Ocean Acidification

Engrailed is a transcription factor required in numerous species for important developmental steps such as neurogenesis, segment formation, preblastoderm organization, and compartment formation. Recent study has proved that engrailed is also a key gene related to shell formation in marine bivalves. In the present study, the expression pattern of an engrailed gene (Cgengrailed-1) in Pacific oyster Crassostrea gigas under CO2-driven acidification was investigated to understand its possible role in the regulation of shell formation and adaptation to ocean acidification (OA). The open reading frame (ORF) of Cgengrailed-1 was obtained, which was of 690 bp encoding a polypeptide of 229 amino acids with a HOX domain. Phylogenetic analysis indicated that the deduced amino acid sequence of Cgengrailed-1 shared high homology with other engraileds from Drosophila melanogaster, Mizuhopecten yessoensi, and Crassostrea virginica. The mRNA transcripts of Cgengrailed-1 were constitutively expressed in various tissues with the highest expression levels detected in labial palp and mantle, which were 86.83-fold (p < 0.05) and 75.87-fold (p < 0.05) higher than that in hepatopancreas. The mRNA expression of Cgengrailed-1 in mantle decreased dramatically after moderate (pH 7.8) and severe (pH 7.4) acidification treatment (0.75- and 0.15-fold of that in control group, p < 0.05). The results of immunofluorescence assay demonstrated that the expression level of Cgengrailed-1 in the middle fold of mantle increased significantly upon moderate and severe acidification treatment. Moreover, after the oyster larvae received acidification treatment at trochophore stage, the mRNA expression levels of Cgengrailed-1 increased significantly in D-shape larvae stages, which was 3.11- (pH 7.8) and 4.39-fold (pH 7.4) of that in control group (p < 0.05). The whole-mount immunofluorescence assay showed that Cgengrailed-1 was mainly expressed on the margin of shell gland, and the periostracum in trochophore, early D-shape larvae and D-shape larvae in both control and acidification treatment groups, and the intensity of positive signals in early D-shape larvae and D-shape larvae increased dramatically under acidification treatment. These results collectively suggested that the expression of Cgengrailed-1 could be triggered by CO2-driven acidification treatment, which might contribute to induce the initial shell formation in oyster larvae and the formation of periostracum in adult oyster to adapt to the acidifying marine environment.

Structural basis for designing an array of engrailed homeodomains

Small DNA-binding proteins that target desired sequences have the potential to act as a scaffold for molecular tools such as genome editing. In this study, an engrailed homeodomain (EHD) was chosen and it was evaluated whether it could be used as a molecular module that can connect to itself to recognize a longer target sequence. It was previously shown that two EHDs connected by a linker (EHD₂) recognize a target sequence twice as long as that recognized by a single EHD in cells only when Arg53 in each EHD in the tandem protein is mutated to alanine {(EHD[R53A])₂}. To investigate the recognition mechanism of (EHD[R53A])₂, the crystal structure of the (EHD[R53A])₂-DNA complex was determined at 1.6 Å resolution. The individual EHDs were found to adopt the typical homeodomain fold. Most importantly, the base-specific interactions in the major groove necessary for the affinity/specificity of wild-type EHD were preserved in (EHD[R53A])₂. Bacterial assays confirmed that the base-specific interactions are retained under cellular conditions. These observations indicate that the R53A mutation only causes a loss of the arginine-phosphate interaction at the protein-DNA interface, which reduces the DNA-binding affinity compared with the wild type. It is therefore concluded that (EHD[R53A])₂ precisely recognizes tandem target sites within cells, enabling the individual EHDs to concurrently bind to the target sites with modest binding affinity. This suggests that modulation of the binding activity of each EHD is vital to construct a protein array that can precisely recognize a sequence with multiple target sites.

Balance between DNA-binding affinity and specificity enables selective recognition of longer target sequences in vivo

Although genome-editing enzymes such as TALEN and CRISPR/Cas9 are being widely used, they have an essential limitation in that their relatively high-molecular weight makes them difficult to be delivered to cells. To develop a novel genome-editing enzyme with a smaller molecular weight, we focused on the engrailed homeodomain (EHD). We designed and constructed proteins composed of two EHDs connected by a linker to increase sequence specificity. In bacterial one-hybrid assays and electrophoresis mobility shift assay analyses, the created proteins exhibited good affinity for DNA sequences consisting of two tandemly aligned EHD target sequences. However, they also bound to individual EHD targets. To avoid binding to single target sites, we introduced amino acid mutations to reduce the protein-DNA affinity of each EHD monomer and successfully created a small protein with high specificity for tandem EHD target sequences.

Phenotypic Nonspecificity as the Result of Limited Specificity of Transcription Factor Function

Drosophila transcription factor (TF) function is phenotypically nonspecific. Phenotypic nonspecificity is defined as one phenotype being induced or rescued by multiple TFs. To explain this unexpected result, a hypothetical world of limited specificity is explored where all TFs have unique random distributions along the genome due to low information content of DNA sequence recognition and somewhat promiscuous cooperative interactions with other TFs. Transcription is an emergent property of these two conditions. From this model, explicit predictions are made. First, many more cases of TF nonspecificity are expected when examined. Second, the genetic analysis of regulatory sequences should uncover cis-element bypass and, third, genetic analysis of TF function should generally uncover differential pleiotropy. In addition, limited specificity provides evolutionary opportunity and explains the inefficiency of expression analysis in identifying genes required for biological processes.

