An invariant asparagine in the POU-specific homeodomain regulates the specificity of the Oct-2 POU motif

Switching DNA-binding specificity by unnatural amino acid substitution

The specificity of protein–nucleic acid recognition is believed to originate largely from hydrogen bonding between protein polar atoms, primarily side-chain and polar atoms of nucleic acid bases. One way to design new nucleic acid binding proteins of novel specificity is by structure-guided alterations of the hydrogen bonding patterns of a nucleic acid–protein complex. We have used cI repressor of bacteriophage λ as a model system. In the λ-repressor–DNA complex, the ɛ-NH₂ group (hydrogen bond donor) of lysine-4 of λ-repressor forms hydrogen bonds with the amide carbonyl atom of asparagine-55 (acceptor) and the O6 (acceptor) of CG6 of operator site O_L1. Substitution of lysine-4 (two donors) by iso-steric S-(2-hydroxyethyl)-cysteine (one donor and one acceptor), by site-directed mutagenesis and chemical modification, leads to switch of binding specificity of λ-repressor from C:G to T:A at position 6 of O_L1. This suggests that unnatural amino acid substitutions could be a simple way of generating nucleic acid binding proteins of altered specificity.

Comparison of X-ray and NMR structures for the Antennapedia homeodomain-DNA complex

Homeodomains are one of the key families of eukaryotic DNA-binding motifs and provide an important model system for studying protein-DNA interactions. We have crystallized the Antennapedia homeodomain-DNA complex and solved this structure at 2.4 A resolution. NMR and molecular dynamics studies had implied that this homeodomain achieves specificity through an ensemble of rapidly fluctuating DNA contacts. The crystal structure is in agreement with the underlying NMR data, but our structure reveals a well-defined set of contacts and also reveals the locations and roles of water molecules at the protein-DNA interface. The synthesis of X-ray and NMR studies provides a unified, general model for homeodomain-DNA interactions.

Conservative Val47 residue of POU homeodomain: role in DNA recognition

Conservative Val47 residue, located in the third recognition helix of the Oct-2 POU domain, was alternately substituted with other 19 amino acids. Affinity and specificity of interaction with oct-site ATGCAAANGA and homeo-specific site ATAANGA were determined for all mutants. The wild type protein (with Val47) has maximal affinity and specificity in POU domain interaction with octamer sequence. However, V47I mutant showed stronger interaction with homeo-specific site. The highest specificity of interaction with homeo-site was recorded for V47S mutant. We conclude that only Val47 provides sequence-specific high-affinity binding of POU proteins with octamer targets other than the homeo-specific site. It is shown also that damages caused by point mutations may be at least partially compensated by participation in the oct-site recognition of both POUh and POUs domains.

Cysteine 50 of the POU H domain determines the range of targets recognized by POU proteins

The best target of POU proteins (Oct-1, Oct-2) is an octamer sequence ATGCAAAT. POU proteins also recognize, with weaker affinity, the TAAT-like targets of another group of regulatory factors, the homeoproteins. Up to now, it has not been known why Cys50 of the POUHdomain is absolutely conserved in contrast to that in homeoproteins. To assess the importance of Cys50 in determining the binding specificity of POU proteins, all possible amino acids were substituted for Cys at position 50, and the resulting mutants were tested with probes containing octamer (ATGCAAATNN) or homeospecific binding sites. Only the wild-type POU was shown to adequately discriminate between the octamer and homeospecific sites, and the protein affinity was only slightly affected by the nucleotide sequence flanking the octamer at the 3'-end. Any amino acid substitution at position 50 resulted in the mutant protein binding efficiently both to the octamer and the TAAT-like sequences. Moreover, in this case the 3'-flanking sequences influenced the binding to a much greater extent.

