Characterization of the minimal DNA-binding domain of the HIV integrase protein.

The human immunodeficiency virus (HIV) integrase (IN) protein mediates an essential step in the retroviral lifecycle, the integration of viral DNA into human DNA. A DNA-binding domain of HIV IN has previously been identified in the C-terminal part of the protein. We tested truncated proteins of the C-terminal region of HIV-1 IN for DNA binding activity in two different assays: UV-crosslinking and southwestern blot analysis. We found that a polypeptide fragment of 50 amino acids (IN220-270) is sufficient for DNA binding. In contrast to full-length IN protein, this domain is soluble under low salt conditions. DNA binding of IN220-270 to both viral DNA and non-specific DNA occurs in an ion-independent fashion. Point mutations were introduced in 10 different amino acid residues of the DNA-binding domain of HIV-2 IN. Mutation of basic amino acid K264 results in strong reduction of DNA binding and of integrase activity.     

LaTBP1: a Leishmania amazonensis DNA-binding protein that associates in vivo with telomeres and GT-rich DNA using a Myb-like domain

Different species of Leishmania can cause a variety of medically important diseases, whose control and treatment are still health problems. Telomere binding proteins (TBPs) have potential as targets for anti-parasitic chemotherapy because of their importance for genome stability and cell viability. Here, we describe LaTBP1 a protein that has a Myb-like DNA-binding domain, a feature shared by most double-stranded telomeric proteins. Binding assays using full-length and truncated LaTBP1 combined with spectroscopy analysis were used to map the boundaries of the Myb-like domain near to the protein only tryptophan residue. The Myb-like domain of LaTBP1 contains a conserved hydrophobic cavity implicated in DNA-binding activity. A hypothetical model helped to visualize that it shares structural homology with domains of other Myb-containing proteins. Competition assays and chromatin immunoprecipitation confirmed the specificity of LaTBP1 for telomeric and GT-rich DNAs, suggesting that LaTBP1 is a new TBP.     

Sequence-specific DNA recognition by the Myb-like domain of plant telomeric protein RTBP1

We have identified a rice gene encoding a DNA-binding protein that specifically recognizes the telomeric repeat sequence TTTAGGG found in plants. This gene, which we refer to as RTBP1 (rice telomere-binding protein 1), encodes a polypeptide with a predicted molecular mass of 70 kDa. RTBP1 is ubiquitously expressed in various organs and binds DNA with two or more duplex TTTAGGG repeats. The predicted protein sequence includes a single domain at the C terminus with extensive homology to Myb-like DNA binding motif. The Myb-like domain of RTBP1 is very closely related to that of other telomere-binding proteins, including TRF1, TRF2, Taz1p, and Tbf1p, indicating that DNA-binding domains of telomere-binding proteins are well conserved among evolutionarily distant species. To obtain precise information on the sequence of the DNA binding site recognized by RTBP1, we analyzed the sequence-specific binding properties of the isolated Myb-like domain of RTBP1. The isolated Myb-like domain was capable of sequence-specific DNA binding as a homodimer. Gel retardation analysis with a series of mutated telomere probes revealed that the internal GGGTTT sequence in the two-telomere repeats is critical for binding of Myb-like domain of RTBP1, which is consistent with the model of the TRF1.DNA complex showing that base-specific contacts are made within the sequence GGGTTA. To the best of our knowledge, RTBP1 is the first cloned gene in which the product is able to bind double-stranded telomeric DNA in plants. Because the Myb-like domain appears to be a significant motif for a large class of proteins that bind the duplex telomeric DNA, RTBP1 may play important roles in plant telomere function in vivo.     

Characterization of a major DNA-binding domain in the herpes simplex virus type 1 DNA-binding protein (ICP8).

We have studied the major DNA-binding protein (ICP8) from herpes simplex virus type 1 to identify its DNA-binding site. Since we obtained our protein from a cell line carrying multiple chromosomally located copies of the ICP8 gene, we first analyzed this protein to assess its similarity to the corresponding viral protein. Our protein resembled the viral protein by molecular weight, response to antibody, preference for binding single-stranded DNA, and ability to lower the melting temperature of poly(dA-dT). To define the DNA-binding domain, we subjected the protein to limited trypsin digestion and separated the peptide products on a sodium dodecyl sulfate-polyacrylamide gel. These fragments were then transferred to a nitrocellulose membrane, renatured in situ, and tested for their ability to bind DNA. From this assay, we identified four fragments which both bound DNA and exhibited the expected binding preference for single-stranded DNA. The sequence of the smallest of these fragments was determined and corresponds to a polypeptide spanning residues 300 to 849 in the intact protein. This peptide contains several regions which may be important for DNA binding based on sequence similarities in single-stranded DNA-binding proteins from other herpesviruses and, in one case, on a conserved sequence found in more distant procaryotic and eucaryotic proteins.     