Phylogenetic and mutational analyses of human LEUTX, a homeobox gene implicated in embryogenesis

Recently, human PAIRED-LIKE homeobox transcription factor (TF) genes were discovered whose expression is limited to the period of embryo genome activation up to the 8-cell stage. One of these TFs is LEUTX, but its importance for human embryogenesis is still subject to debate. We confirmed that human LEUTX acts as a TAATCC-targeting transcriptional activator, like other K50-type PAIRED-LIKE TFs. Phylogenetic comparisons revealed that Leutx proteins are conserved across Placentalia and comprise two conserved domains, the homeodomain, and a Leutx-specific domain containing putative transcriptional activation motifs (9aaTAD). Examination of human genotype resources revealed 116 allelic variants in LEUTX. Twenty-four variants potentially affect function, but they occur only heterozygously at low frequency. One variant affects a DNA-specificity determining residue, mutationally reachable by a one-base transition. In vitro and in silico experiments showed that this LEUTX mutation (alanine to valine at position 54 in the homeodomain) results in a transactivational loss-of-function to a minimal TAATCC-containing promoter and a 36 bp motif enriched in genes involved in embryo genome activation. A compensatory change in residue 47 restores function. The results support the notion that human LEUTX functions as a transcriptional activator important for human embryogenesis.

Lysine Side-Chain Dynamics in the Binding Site of Homeodomain/DNA Complexes As Observed by NMR Relaxation Experiments and Molecular Dynamics Simulations

An important but poorly characterized contribution to the thermodynamics of protein-DNA interactions is the loss of entropy that occurs from restricting the conformational freedom of amino acid side chains. The effect of restricting the flexibility of several side chains at a protein-DNA interface may be comparable in many cases to the other factors that determine the binding thermodynamics and may, therefore, play a key role in dictating the binding affinity and/or specificity. Because the entropic contributions, including the presence and influence of side-chain dynamics, are especially difficult to estimate based on structural information, it is important to pursue experimental and theoretical studies that can provide direct information regarding these issues. We report on studies of a model system, the homeodomain/DNA complex, focusing on the Lys50 class of homeodomains where a key lysine residue in position 50 was shown previously to be critical for binding site specificity. NMR methodology was employed for determining the dynamics of lysine side-chain amino groups via 15N relaxation measurements in the Lys50-class homeodomains from the Drosophila protein Bicoid and the human protein Pitx2. In the case of Pitx2, complexes with both a consensus and a nonconsensus DNA binding site were examined. NMR-derived order parameters indicated moderate to substantial conformational freedom for the lysine NH3+ group in the complexes studied. To complement the experimental NMR measurements, molecular dynamics simulations were performed for the consensus complexes to gain further, detailed insights regarding the dynamics of the Lys50 side chain and other important residues in the protein-DNA interface.

Structure of human heat-shock transcription factor 1 in complex with DNA

Heat-shock transcription factor 1 (HSF1) has a central role in mediating the protective response to protein conformational stresses in eukaryotes. HSF1 consists of an N-terminal DNA-binding domain (DBD), a coiled-coil oligomerization domain, a regulatory domain and a transactivation domain. Upon stress, HSF1 trimerizes via its coiled-coil domain and binds to the promoters of heat shock protein-encoding genes. Here, we present cocrystal structures of the human HSF1 DBD in complex with cognate DNA. A comparative analysis of the HSF1 paralog Skn7 from Chaetomium thermophilum showed that single amino acid changes in the DBD can switch DNA binding specificity, thus revealing the structural basis for the interaction of HSF1 with cognate DNA. We used a crystal structure of the coiled-coil domain of C. thermophilum Skn7 to develop a model of the active human HSF1 trimer in which HSF1 embraces the heat-shock-element DNA.

Role of DNA conformation & energetic insights in Msx-1-DNA recognition as revealed by molecular dynamics studies on specific and nonspecific complexes

In most of homeodomain-DNA complexes, glutamine or lysine is present at 50th position and interacts with 5th and 6th nucleotide of core recognition region. Molecular dynamics simulations of Msx-1-DNA complex (Q50-TG) and its variant complexes, that is specific (Q50K-CC), nonspecific (Q50-CC) having mutation in DNA and (Q50K-TG) in protein, have been carried out. Analysis of protein-DNA interactions and structure of DNA in specific and nonspecific complexes show that amino acid residues use sequence-dependent shape of DNA to interact. The binding free energies of all four complexes were analysed to define role of amino acid residue at 50th position in terms of binding strength considering the variation in DNA on stability of protein-DNA complexes. The order of stability of protein-DNA complexes shows that specific complexes are more stable than nonspecific ones. Decomposition analysis shows that N-terminal amino acid residues have been found to contribute maximally in binding free energy of protein-DNA complexes. Among specific protein-DNA complexes, K50 contributes more as compared to Q50 towards binding free energy in respective complexes. The sequence dependence of local conformation of DNA enables Q50/Q50K to make hydrogen bond with nucleotide(s) of DNA. The changes in amino acid sequence of protein are accommodated and stabilized around TAAT core region of DNA having variation in nucleotides.

Structure-based modeling of protein: DNA specificity

Protein:DNA interactions are essential to a range of processes that maintain and express the information encoded in the genome. Structural modeling is an approach that aims to understand these interactions at the physicochemical level. It has been proposed that structural modeling can lead to deeper understanding of the mechanisms of protein:DNA interactions, and that progress in this field can not only help to rationalize the observed specificities of DNA-binding proteins but also to allow researchers to engineer novel DNA site specificities. In this review we discuss recent developments in the structural description of protein:DNA interactions and specificity, as well as the challenges facing the field in the future.

