Interaction with Suv39H1 is critical for Snail-mediated E-cadherin repression in breast cancer.

Expression of E-cadherin, a hallmark of epithelial-mesenchymal transition (EMT), is often lost due to promoter DNA methylation in basal-like breast cancer (BLBC), which contributes to the metastatic advantage of this disease; however, the underlying mechanism remains unclear. Here, we identified that Snail interacted with Suv39H1 (suppressor of variegation 3-9 homolog 1), a major methyltransferase responsible for H3K9me3 that intimately links to DNA methylation. We demonstrated that the SNAG domain of Snail and the SET domain of Suv39H1 were required for their mutual interactions. We found that H3K9me3 and DNA methylation on the E-cadherin promoter were higher in BLBC cell lines. We showed that Snail interacted with Suv39H1 and recruited it to the E-cadherin promoter for transcriptional repression. Knockdown of Suv39H1 restored E-cadherin expression by blocking H3K9me3 and DNA methylation and resulted in the inhibition of cell migration, invasion and metastasis of BLBC. Our study not only reveals a critical mechanism underlying the epigenetic regulation of EMT, but also paves a way for the development of new treatment strategies against this disease.

Identification and functional characterization of a novel mutation of hepatocyte nuclear factor-1alpha gene in a Korean family with MODY3.

AIMS/HYPOTHESIS: After screening 16 Korean families with early onset Type 2 diabetes in search for hepatocyte nuclear factor (HNF) -1alpha gene mutation, we identified a novel missense mutation (R263L) associated with MODY phenotype. We studied the biological characteristics of the mutation and the potential functional consequences based on the crystallographic structure of HNF-1alpha in complex with DNA. METHODS: DNA from subjects with a familial form of early onset diabetes was isolated and HNF-1alpha was sequenced. The R263L substitution was generated by PCR-based sited-directed mutagenesis. Functional and biochemical studies were conducted by reporter assay using glucose-transporter type 2 (GLUT2) or insulin promoters and electrophoretic mobility shift assay, respectively. RESULTS: Transfection of wild-type HNF-1alpha increased the reporter activities of GLUT2 and insulin promoters in NIH3T3 and SK-Hep1 cells, while R263L mutant was defective in transactivation of those promoters. Both wild-type HNF-1alpha and R263L mutant could not transactivate GLUT2 and insulin promoters in MIN6N8 insulinoma cells. R263L mutant had a defective cooperation with its heterodimeric partner HNF-1beta or coactivator p300. R263L mutant protein displayed greatly reduced DNA binding ability, despite its comparable protein stability to the wild-type HNF-1alpha. CONCLUSION/INTERPRETATION: These results suggest that the mutation of HNF-1alpha at codon 263 from arginine to leucine leads to the development of MODY3 through decreased insulin production and defective glucose sensing. These findings are in good agreement with the crystal structure in which R263 makes hydrogen bonds with phosphorus atoms of DNA backbone to mediate the stable binding of HNF-1alpha homeodomain to the promoter.

Crystal structure of human frataxin.

Friedreich's ataxia, an autosomal recessive neurodegenerative disorder characterized by progressive gait and limb ataxia, cardiomyopathy, and diabetes mellitus, is caused by decreased frataxin production or function. The structure of human frataxin, which we have determined at 1.8-A resolution, reveals a novel protein fold. A five-stranded, antiparallel beta sheet provides a flat platform, which supports a pair of parallel alpha helices, to form a compact alphabeta sandwich. A cluster of 12 acidic residues from the first helix and the first strand of the large sheet form a contiguous anionic surface on the protein. The overall protein structure and the anionic patch are conserved in eukaryotes, including animals, plants, and yeast, and in prokaryotes. Additional conserved residues create an extended 1008-A(2) patch on a distinct surface of the protein. Side chains of disease-associated mutations either contribute to the anionic patch, help create the second conserved surface, or point toward frataxin's hydrophobic core. These structural findings predict potential modes of protein-protein and protein-iron binding.

X-ray structure of a truncated form of cytochrome f from chlamydomonas reinhardtii.

A truncated form of cytochrome f from Chlamydomonas reinhardtii (an important eukaryotic model organism for photosynthetic electron transfer studies) has been crystallized (space group P2(1)2(1)2(1); three molecules/asymmetric unit) and its structure determined to 2.0 A resolution by molecular replacement using the coordinates of a truncated turnip cytochrome f as a model. The structure displays the same folding and detailed features as turnip cytochrome f, including (a) an unusual heme Fe ligation by the alpha-amino group of tyrosine 1, (b) a cluster of lysine residues (proposed docking site of plastocyanin), and (c) the presence of a chain of seven water molecules bound to conserved residues and extending between the heme pocket and K58 and K66 at the lysine cluster. For this array of waters, we propose a structural role. Two cytochrome f molecules are related by a noncrystallographic symmetry operator which is a distorted proper 2-fold rotation. This may represent the dimeric relation of the monomers in situ; however, the heme orientation suggested by this model is not consistent with previous EPR measurements on oriented membranes.