Linker length and composition influence the flexibility of Oct-1 DNA binding

POU domain transcription factors have two separate helix-turn-helix DNA-binding subdomains, the POU homeodomain (POUhd) and the POU-specific domain (POUs). Each subdomain recognizes a specific subsite of 4 or 5 bp in the octamer recognition sequence. The Oct-1 POU subdomains are connected by a 23 amino acid unstructured linker region. To investigate the requirements for the linker and its role in DNA recognition, we constructed POU domains in which the subdomains are connected with linkers varying in length between 2 and 37 amino acids. Binding to the natural octamer site required a minimal linker length of between 10 and 14 amino acids. A POU domain with an eight amino acid linker, however, had a high affinity for a site in which the POUs recognition sequence was inverted. Computer modelling shows that inversion of the POUs subdomain shortens the distance between the subdomains sufficiently to enable an eight amino acid linker to bridge the distance. DNase I footprinting as well as mutation of the POUs-binding site confirms the inverted orientation of the POUs domain. Switching of the POUs and POUhd subdomains and separation by 3 bp leads to a large distance which could only be bridged effectively by a long 37 amino acid linker. In addition to linker length, mutation of a conserved glutamate residue in the linker affected binding. As shown by surface plasmon resonance measurements, this was caused by a decrease in the on-rate. Our data indicate that there are both length and sequence requirements in the linker region which allow flexibility leading to selective binding to differently spaced and oriented subsites.

DNA binding properties of the Saccharomyces cerevisiae DAT1 gene product

The DAT1 gene of Saccharomyces cerevisiae encodes a DNA binding protein (Dat1p) that specifically recognizes the minor groove of non-alternating oligo(A).oligo(T) tracts. Sequence-specific recognition requires arginine residues found within three perfectly repeated pentads (G-R-K-P-G) of the Dat1p DNA binding domain [Reardon, B. J., Winters, R. S., Gordon, D., and Winter, E. (1993) Proc. Natl. Acad. Sci. USA 90, 11327-1131]. This report describes a rapid and simple method for purifying the Dat1p DNA binding domain and the biochemical characterization of its interaction with oligo(A).oligo(T) tracts. Oligonucleotide binding experiments and the characterization of yeast genomic Dat1p binding sites show that Dat1p specifically binds to any 11 base sequence in which 10 bases conform to an oligo(A).oligo(T) tract. Binding studies of different sized Dat1p derivatives show that the Dat1p DNA binding domain can function as a monomer. Competition DNA binding assays using poly(I).poly(C), demonstrate that the minor groove oligo(A).oligo(T) constituents are not sufficient for high specificity DNA binding. These data constrain the possible models for Dat1p/oligo(A).oligo(T) complexes, suggest that the DNA binding domain is in an extended structure when complexed to its cognate DNA, and show that Dat1p binding sites are more prevalent than previously thought.

Solution structure of a POU-specific homeodomain: 3D-NMR studies of human B-cell transcription factor Oct-2

The POU DNA-binding motif defines a conserved family of eukaryotic transcription factors involved in regulation of gene expression. This bipartite motif consists of an N-terminal POU-specific domain (POUs), a flexible linker, and a C-terminal POU-specific homeodomain (POUHD). Here we describe the solution structure of a POU-specific homeodomain. An NMR model is obtained from Oct-2, a human B-cell specific transcription factor which participates in the regulation of immunoglobulin genes. A fragment of Oct-2 containing POUHD and an adjoining linker was expressed in Escherichia coli and characterized by three-dimensional nuclear magnetic resonance (3D-NMR) spectroscopy. Complete 1H and 15N resonance assignment of the POUHD moiety is presented. The POUHD solution structure, as calculated by distance geometry and simulated annealing (DG/SA), is similar to that of canonical homeodomains. A salient difference between solution and crystal structures is observed in the C-terminal segment of alpha-helix 3 (the HTH recognition helix), which is not well ordered in solution. Because this segment presumably folds upon specific DNA binding, its flexibility in solution may reduce the intrinsic DNA affinity of POUHD in the absence of POUs.