Structure solution of DNA-binding proteins and complexes with ARCIMBOLDO libraries

Protein-DNA interactions play a major role in all aspects of genetic activity within an organism, such as transcription, packaging, rearrangement, replication and repair. The molecular detail of protein-DNA interactions can be best visualized through crystallography, and structures emphasizing insight into the principles of binding and base-sequence recognition are essential to understanding the subtleties of the underlying mechanisms. An increasing number of high-quality DNA-binding protein structure determinations have been witnessed despite the fact that the crystallographic particularities of nucleic acids tend to pose specific challenges to methods primarily developed for proteins. Crystallographic structure solution of protein-DNA complexes therefore remains a challenging area that is in need of optimized experimental and computational methods. The potential of the structure-solution program ARCIMBOLDO for the solution of protein-DNA complexes has therefore been assessed. The method is based on the combination of locating small, very accurate fragments using the program Phaser and density modification with the program SHELXE. Whereas for typical proteins main-chain α-helices provide the ideal, almost ubiquitous, small fragments to start searches, in the case of DNA complexes the binding motifs and DNA double helix constitute suitable search fragments. The aim of this work is to provide an effective library of search fragments as well as to determine the optimal ARCIMBOLDO strategy for the solution of this class of structures.

Selective DNA-binding by designed bisbenzamidine-homeodomain chimeras

We report the construction of conjugates between three variants of the helix 3 region of a Q50K engrailed homeodomain and bisbenzamidine minor-groove DNA binders. The hybrid featuring the sequence of the native protein failed to bind to DNA; however, modifications that increased the α-helical folding propensity of the peptide allowed specific DNA binding by a bipartite (major/minor groove) interaction.

Discovery, optimization and validation of an optimal DNA-binding sequence for the Six1 homeodomain transcription factor

The Six1 transcription factor is a homeodomain protein involved in controlling gene expression during embryonic development. Six1 establishes gene expression profiles that enable skeletal myogenesis and nephrogenesis, among others. While several homeodomain factors have been extensively characterized with regards to their DNA-binding properties, relatively little is known of the properties of Six1. We have used the genomic binding profile of Six1 during the myogenic differentiation of myoblasts to obtain a better understanding of its preferences for recognizing certain DNA sequences. DNA sequence analyses on our genomic binding dataset, combined with biochemical characterization using binding assays, reveal that Six1 has a much broader DNA-binding sequence spectrum than had been previously determined. Moreover, using a position weight matrix optimization algorithm, we generated a highly sensitive and specific matrix that can be used to predict novel Six1-binding sites with highest accuracy. Furthermore, our results support the idea of a mode of DNA recognition by this factor where Six1 itself is sufficient for sequence discrimination, and where Six1 domains outside of its homeodomain contribute to binding site selection. Together, our results provide new light on the properties of this important transcription factor, and will enable more accurate modeling of Six1 function in bioinformatic studies.

Exploring the DNA-recognition potential of homeodomains

The recognition potential of most families of DNA-binding domains (DBDs) remains relatively unexplored. Homeodomains (HDs), like many other families of DBDs, display limited diversity in their preferred recognition sequences. To explore the recognition potential of HDs, we utilized a bacterial selection system to isolate HD variants, from a randomized library, that are compatible with each of the 64 possible 3' triplet sites (i.e., TAANNN). The majority of these selections yielded sets of HDs with overrepresented residues at specific recognition positions, implying the selection of specific binders. The DNA-binding specificity of 151 representative HD variants was subsequently characterized, identifying HDs that preferentially recognize 44 of these target sites. Many of these variants contain novel combinations of specificity determinants that are uncommon or absent in extant HDs. These novel determinants, when grafted into different HD backbones, produce a corresponding alteration in specificity. This information was used to create more explicit HD recognition models, which can inform the prediction of transcriptional regulatory networks for extant HDs or the engineering of HDs with novel DNA-recognition potential. The diversity of recovered HD recognition sequences raises important questions about the fitness barrier that restricts the evolution of alternate recognition modalities in natural systems.

DNA targeting and cleavage by an engineered metalloprotein dimer

Nature has illustrated through numerous examples that protein dimerization has structural and functional advantages. We previously reported the design and characterization of an engineered "metallohomeodomain" protein (C2) based on a chimera of the EF-hand Ca-binding motif and the helix-turn-helix motif of homeodomains (Lim and Franklin in Protein Sci. 15:2159-2165, 2004). This small metalloprotein binds the hard metal ions Ca(II) and Ln(III) and interacts with DNA with modest sequence preference and affinity, yet exhibits only residual DNA cleavage activity. Here we have achieved substantial improvement in function by constructing a covalent dimer of this C2 module (F2) to create a larger multidomain protein. As assayed via fluorescence spectroscopy, this F2 protein binds Ca(II) more avidly (25-fold) than C2 on a per-domain basis; in gel shift selection experiments, metallated F2 exhibits a specificity toward 5'-TAATTA-3' sequences. Finally, Ca(2)F2 cleaves plasmid DNA and generates a linear product in a Ca(II)-dependent way, unlike the CaC2 monomer. To the best of our knowledge this activation of Ca(II) in the context of an EF-hand binding motif is unique and represents a significant step forward in the design of artificial metallonucleases by utilizing biologically significant metal ions.

Origins of specificity in protein-DNA recognition

Specific interactions between proteins and DNA are fundamental to many biological processes. In this review, we provide a revised view of protein-DNA interactions that emphasizes the importance of the three-dimensional structures of both macromolecules. We divide protein-DNA interactions into two categories: those when the protein recognizes the unique chemical signatures of the DNA bases (base readout) and those when the protein recognizes a sequence-dependent DNA shape (shape readout). We further divide base readout into those interactions that occur in the major groove from those that occur in the minor groove. Analogously, the readout of the DNA shape is subdivided into global shape recognition (for example, when the DNA helix exhibits an overall bend) and local shape recognition (for example, when a base pair step is kinked or a region of the minor groove is narrow). Based on the >1500 structures of protein-DNA complexes now available in the Protein Data Bank, we argue that individual DNA-binding proteins combine multiple readout mechanisms to achieve DNA-binding specificity. Specificity that distinguishes between families frequently involves base readout in the major groove, whereas shape readout is often exploited for higher resolution specificity, to distinguish between members within the same DNA-binding protein family.