Crystal structure of the beta-glycosidase from the hyperthermophile Thermosphaera aggregans: insights into its activity and thermostability.

The glycosyl hydrolases are an important group of enzymes that are responsible for cleaving a range of biologically significant carbohydrate compounds. Structural information on these enzymes has provided useful information on their molecular basis for the functional variations, while the characterization of the structural features that account for the high thermostability of proteins is of great scientific and biotechnological interest. To these ends we have determined the crystal structure of the beta-glycosidase from a hyperthermophilic archeon Thermosphaera aggregans. The structure is a (beta/alpha)8 barrel (TIM-barrel), as seen in other glycosyl hydrolase family 1 members, and forms a tetramer. Inspection of the active site and the surrounding area reveals two catalytic glutamate residues consistent with the retaining mechanism and the surrounding polar and aromatic residues consistent with a monosaccharide binding site. Comparison of this structure with its mesophilic counterparts implicates a variety of structural features that could contribute to the thermostability. These include an increased number of surface ion pairs, an increased number of internal water molecules and a decreased surface area upon forming an oligomeric quaternary structure.

Structure of an RNA internal loop consisting of tandem C-A+ base pairs.

The crystal structure of the RNA octamer 5'-CGC(CA)GCG-3' has been determined from X-ray diffraction data to 2.3 A resolution. In the crystal, this oligomer forms a self-complementary double helix in the asymmetric unit. Tandem non-Watson-Crick C-A and A-C base pairs comprise an internal loop in the middle of the duplex, which is incorporated with little distortion of the A-form double helix. From the geometry of the C-A base pairs, it is inferred that the adenosine imino group is protonated and donates a hydrogen bond to the carbonyl group of the cytosine. The wobble geometry of the C-A+ base pairs is very similar to that of the common U-G non-Watson-Crick pair.

Electron transfer by domain movement in cytochrome bc1.

The cytochrome bc1 is one of the three major respiratory enzyme complexes residing in the inner mitochondrial membrane. Cytochrome bc1 transfers electrons from ubiquinol to cytochrome c and uses the energy thus released to form an electrochemical gradient across the inner membrane. Our X-ray crystal structures of the complex from chicken, cow and rabbit in both the presence and absence of inhibitors of quinone oxidation, reveal two different locations for the extrinsic domain of one component of the enzyme, an iron-sulphur protein. One location is close enough to the supposed quinol oxidation site to allow reduction of the Fe-S protein by ubiquinol. The other site is close enough to cytochrome c1 to allow oxidation of the Fe-S protein by the cytochrome. As neither location will allow both reactions to proceed at a suitable rate, the reaction mechanism must involve movement of the extrinsic domain of the Fe-S component in order to shuttle electrons from ubiquinol to cytochrome c1. Such a mechanism has not previously been observed in redox protein complexes.

Apo structure of the ligand-binding domain of aspartate receptor from Escherichia coli and its comparison with ligand-bound or pseudoligand-bound structures.

The aspartate receptor from E. coli is a dimeric transmembrane-signaling protein that mediates chemotaxis behavior and is the most studied system among the chemotaxis receptors to understand the molecular mechanism for transmembrane signaling. However, there is an unresolved issue for the structural event which initiates the transmembrane signal upon binding to the ligand. Biochemical and genetic evidence implies an intrasubunit mechanism (monomeric model) whereas crystallographic evidence implies an intersubunit mechanism (dimeric model). Crystallographic evidence has been ambiguous because all the apo protein structures contained a pseudoligand sulfate, and a completely ligand-free structure has not been available thus far. Here we present the crystal structure of the ligand binding domain of the aspartate receptor free of the ligand aspartate or pseudoligand sulfate. The structural comparison of this structure with those of ligand-bound and pseudoligand-bound forms revealed that, on ligand or pseudoligand binding, the conformational change in the ligand-binding domain is relatively small, but there is a considerable rotation between two subunits, supporting the dimeric model.

Molecular structure of staphylococcus and streptococcus superantigens.

Staphylococcus aureus and streptococci, notably those belonging to group A, make up a large family of true exotoxins referred to as pyrogenic toxin superantigens. These toxins cause toxic shock-like syndromes and have been implicated in several allergic and autoimmune diseases. Included within this group of proteins are the staphylococcal enterotoxins, designated serotypes A, B, Cn, D, E, and G; two forms of toxic shock syndrome toxin-1 also made by Staphylococcus aureus; the group A streptococcal pyrogenic exotoxins, serotypes A, B, and C; and recently described toxins associated with groups B, C, F, and G streptococci. The nucleotide sequences of the genes for all of the toxins except those from the groups B, C, F, and G streptococcal strains have been sequenced. The sequencing studies indicate that staphylococcal enterotoxins B and C and streptococcal pyrogenic exotoxin A share highly significant sequence similarity; staphylococcal enterotoxins A, D, and E share highly significant sequence similarity; and toxic shock syndrome toxin-1 and streptococcal pyrogenic exotoxin B and C share little, if any, sequence similarity with any of the toxins. Despite the dissimilarities seen in primary amino acid sequence among some members of the toxin family, it was hypothesized that there was likely to be significant three-dimensional structure similarity among all the toxins. The three-dimensional structures of three of the pyrogenic toxin superantigens have been determined recently. The structural features of two of these, toxic shock syndrome toxin-1 and enterotoxin C3, are presented. Toxic shock syndrome-1 exists as a protein with two major domains, referred to as A and B. The molecule begins with a short N-terminal alpha-helix that then leads into a clawshaped structure in domain B that is made up of beta strands.