A putative DNA-binding domain in the NUCKS protein

We have studied the DNA-binding properties of a NUCKS-derived, synthetic peptide containing an extended GRP motif. This peptide binds to random-sequence DNA, but did not bind preferentially to poly(dA-dT). A synthetic peptide with the same amino acid composition but with a random sequence did not bind to DNA, suggesting that the structure of the DNA-binding domain plays a pivotal role in the interaction with DNA. NMR and graphic modeling were employed to investigate the structure of the synthetic peptide. It was shown that the DNA-binding peptide constituted an alpha helix in phosphate buffer at pH 5.5. Docking results indicated an almost perfect fit for this small, helical peptide into the major groove of DNA with the possibility of four basic residues interacting with the phosphate backbone of DNA. One consensus site for phosphorylation by Cdk1 is located in the N-terminal end of the DNA-binding peptide. Upon phosphorylation of this site, the binding to DNA was completely prohibited. Immunofluorescence experiments showed that NUCKS was located in the nuclei in proliferating cells in interphase of the cell cycle, but was distributed throughout the cytoplasm in mitotic cells.     

DNA-binding domain in adenovirus core protein VII

Major core protein, VII of adenovirus type 2 can be cleaved with BrCN into four fragments A, B, C and D according to the electrophoretic mobility on SDS-polyacrylamide gel. The sequential order of the fragments was ACBD from NH2-terminus to COOH-end. From the results of protein blotting experiment and nitrocellulose filter binding assay, fragments A and C, the amino end half of VII, was suggested to be DNA-binding domain. Furthermore, the binding of VII to DNA is suggested to become tighter as the fragment A moiety of pVII is processed in the virion.     

Analysis of the DNA-binding domain of Escherichia coli DnaA protein

The DNA-binding domain of the Escherichia coli DnaA protein is represented by the 94 C-terminal amino acids (domain 4, aa 374-467). The isolated DNA-binding domain acts as a functional repressor in vivo, as monitored with a mioC:lacZ translational fusion integrated into the chromosome of the indicator strain. In order to identify residues required for specific DNA binding, site-directed and random PCR mutagenesis were performed, using the mioC:lacZ construct for selection. Mutations defective in DNA binding were found all over the DNA-binding domain with some clustering in the basic loop region, within presumptive helix B and in a highly conserved region at the N-terminus of presumptive helix C. Surface plasmon resonance (SPR) analysis revealed different binding classes of mutant proteins. No or severely reduced binding activity was demonstrated for amino acid substitutions at positions R399, R407, Q408, H434, T435, T436 and A440. Altered binding specificity was found for mutations in a 12 residue region close to the N-terminus of helix C. The defects of the classical temperature sensitive mutants dnaA204, dnaA205 and dnaA211 result from instability of the proteins at higher temperatures. dnaX suppressors dnaA71 and dnaA721 map to the region close to helix C and bind DNA non-specifically.     

Functional analyses of the Dof domain, a zinc finger DNA-binding domain, in a pumpkin DNA-binding protein AOBP

AOBP, a DNA-binding protein in pumpkin, contains a Dof domain that is composed of 52 amino acid residues and is highly conserved in several DNA-binding proteins of higher plants. The Dof domain has a significant resemblance to Cys2/Cys2 zinc finger DNA-binding domains of steroid hormone receptors and GATA1, but has a longer putative loop where an extra Cys residue is conserved. We show that the Dof domain in AOBP functions as a zinc finger DNA-binding domain and suggest that the Cys residue uniquely conserved in the putative loop might negatively regulate the binding to DNA.     

DNA-binding specificity and dimerization of the DNA-binding domain of the PEND protein in the chloroplast envelope membrane

The PEND protein is a DNA-binding protein in the inner envelope membrane of a developing chloroplast, which may anchor chloroplast nucleoids. Here we report the DNA-binding characteristics of the N-terminal basic region plus leucine zipper (bZIP)-like domain of the PEND protein that we call cbZIP domain. The basic region of the cbZIP domain diverges significantly from the basic region of known bZIP proteins that contain a bipartite nuclear localization signal. However, the cbZIP domain has the ability to dimerize in vitro. Selection of binding sites from a random sequence pool indicated that the cbZIP domain preferentially binds to a canonical sequence, TAAGAAGT. The binding site was also confirmed by gel mobility shift analysis using a representative binding site within the chloroplast DNA. These results suggest that the cbZIP domain is a unique DNA-binding domain of the chloroplast protein.     