Novel structural features in two ZHX homeodomains derived from a systematic study of single and multiple domains

BACKGROUND

Zhx1 to 3 (zinc-fingers and homeoboxes) form a set of paralogous genes encoding multi-domain proteins. ZHX proteins consist of two zinc fingers followed by five homeodomains. ZHXs have biological roles in cell cycle control by acting as co-repressors of the transcriptional regulator Nuclear Factor Y. As part of a structural genomics project we have expressed single and multi-domain fragments of the different human ZHX genes for use in structure determination.

RESULTS

A total of 30 single and multiple domain ZHX1-3 constructs selected from bioinformatics protocols were screened for soluble expression in E. coli using high throughput methodologies. Two homeodomains were crystallized leading to structures for ZHX1 HD4 and ZHX2 HD2. ZHX1 HD4, although closest matched to homeodomains from 'homez' and 'engrailed', showed structural differences, notably an additional C-terminal helix (helix V) which wrapped over helix I thereby making extensive contacts. Although ZHX2 HD2-3 was successfully expressed and purified, proteolysis occurred during crystallization yielding crystals of just HD2. The structure of ZHX2 HD2 showed an unusual open conformation with helix I undergoing 'domain-swapping' to form a homodimer.

CONCLUSIONS

Although multiple-domain constructs of ZHX1 selected by bioinformatics studies could be expressed solubly, only single homeodomains yielded crystals. The crystal structure of ZHX1 HD4 showed additional hydrophobic interactions relative to many known homeodomains via extensive contacts formed by the novel C-terminal helix V with, in particular, helix I. Additionally, the replacement of some charged covariant residues (which are commonly observed to form salt bridges in non-homeotherms such as the Drosophila 'engrailed' homeodomain), by apolar residues further increases hydrophobic contacts within ZHX1 HD4, and potentially stability, relative to engrailed homeodomain. ZHX1 HD4 helix V points away from the normally observed DNA major groove binding site on homeodomains and thus would not obstruct the putative binding of nucleic acid. In contrast, for ZHX2 HD2 the observed altered conformation involving rearrangement of helix I, relative to the canonical homeodomain fold, disrupts the normal DNA binding site, although protein-protein binding is possible as observed in homodimer formation.

Cooperative DNA-binding and sequence-recognition mechanism of aristaless and clawless

To achieve accurate gene regulation, some homeodomain proteins bind cooperatively to DNA to increase those site specificities. We report a ternary complex structure containing two homeodomain proteins, aristaless (Al) and clawless (Cll), bound to DNA. Our results show that the extended conserved sequences of the Cll homeodomain are indispensable to cooperative DNA binding. In the Al-Cll-DNA complex structure, the residues in the extended regions are used not only for the intermolecular contacts between the two homeodomain proteins but also for the sequence-recognition mechanism of DNA by direct interactions. The residues in the extended N-terminal arm lie within the minor groove of DNA to form direct interactions with bases, whereas the extended conserved region of the C-terminus of the homeodomain interacts with Al to stabilize and localize the third alpha helix of the Cll homeodomain. This structure suggests a novel mode for the cooperativity of homeodomain proteins.

Autoregulatory binding sites in the zebrafish six3a promoter region define a new recognition sequence for Six3 proteins

The homeodomain (HD) transcription factor Six3, which is a member of the Six/Sine oculis family, is essential for development of the eyes and forebrain in vertebrates. It has recently been claimed that the HDs of Six3 and other members of the Six family have a common recognition sequence, TGATAC. However, a different recognition sequence including the typical TAAT core motif, which has not yet been fully defined, has also been proposed for the Six3 HD in mice. Our study of the zebrafish orthologue six3a, which has an identical HD, shows that it binds in vitro to multiple TAAT-containing sites within its promoter region. Comparison of the different binding affinities for these sequences identifies three high-affinity sites with a common TAATGTC motif. Notably, this new recognition sequence, which is supported by our analysis of the influence of single-nucleotide substitutions on the DNA-binding affinity, is distinct from all of the DNA-binding specificities previously described in surveys of HDs. In addition, our comparison of Six3a HD binding to the novel TAATGTC motif and the common recognition sequence of Six family HDs (TGATAC) shows very similar affinities, suggesting two distinct DNA-binding modes. Transient reporter assays of the six3a promoter in zebrafish embryos also indicate that the three high-affinity sites are involved in autoregulation. In support of this, chromatin immunoprecipitation experiments show enrichment of Six3a binding to a six3a promoter fragment containing two clustered high-affinity sites. These findings provide strong evidence that the TAATGTC motif is an important target sequence for vertebrate Six3 proteins in vivo.

Variation in homeodomain DNA binding revealed by high-resolution analysis of sequence preferences

Most homeodomains are unique within a genome, yet many are highly conserved across vast evolutionary distances, implying strong selection on their precise DNA-binding specificities. We determined the binding preferences of the majority (168) of mouse homeodomains to all possible 8-base sequences, revealing rich and complex patterns of sequence specificity and showing that there are at least 65 distinct homeodomain DNA-binding activities. We developed a computational system that successfully predicts binding sites for homeodomain proteins as distant from mouse as Drosophila and C. elegans, and we infer full 8-mer binding profiles for the majority of known animal homeodomains. Our results provide an unprecedented level of resolution in the analysis of this simple domain structure and suggest that variation in sequence recognition may be a factor in its functional diversity and evolutionary success.

