Solution structure of the epithelial cadherin domain responsible for selective cell adhesion.

Cadherins are calcium-dependent cell adhesion molecules containing extracellular repeats of approximately 110 amino acids. The three-dimensional structure of the amino-terminal repeat of mouse epithelial cadherin was determined by multidimensional heteronuclear magnetic resonance spectroscopy. The calcium ion was bound by a short alpha helix and by loops at one end of the seven-stranded beta-barrel structure. An exposed concave face is in a position to provide homophilic binding specificity and was also sensitive to calcium ligation. Unexpected structural similarities with the immunoglobulin fold suggest an evolutionary relation between calcium-dependent and calcium-independent cell adhesion molecules.

Purification and spectroscopic characterization of a recombinant amino-terminal polypeptide fragment of mouse epithelial cadherin.

Cadherins are a family of Ca(2+)-dependent cell adhesion molecules containing four extracellular tandem repeats each of 110 amino acids. The most amino-terminal repeat is believed to confer the specificity of cell adhesion. A polypeptide containing the amino-terminal repeat of mouse epithelial cadherin has been over-expressed in E. coli and purified to homogeneity. This polypeptide binds Ca2+ with a dissociation constant of 1.6 x 10(-4) M. CD and NMR experiments indicate that the polypeptide adopts a predominantly beta-sheet conformation and that binding of Ca2+ induces only small conformational changes.

Lateral self-assembly of E-cadherin directed by cooperative calcium binding.

We report the Ca2+ binding characteristics of recombinant Ecad12, a construct spanning the first two repeats of epithelial cadherin, and demonstrate the links between Ca2+ binding and dimer formation. Sedimentation equilibrium and dynamic light scattering experiments show that weak dimerization of Ecad12 occurs in the presence of 10 mM Ca2+ (KdP = 0.17 mM), while no appreciable dimer formation was detected in the absence of Ca2+. Ca2+-induced dimerization was also observed in electron microscopy images of Ecad12. We conclude from Ca2+ titration experiments monitored by tryptophan fluorescence and flow dialysis that dimerization does not affect the equilibrium binding constant for Ca2+. However, the value of the Hill coefficient for Ca2+ binding increases from 1.5 to 2.4 as the protein concentration increases, showing that dimer formation largely contributes to the cooperativity in Ca2+ binding. Based on these observations and previous crystallographic studies, we propose that calcium acts more likely as a geometrical aligner ensuring the proper assembly of cadherin molecules, rather than a simple adhesive.

HLA-B-associated transcript 3 (Bat3/Scythe) negatively regulates Smad phosphorylation in BMP signaling.

Members of the transforming growth factor-ß (TGF-ß) superfamily participate in numerous biological phenomena in multiple tissues, including in cell proliferation, differentiation, and migration. TGF-ß superfamily proteins therefore have prominent roles in wound healing, fibrosis, bone formation, and carcinogenesis. However, the molecular mechanisms regulating these signaling pathways are not fully understood. Here, we describe the regulation of bone morphogenic protein (BMP) signaling by Bat3 (also known as Scythe or BAG6). Bat3 overexpression in murine cell lines suppresses the activity of the Id1 promoter normally induced by BMP signaling. Conversely, Bat3 inactivation enhances the induction of direct BMP target genes, such as Id1, Smad6, and Smad7. Consequently, Bat3 deficiency accelerates the differentiation of primary osteoblasts into bone, with a concomitant increase in the bone differentiation markers Runx2, Osterix, and alkaline phosphatase. Using biochemical and cell biological analyses, we show that Bat3 inactivation sustains the C-terminal phosphorylation and nuclear localization of Smad1, 5, and 8 (Smad1/5/8), thereby enhancing biological responses to BMP treatment. At the mechanistic level, we show that Bat3 interacts with the nuclear phosphatase small C-terminal domain phosphatase (SCP) 2, which terminates BMP signaling by dephosphorylating Smad1/5/8. Notably, Bat3 enhances SCP2-Smad1 interaction only when the BMP signaling pathway is activated. Our results demonstrate that Bat3 is an important regulator of BMP signaling that functions by modulating SCP2-Smad interaction.