Crystallization of HMG-CoA reductase from Pseudomonas mevalonii.

Crystals of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase from Pseudomonas mevalonii have been grown by vapor diffusion in hanging drops at pH 6.7 using ammonium sulfate as the precipitant. Serial dilution seeding and manipulation of glycerol concentration were both used to obtain crystals larger than 1.0 mm. The crystals are cubic, space group I4(1)32, with a = 229.4 A. A V(m) value of 2.71 A(3) Da(-l) indicates 96 molecules per unit cell with two molecules in the asymmetric unit. These crystals diffract to 2.8 A with conventional X-ray sources, and beyond 2.4 A with synchrotron radiation.

Investigation of the role of the disulphide bond in the activity and structure of staphylococcal enterotoxin C1.

The goal of this study was to investigate the role of the disulphide bond of staphylococcal enterotoxin C1 (SEC1) in the structure and activity of the toxin. Mutants unable to form a disulphide bond were generated by substituting alanine or serine for cysteine at positions 93 and/or 110. Although we did not directly investigate the residues between the disulphide linkage, tryptic lability showed that significant native structure in the cystine loop is preserved in the absence of covalent bonding between residues 93 and 110. Since no correlation was observed between the behaviour of these mutants with regard to toxin stability, emesis and T cell proliferation we conclude that SEC1-induced emesis and T cell proliferation are dependent on separate regions of the molecule. The disulphide bond itself is not an absolute requirement for either activity. However, conformation within or adjacent to the loop is important for emesis. Although mutants with alanine substitutions were not emetic, those with serine substitutions retained this activity, suggesting that the disulphide linkage stabilizes a crucial conformation but can be replaced by residues which hydrogen bond.

Predictions of T-cell receptor- and major histocompatibility complex-binding sites on staphylococcal enterotoxin C1.

We have focused on regions of staphylococcal enterotoxin C1 (SEC1) causing immunomodulation. N-terminal deletion mutants lacking residues 6 through 13 induced T-cell proliferation similar to that induced by native toxin. However, mutants with residues deleted between positions 19 and 33, although nonmitogenic themselves, were able to inhibit both SEC1-induced T-cell proliferation and binding of the native toxin to major histocompatibility complex (MHC) class II. Presumably, these deletions define a part of SEC1 that interacts with the T-cell receptor. Three synthetic peptides containing residues located in a region analogous to the alpha 5 groove of SEC3 had residual mitogenic activity or blocked T-cell proliferation induced by SEC1 and appear to recognize the same site as SEC1 on a receptor for the toxin, presumably MHC class II. We conclude that isolated portions of the SEC1 molecule can retain residual mitogenic activity but that the entire protein is needed to achieve maximal superantigenic stimulation. Our results, together with the results of other investigators, support a model in which SEC1 binds to an alpha helix of MHC class II through a central groove in the toxin and thereby promotes or stabilizes the interaction between antigen-presenting cells and T cells.

Three-dimensional structure of a human class II histocompatibility molecule complexed with superantigen.

The structure of a bacterial superantigen, Staphylococcus aureus enterotoxin B, bound to a human class II histocompatibility complex molecule (HLA-DR1) has been determined by X-ray crystallography. The superantigen binds as an intact protein outside the conventional peptide antigen-binding site of the class II major histocompatibility complex (MHC) molecule. No large conformational changes occur upon complex formation in either the DR1 or the enterotoxin B molecules. The structure of the complex helps explain how different class II molecules and superantigens associate and suggests a model for ternary complex formation with the T-cell antigen receptor (TCR), in which unconventional TCR-MHC contacts are possible.

Crystallization and preliminary X-ray diffraction analysis of staphylococcal enterotoxin type C.

The Type C staphylococcal enterotoxin produced by Staphylococcus aureus strain FRI-909 has been crystallized using a combination of two precipitants, ammonium sulfate and polyethylene glycol 400, with the addition of small amounts of detergent. Two related crystal forms have been obtained, one triclinic, and one tetragonal, both with one toxin molecule per asymmetric unit. These crystals are stable for at least 75 hr in the X-ray beam and diffract to at least 2.2 and 2.6 A, respectively. The triclinic crystals have unit cell parameters a = 38.5 A, b = 43.7 A, c = 36.9 A, and interaxial angles alpha = 99.9 degrees, beta = 95.8 degrees, and gamma = 98.5 degrees. The tetragonal crystals are of space group P4(1)22 with unit cell parameters a = 43.4 A and c = 278.0 A.