Analysis of homeodomain specificities allows the family-wide prediction of preferred recognition sites

We describe the comprehensive characterization of homeodomain DNA-binding specificities from a metazoan genome. The analysis of all 84 independent homeodomains from D. melanogaster reveals the breadth of DNA sequences that can be specified by this recognition motif. The majority of these factors can be organized into 11 different specificity groups, where the preferred recognition sequence between these groups can differ at up to four of the six core recognition positions. Analysis of the recognition motifs within these groups led to a catalog of common specificity determinants that may cooperate or compete to define the binding site preference. With these recognition principles, a homeodomain can be reengineered to create factors where its specificity is altered at the majority of recognition positions. This resource also allows prediction of homeodomain specificities from other organisms, which is demonstrated by the prediction and analysis of human homeodomain specificities.

Comparison of multiple crystal structures with NMR data for engrailed homeodomain

Two methods are currently available to solve high resolution protein structures-X-ray crystallography and nuclear magnetic resonance (NMR). Both methods usually produce highly similar structures, but small differences between both solutions are always observed. Here the raw NMR data as well as the solved NMR structure were compared to the multiple crystal structures solved for the WT 60 residue three helix bundle engrailed homeodomain (EnHD) and single point mutants. There was excellent agreement between TALOS-predicted and crystal structure-observed dihedral angles and a good agreement for the (3) J(H ( N ) H ( alpha )) couplings for the multiple crystal structures. Around 1% of NOEs were violated for any crystal structure, but no NOE was inconsistent with all of the crystal structures. Violations usually occurred for surface residues or for residues for which multiple discreet conformations were observed between the crystal structures. Comparison of the disorder shown in the multiple crystal structures shows little correlation with dynamics under native conditions for this protein.

Functional specificity of a Hox protein mediated by the recognition of minor groove structure

The recognition of specific DNA-binding sites by transcription factors is a critical yet poorly understood step in the control of gene expression. Members of the Hox family of transcription factors bind DNA by making nearly identical major groove contacts via the recognition helices of their homeodomains. In vivo specificity, however, often depends on extended and unstructured regions that link Hox homeodomains to a DNA-bound cofactor, Extradenticle (Exd). Using a combination of structure determination, computational analysis, and in vitro and in vivo assays, we show that Hox proteins recognize specific Hox-Exd binding sites via residues located in these extended regions that insert into the minor groove but only when presented with the correct DNA sequence. Our results suggest that these residues, which are conserved in a paralog-specific manner, confer specificity by recognizing a sequence-dependent DNA structure instead of directly reading a specific DNA sequence.

A one-dimensional reaction coordinate for identification of transition states from explicit solvent P(fold)-like calculations

A properly identified transition state ensemble (TSE) in a molecular dynamics (MD) simulation can reveal a tremendous amount about how a protein folds and offer a point of comparison to experimentally derived Phi(F) values, which reflect the degree of structure in these transient states. In one such method of TSE identification, dubbed P(fold), MD simulations of individual protein structures taken from an unfolding trajectory are used to directly assess an input structure's probability of folding before unfolding, and P(fold) is, by definition, 0.5 for the TSE. Other, less computationally intensive methods, such as multidimensional scaling (MDS) of the pairwise root mean-squared deviation (RMSD) matrix of the conformations sampled in a thermal unfolding trajectory, have also been used to identify the TSE. Identification of the TSE is made from the original MD simulation without the need to run further simulations. Here we present a P(fold)-like study and describe methods for identification of the TSE through the derivation of a high fidelity, bounded, one-dimensional reaction coordinate for protein folding. These methods are applied to the engrailed homeodomain. The TSE identified by this approach is essentially identical to the TSE identified previously by MDS of the pairwise RMSD matrix. However, the cost of performing P(fold), or even our reduced P(fold)-like calculations, is at least 36,000 times greater than the MDS method.

Defining functional distance using manifold embeddings of gene ontology annotations

Although rigorous measures of similarity for sequence and structure are now well established, the problem of defining functional relationships has been particularly daunting. Here, we present several manifold embedding techniques to compute distances between Gene Ontology (GO) functional annotations and consequently estimate functional distances between protein domains. To evaluate accuracy, we correlate the functional distance to the well established measures of sequence, structural, and phylogenetic similarities. Finally, we show that manual classification of structures into folds and superfamilies is mirrored by proximity in the newly defined function space. We show how functional distances place structure-function relationships in biological context resulting in insight into divergent and convergent evolution. The methods and results in this paper can be readily generalized and applied to a wide array of biologically relevant investigations, such as accuracy of annotation transference, the relationship between sequence, structure, and function, or coherence of expression modules.

Structural basis of DNA recognition by the alternative sigma-factor, sigma54

The sigma subunit of bacterial RNA polymerase (RNAP) regulates gene expression by directing RNAP to specific promoters. Unlike sigma(70)-type proteins, the alternative sigma factor, sigma(54), requires interaction with an ATPase to open DNA. We present the solution structure of the C-terminal domain of sigma(54) bound to the -24 promoter element, in which the conserved RpoN box motif inserts into the major groove of the DNA. This structure elucidates the basis for sequence specific recognition of the -24 element, orients sigma(54) on the promoter, and suggests how the C-terminal domain of sigma(54) interacts with RNAP.