Characterization of the chymotryptic core of the adenovirus DNA-binding protein

A fragment of the DNA-binding protein of adenovirus type 5 has been obtained by controlled chymotryptic digestion of the entire molecule. Partial sequence determination indicates that the fragment consists of amino acids 174-525. The fragment is biologically active as measured by its ability to substitute for the entire molecule in a reconstituted DNA replication system. Crystals have been obtained that show diffraction to 2 A.     

Insect proteins homologous to mammalian high mobility group protein 1. Characterization and DNA-binding properties.

Two chromosomal high mobility group (HMG) proteins from larvae of Chironomus thummi (Diptera) and from an epithelial cell line of Chironomus tentans were purified to homogeneity and chemically characterized. cDNA clones encoding these proteins were isolated from an expression library using an immunoscreening approach and were sequenced. The deduced amino acid sequences revealed their homology to HMG protein 1 of vertebrates. These insect proteins have therefore been designated cHMG1a and cHMG1b. They have a molecular mass of 12,915 and 12,019 kDa, respectively, and preferentially bind to AT-rich DNA. Indirect immunofluorescence microscopy with a polyclonal antibody showed the presence of cHMG1a and cHMG1b in condensed chromomeres but not in puffs, nucleoli, and cytoplasm. The cHMG1a and cHMG1b genes were both localized to a single band in region 14 of chromosome 1 of C. tentans and appear to be single copy genes. An immunologically related protein was purified from Drosophila melanogaster Kc cells. Its size and amino acid composition indicate that it is an HMG1 of D. melanogaster. On the other hand, our antibody did not recognize calf HMG1. The identification and characterization of HMG1 proteins in insects with polytene chromosomes opens new possibilities for studying function(s) of this group of chromosomal proteins.     

Structure and DNA-binding activity of the Pyrococcus furiosus SMC protein hinge domain

Structural Maintenance of Chromosomes (SMC) proteins are essential for a wide range of processes including chromosome structure and dynamics, gene regulation, and DNA repair. While bacteria and archaea have one SMC protein that forms a homodimer, eukaryotes possess three distinct SMC complexes, consisting of heterodimeric pairs of six different SMC proteins. SMC holocomplexes additionally contain several specific regulatory subunits. The bacterial SMC complex is required for chromosome condensation and segregation. In eukaryotes, this function is carried out by the condensin (SMC2-SMC4) complex. SMC proteins consist of N-terminal and C-terminal domains that fold back onto each other to create an ATPase "head" domain, connected to a central "hinge" domain via a long coiled-coil region. The hinge domain mediates dimerization of SMC proteins and binds DNA. This activity implicates a direct involvement of the hinge domain in the action of SMC proteins on DNA. We studied the SMC hinge domain from the thermophilic archaeon Pyrococcus furiosus. Its crystal structure shows that the SMC hinge domain fold is largely conserved between archaea and bacteria as well as eukarya. Like the eukaryotic condensin hinge domain, the P. furiosus SMC hinge domain preferentially binds single-stranded DNA (ssDNA), but its affinity for DNA is weaker than that of its eukaryotic counterpart, and point mutations reveal that its DNA-binding surface is more confined. The ssDNA-binding activity of its hinge domain might play a role in the DNA-loading process of the prokaryotic SMC complex during replication.     

Characterization of DNA recognition by the human UV-damaged DNA-binding protein.

The UV-damaged DNA-binding (UV-DDB) protein is the major factor that binds DNA containing damage caused by UV radiation in mammalian cells. We have investigated the DNA recognition by this protein in vitro, using synthetic oligonucleotide duplexes and the protein purified from a HeLa cell extract. When a 32P-labeled 30-mer duplex containing the (6-4) photoproduct at a single site was used as a probe, only a single complex was detected in an electrophoretic mobility shift assay. It was demonstrated by Western blotting that both of the subunits (p48 and p127) were present in this complex. Electrophoretic mobility shift assays using various duplexes showed that the UV-DDB protein formed a specific, high affinity complex with the duplex containing an abasic site analog, in addition to the (6-4) photoproduct. By circular permutation analyses, these DNA duplexes were found to be bent at angles of 54 degrees and 57 degrees in the complexes with this protein. From the previously reported NMR studies and the fluorescence resonance energy transfer experiments in the present study, it can be concluded that the UV-DDB protein binds DNA that can be bent easily at the above angle.