Partial purification and characterization of two extracellular N-acetyl-D-glucosaminidases produced by the entomopathogenic fungus Beauveria bassiana.

Beauveria bassiana grown in a liquid medium containing N-acetyl-D-glucosamine and colloidal chitin produced two distinct N-acetyl-D-glucosaminidases, NAGase 1 and NAGase 2. NAGase 1 had a molecular weight of 97,000 and NAGase 2 was comprised of two subunits, of molecular weights 64,000 and 66,000. The optimal temperature and pH for NAGase 1 were 57 degrees C and pH 5 and for NAGase 2 they were 37 degrees C and pH 5. NAGase 1 was more thermostable than NAGase 2. The isolectric points of NAGase 1 and 2 were ca. pH 9.5 and 5.5, respectively. NAGase 1 and 2 were unaffected by a 10 mM concentration of chloride salts of the ions Ca2+, Mg2+, or Zn2+, by 10 mM EDTA, and by 0.25-1 mM of short chain fatty acids. Dithiothreitol caused some inactivation of NAGase 2 while stimulating activity of NAGase 1. NAGase 1 had a Km of 0.38 mM and a Kcat/Km of 3923.88 M-1.s-1.NAGase 2 had a Km of 2.095 mM and a Kcat/Km of 411.88 M-1.s-1 when p-nitrophenyl-beta-N-acetylglucosaminide was used as the substrate.

Solution structure of the C-terminal core domain of human TFIIB: similarity to cyclin A and interaction with TATA-binding protein.

TFIIB is an essential component of the machinery that transcribes protein-coding genes. The three-dimensional structure of the human TFIIB core domain (TFIIBc) has been determined using multidimensional heteronuclear magnetic resonance spectroscopy. The molecule consists of two direct repeats that adopt similar alpha-helical folds, conferring pseudo-twofold symmetry. An extensive, central basic surface including an amphipathic alpha helix is critical to the function of TFIIB as a bridge between the TBP-promoter complex and RNA polymerase II and associated general and regulatory transcription factors. Similarities between the TFIIBc and cyclin A folds indicate that elements of the eukaryotic cell cycle control apparatus evolved from more fundamental transcriptional control components, demonstrating a link between the transcription and cell cycle molecular machineries.

1H, 15N and 13C resonance assignments and monomeric structure of the amino-terminal extracellular domain of epithelial cadherin.

E-cadherin is a transmembrane protein that provides Ca(2+)-dependent cell adhesion to epithelial cells. The large majority of the 1H, 15N, 13C and 13CO resonances of a 146-amino acid polypeptide from epithelial (E-) cadherin have been assigned using multidimensional NMR spectroscopy. The structure of the amino-terminal 100 amino acids, corresponding to the first extracellular repeat of E-cadherin [Overduin et al. (1995) Science, 267, 386-389], has been refined. The monomeric state of this isolated domain is demonstrated by light scattering and sedimentation analysis. Seven beta-strands and two short helices were identified by patterns of NOE cross-peaks, vicinal coupling constants and chemical shift indices. A novel structural motif termed a quasi-beta-helix found in the crystal structure of a neural (N-) cadherin domain [Shapiro et al. (1995) Nature, 374, 327-337] is characterized in detail for the first time by NMR. Slowly exchanging amides were concentrated in the beta-sheet region and quasi-beta-helix. The beta-barrel fold of the cadherin domain is topologically similar to the immunoglobulin fold. Comparison of this solution structure to the crystallized dimers of the N-terminal pair of E-cadherin domains [Nagar et al. (1996) Nature, 380, 360-364] and of the homologous single domain of N-cadherin reveals a conserved cadherin fold with minor structural differences, which can be accounted for by differences in metal ligation and oligomeric state.

The button test: a small scale method using microdialysis cells for assessing protein solubility at concentrations suitable for NMR.