Energetics of the protein-DNA-water interaction

Background

To understand the energetics of the interaction between protein and DNA we analyzed 39 crystallographically characterized complexes with the HINT (Hydropathic INTeractions) computational model. HINT is an empirical free energy force field based on solvent partitioning of small molecules between water and 1-octanol. Our previous studies on protein-ligand complexes demonstrated that free energy predictions were significantly improved by taking into account the energetic contribution of water molecules that form at least one hydrogen bond with each interacting species.

Results

An initial correlation between the calculated HINT scores and the experimentally determined binding free energies in the protein-DNA system exhibited a relatively poor r² of 0.21 and standard error of ± 1.71 kcal mol^-1. However, the inclusion of 261 waters that bridge protein and DNA improved the HINT score-free energy correlation to an r² of 0.56 and standard error of ± 1.28 kcal mol^-1. Analysis of the water role and energy contributions indicate that 46% of the bridging waters act as linkers between amino acids and nucleotide bases at the protein-DNA interface, while the remaining 54% are largely involved in screening unfavorable electrostatic contacts.

Conclusion

This study quantifies the key energetic role of bridging waters in protein-DNA associations. In addition, the relevant role of hydrophobic interactions and entropy in driving protein-DNA association is indicated by analyses of interaction character showing that, together, the favorable polar and unfavorable polar/hydrophobic-polar interactions (i.e., desolvation) mostly cancel.

Structure and properties of a re-engineered homeodomain protein-DNA interface

The homeodomain (HD)-DNA interface has been conserved over 500 million years of evolution. Despite this conservation, we have successfully re-engineered the engrailed HD to specifically recognize an unnatural nucleotide using a phage display selection. Here we report the synthesis of novel nucleosides and the selection of mutant HDs that bind these nucleotides using phage display. The high-resolution crystal structure of one mutant in complex with modified and unmodified DNA demonstrates that, even with the substantial perturbation to the interface, this selected mutant retains a canonical HD structure. Dissection of the contributions due to each of the selected mutations reveals that the majority of the modification-specific binding is accomplished by a single mutation (I47G) but that the remaining mutations retune the stability of the HD. These results afford a detailed look at a re-engineered protein-DNA interaction and provide insight into the opportunities for re-engineering highly conserved interfaces.

The solution structure of the native K50 Bicoid homeodomain bound to the consensus TAATCC DNA-binding site

The solution structure of the homeodomain of the Drosophila morphogenic protein Bicoid (Bcd) complexed with a TAATCC DNA site is described. Bicoid is the only known protein that uses a homeodomain to regulate translation, as well as transcription, by binding to both RNA and DNA during early Drosophila development; in addition, the Bcd homeodomain can recognize an array of different DNA sites. The dual functionality and broad recognition capabilities signify that the Bcd homeodomain may possess unique structural/dynamic properties. Bicoid is the founding member of the K50 class of homeodomain proteins, containing a lysine residue at the critical 50th position (K50) of the homeodomain sequence, a residue required for DNA and RNA recognition; Bcd also has an arginine residue at the 54th position (R54), which is essential for RNA recognition. Bcd is the only known homeodomain with the K50/R54 combination of residues. The Bcd structure indicates that this homeodomain conforms to the conserved topology of the homeodomain motif, but exhibits a significant variation from other homeodomain structures at the end of helix 1. A key result is the observation that the side-chains of the DNA-contacting residues K50, N51 and R54 all show strong signs of flexibility in the protein-DNA interface. This finding is supportive of the adaptive-recognition theory of protein-DNA interactions.

Protein-DNA binding specificity predictions with structural models

Protein-DNA interactions play a central role in transcriptional regulation and other biological processes. Investigating the mechanism of binding affinity and specificity in protein-DNA complexes is thus an important goal. Here we develop a simple physical energy function, which uses electrostatics, solvation, hydrogen bonds and atom-packing terms to model direct readout and sequence-specific DNA conformational energy to model indirect readout of DNA sequence by the bound protein. The predictive capability of the model is tested against another model based only on the knowledge of the consensus sequence and the number of contacts between amino acids and DNA bases. Both models are used to carry out predictions of protein-DNA binding affinities which are then compared with experimental measurements. The nearly additive nature of protein-DNA interaction energies in our model allows us to construct position-specific weight matrices by computing base pair probabilities independently for each position in the binding site. Our approach is less data intensive than knowledge-based models of protein-DNA interactions, and is not limited to any specific family of transcription factors. However, native structures of protein-DNA complexes or their close homologs are required as input to the model. Use of homology modeling can significantly increase the extent of our approach, making it a useful tool for studying regulatory pathways in many organisms and cell types.

Photocontrol of DNA Binding Specificity of a Miniature Engrailed Homeodomain

Control of DNA binding of HDH-3, a 18-residue polypeptide based on the recognition helix of the Q50K engrailed homeodomain, has been achieved. HDH-3 was linked to an azobenzene cross-linker through two cysteine residues in an i, i + 11 spacing. For the thermodynamically stable trans configuration of the cross-linker, the dark-adapted peptide (dad-HDH-3) adopted a mainly alpha-helical structure as judged by circular dichroism (CD) spectroscopy. After irradiation with light of 360 nm, the helical content of the peptide (irrad-HDH-3) was reduced significantly and the CD spectrum of the irradiated peptide resembled that of the largely unstructured, unalkylated peptide. Despite lacking helices-1 and -2 and the N-terminal arm of Q50K engrailed, dad-HDH-3 bound to its natural DNA target sequence TAATCC (QRE) with high affinity (K(D) = 7.5 +/- 1.3 nM). The binding affinity for the mutant DNA sequence, TAATTA (ERE), was reduced significantly (K(D) = 140 +/- 11 nM). Unlike irrad-HDH-3, which like the unalkylated parent peptide displayed only marginal DNA binding specificity, dad-HDH-3 specified base pairs 5 and 6 of QRE with an accuracy rivaling that of the intact wild-type Q50K engrailed homeodomain, making dad-HDH-3 the most specific designed DNA binding miniature homeodomain reported to date. Moreover, DNA binding affinity and specificity of HDH-3 could be controlled externally by irradiation with light.

Integration of bioinformatics and computational biology to understand protein-DNA recognition mechanism

Transcription factors play essential role in the gene regulation in higher organisms, binding to multiple target sequences and regulating multiple genes in a complex manner. In order to decipher the mechanism of gene regulation, it is important to understand the molecular mechanism of protein-DNA recognition. Here we describe a strategy to approach this problem, using various methods in bioinformatics and computational biology. We have used a knowledge-based approach, utilizing rapidly increasing structural data of protein-DNA complexes, to derive empirical potential functions for the specific interactions between bases and amino acids as well as for DNA conformation, from the statistical analyses on the structural data. Then these statistical potentials are used to quantify the specificity of protein-DNA recognition. The quantification of specificity has enabled us to establish the structure-function analysis of transcription factors, such as the effects of binding cooperativity on target recognition. The method is also applied to real genome sequences, predicting potential target sites. We are also using computer simulations of protein-DNA interactions and DNA conformation in order to complement the empirical method. The integration of these approaches together will provide deeper insight into the mechanism of protein-DNA recognition and improve the target prediction of transcription factors.

Dissecting the Engrailed homeodomain-DNA interaction by phage-displayed shotgun scanning

Phage-displayed alanine shotgun scanning was used to dissect contributions by engrailed homedomain (En-HD) residues 17 through 46, which indirectly influence recognition of DNA. The relative contributions of such indirect contacts, quantified by shotgun scanning, highlight previously unexplored En-HD residues. Two motifs dominate En-HD function in this region. First, two surface-exposed aromatic residues (F20 and Y25) bracket the hydrophobic core. Second, two sets of turn-forming residues are highlighted, including carboxamide-requiring residues E22/N23 and a leucine/isoleucine splint. The En-HD hydrophobic core exhibits a surprising degree of malleability, as demonstrated by homolog shotgun scanning. Most selectants from in vitro shotgun scanning mirror the consensus human homeodomain sequence. Thus, natural evolution and in vitro selection use similar selection criteria: affinity, specificity, and stability. However, homolog shotgun scanning identified mutations capable of improving the affinity and specificity of En-HD.

Homeodomain revisited: a lesson from disease-causing mutations

The homeodomain is a highly conserved DNA-binding motif that is found in numerous transcription factors throughout a large variety of species from yeast to humans. These gene-specific transcription factors play critical roles in development and adult homeostasis, and therefore, any germline mutations associated with these proteins can lead to a number of congenital abnormalities. Although much has been revealed concerning the molecular architecture and the mechanism of homeodomain-DNA interactions, the study of disease-causing mutations can further provide us with instructive information as to the role of particular residues in a conserved mode of action. In this paper, I have compiled the homeodomain missense mutations found in various human diseases and re-examined the functional role of the mutational "hot spot" residues in light of the structures obtained from crystallography. These findings should be useful in understanding the essential components of the homeodomain and in attempts to design agonist or antagonists to modulate their activity and to reverse the effects caused by the mutations.

A phage display selection of engrailed homeodomain mutants and the importance of residue Q50

Mutants of engrailed homeodomain (HD) that retain DNA-binding activity were isolated using a phage display selection. This selection was used to enrich for active DNA-binding clones from a complex library consisting of over a billion members. A more focused library of mutant homeodomains consisting of all possible amino acid combinations at two DNA-contacting residues (I47 and Q50) was constructed and screened for members capable of binding tightly and specifically to the engrailed consensus sequence, TAATTA. The isolated mutants largely recapitulated the distribution of amino acids found at these positions in natural homeodomains thus validating the in vitro selection conditions. In particular, the unequivocal advantage enjoyed by glutamine at residue 50 is surprising in light of reports that minimize the importance of this residue. Here, the subtle contributions of residue Q50 are demonstrated to play a functionally important role in specific recognition of DNA. These results highlight the complex subtlety of protein-DNA interactions, underscoring the value of the first reported in vitro selection of a homeodomain.

Specific DNA recognition by theAntp homeodomain: MD simulations of specific and nonspecific complexes

Four molecular dynamics simulation trajectories of complexes between the wild-type or a mutant Antennapedia homeodomain and 2 DNA sequences were generated in order to probe the mechanisms governing the specificity of DNA recognition. The starting point was published affinity measurements showing that a single protein mutation combined with a replacement of 2 base pairs yields a new high-affinity complex, whereas the other combinations, with changes on only 1 macromolecule, exhibited lower affinity. The simulations of the 4 complexes yielded fluctuating networks of interaction. On average, these networks differ significantly, explaining the switch of affinity caused by the alterations in the macromolecules. The network of mostly hydrogen-bonding interactions involving several water molecules, which was suggested both by X-ray and NMR structures of the wild-type homeodomain and its DNA operator sequence, could be reproduced in the trajectory. More interestingly, the high-affinity complex with alterations in both the protein and the DNA yielded again a dynamic but very tight network of intermolecular interactions, however, attributing a significantly stronger role to direct hydrophobic interactions at the expense of water bridges. The other 2 homeodomain-DNA complexes, with only 1 molecule altered, show on average over the trajectories a clearly reduced number of protein-DNA interactions. The observations from these simulations suggest specific experiments and thus close the circle formed by biochemical, structural, and computational studies. The shift from a water-dominated to a more "dry" interface may prove important in the design of proteins binding DNA in a specific manner.

Insights into nonspecific binding of homeodomains from a structure of MATalpha2 bound to DNA

The 2.1-A resolution crystal structure of the MATalpha2 homeodomain bound to DNA reveals the unexpected presence of two nonspecifically bound alpha2 homeodomains, in addition to the two alpha2 homeodomains bound to canonical alpha2 binding sites. One of the extra homeodomains makes few base-specific contacts, while the other extra homeodomain binds to DNA in a previously unobserved manner. This unusually bound homeodomain is rotated on the DNA, making possible major groove contacts by side-chains that normally do not contact the DNA. This alternate docking may represent one way in which homeodomains sample nonspecific DNA sequences.

Lanthanide-binding helix-turn-helix peptides: solution structure of a designed metallonuclease

A designed lanthanide-binding chimeric peptide based on the strikingly similar geometries of the EF-hand and helix-turn-helix (HTH) motifs was investigated by NMR and CD spectroscopy and found to retain the same overall solution structure of the parental motifs. CD spectroscopy showed that the 33-mer peptide P3W folds on binding lanthanides, with an increase in alpha-helicity from 20% in the absence of metal to 38% and 35% in the presence of excess Eu(III) and La(III) ions, respectively. The conditional binding affinities of P3W for La(III) (5.9 +/- 0.3 microM) and for Eu(III) (6.2 +/- 0.3 microM) (pH 7.8, 5 mM Tris) were determined by tryptophan fluorescence titration. The La(III) complex of peptide P3, which differs from P3W by only one Trp-to-His substitution, has much less signal dispersion in the proton NMR spectra than LaP3W, indicating that the Trp residue is a critical hydrophobic anchor for maintaining a well-folded helix-turn-helix structure. A chemical-shift index analysis indicates the metallopeptide has a helix-loop-helix secondary structure. A structure calculated by using nuclear Overhauser effect and other NMR constraints reveals that P3W not only has a tightly folded metal-binding loop but also retains the alpha-alpha corner supersecondary structure of the parental motifs. Although the solution structure is undefined at both the N and C termini, the NMR structure confirms the successful incorporation of a metal-binding loop into a HTH sequence.

Miniature homeodomains: high specificity without an N-terminal arm

Recently, we described a strategy for the design of miniature proteins that bind DNA and protein surfaces with high affinity and selectivity. This strategy involves identifying the functional epitope required for macromolecular recognition by a natural protein and presenting it on a small, stable protein scaffold. In previous work, high-affinity DNA recognition was achieved only when the miniature protein contained the complete functional epitope. Here we report a miniature homeodomain that recognizes its 6-bp target site in the nanomolar concentration range at 25 degrees C, despite the absence of DNA contact residues located along the homeodomain N-terminal arm. We conclude that miniature proteins can achieve high affinity and selectivity for DNA by design even when the functional epitope is incomplete by using pre-organization to effectively compensate for lost protein-DNA contacts. In this case it has been possible to miniaturize both the recognition surface and the structural framework of a globular protein fold.

Structure of the DNA binding region of prospero reveals a novel homeo-prospero domain

The Prospero transcription factor promotes neural differentiation in Drosophila, and its activity is tightly regulated by modulating its subcellular localization. Prospero is exported from the nucleus of neural precursors but imported into the nucleus of daughter cells, which is necessary for their proper differentiation. Prospero has a highly divergent putative homeodomain adjacent to a conserved Prospero domain; both are required for sequence-specific DNA binding. Here we show that the structure of these two regions consists of a single structural unit (a homeo-prospero domain), in which the Prospero domain region is in position to contribute to DNA binding and also to mask a defined nuclear export signal that is within the putative homeodomain region. We propose that the homeo-prospero domain coordinately regulates Prospero nuclear localization and DNA binding specificity.

Hydrolytically active Eu(III) and Ce(IV) EF-hand peptides

A chimeric peptide (P4) has been designed to incorporate an EF-hand metal-binding loop into the context of the helix-turn-helix DNA binding motif of the engrailed homeodomain. This construct binds lanthanides, and in the presence of these metals, promotes the cleavage of supercoiled DNA and model phosphate esters (bisnitrophenyl phosphate). P4 binds lanthanides with moderate affinities (Eu(III), log K(a)=4.85; and Ce(IV), log K(a)=5.23). The structure of P4 is enhanced by metal binding, but the increase in secondary structure observed by CD is small, and suggests the metallopeptide is also quite flexible. Despite this flexibility, the efficient cleavage of DNA at low concentrations is dependent on the metallopeptide, and not on peptide or metal alone. This enhanced reactivity suggests the designed DNA-binding EF-hand peptides deliver the metal to the DNA for catalysis, even without rigid secondary structure.

Structural and thermodynamic characterization of the DNA binding properties of a triple alanine mutant of MATalpha2

Triply mutated MATalpha2 protein, alpha2-3A, in which all three major groove-contacting residues are mutated to alanine, is defective in binding DNA alone or in complex with Mcm1 yet binds with MATa1 with near wild-type affinity and specificity. To gain insight into this unexpected behavior, we determined the crystal structure of the a1/alpha2-3A/DNA complex. The structure shows that the triple mutation causes a collapse of the alpha2-3A/DNA interface that results in a reorganized set of alpha2-3A/DNA contacts, thereby enabling the mutant protein to recognize the wild-type DNA sequence. Isothermal titration calorimetry measurements reveal that a much more favorable entropic component stabilizes the a1/alpha2-3A/DNA complex than the alpha2-3A/DNA complex. The combined structural and thermodynamic studies provide an explanation of how partner proteins influence the sequence specificity of a DNA binding protein.