A simple method has been developed for screening solution conditions to determine conditions under which a protein is soluble at the high concentrations typically used for NMR spectroscopy. The method employs microdialysis cells or 'buttons'. The low sample volume (5 microliters) required for each microdialysis button permits testing of a wide range of solution conditions and temperatures with high protein concentrations, using a small amount of protein. Following precipitation of several NMR samples of the C-terminal core domain of human TFIIB, the microdialysis button screen facilitated identification of conditions in which precipitation of the TFIIB core domain was eliminated. The microdialysis button method for screening solution conditions is generally applicable and has been used to permit rapid identification of suitable NMR sample solution conditions for proteins involved in transcription and cell adhesion.

Human general transcription factor TFIIB: conformational variability and interaction with VP16 activation domain.

Human TFIIB, an essential factor in transcription of protein-coding genes by RNA polymerase II, consists of an amino-terminal zinc binding domain (TFIIBn) connected by a linker of about 60 residues to a carboxy-terminal core domain (TFIIBc). The TFIIB core domain has two internally repeated motifs, each comprising five alpha-helices arranged as in the cyclin box. Compared to the crystal structure of TFIIBc in complex with TBP and a TATA-containing oligonucleotide, the NMR-derived solution structure of free TFIIBc is more compact, with a different repeat-repeat orientation and a significantly shorter first helix in the second repeat. Analysis of backbone 15N relaxation parameters indicates the presence of relatively large amplitude, nanosecond time-scale motions in the TFIIBc interrepeat linker and structural fluctuations throughout the backbone. Interaction of TFIIBc with the acidic activation domain of VP16 or with TFIIBn induces 1H-15N chemical shift and line width changes concentrated in the first repeat, interrepeat linker and the first helix of the second repeat. These results suggest that TFIIB is somewhat pliable and that the conformation of the C-terminal core domain can be modulated by interaction with the N-terminal zinc binding domain. Furthermore, binding of the VP16 activation domain may promote TFIIBc conformations primed for binding to a TBP-DNA complex.

FRET-based in vivo Ca2+ imaging by a new calmodulin-GFP fusion molecule.

Intracellular Ca2+ acts as a second messenger that regulates numerous physiological cellular phenomena including development, differentiation and apoptosis. Cameleons, a class of fluorescent indicators for Ca2+ based on green fluorescent proteins (GFPs) and calmodulin (CaM), have proven to be a useful tool in measuring free Ca2+ concentrations in living cells. Traditional cameleons, however, have a small dynamic range of fluorescence resonance energy transfer (FRET), making subtle changes in Ca2+ concentrations difficult to detect and study in some cells and organelles. Using the NMR structure of CaM bound to the CaM binding peptide derived from CaM-dependent kinase kinase (CKKp), we have rationally designed a new cameleon that displays a two-fold increase in the FRET dynamic range within the physiologically significant range of cytoplasmic Ca2+ concentration of 0.05-1 microM.

Solution structure of a TBP-TAF(II)230 complex: protein mimicry of the minor groove surface of the TATA box unwound by TBP.

General transcription factor TFIID consists of TATA box-binding protein (TBP) and TBP-associated factors (TAF(II)s), which together play a central role in both positive and negative regulation of transcription. The N-terminal region of the 230 kDa Drosophila TAF(II) (dTAF(II)230) binds directly to TBP and inhibits TBP binding to the TATA box. We report here the solution structure of the complex formed by dTAF(II)230 N-terminal region (residues 11-77) and TBP. dTAF(II)230(11-77) comprises three alpha helices and a beta hairpin, forming a core that occupies the concave DNA-binding surface of TBP. The TBP-binding surface of dTAF(II)230 markedly resembles the minor groove surface of the partially unwound TATA box in the TBP-TATA complex. This protein mimicry of the TATA element surface provides the structural basis of the mechanism by which dTAF(II)230 negatively controls the TATA box-binding activity within the TFIID complex.

Calcium-regulated DNA binding and oligomerization of the neuronal calcium-sensing protein, calsenilin/DREAM/KChIP3.

Calsenilin/DREAM/KChIP3, a member of the recoverin branch of the EF-hand superfamily, interacts with presenilins, serves as a calcium-regulated transcriptional repressor, and interacts with A-type potassium channels. Here we report physicochemical characterization of calcium binding, oligomerization, and DNA binding of human calsenilin/DREAM/KChIP3. Equilibrium Ca(2+) binding measurements indicate that the protein binds 3 Ca(2+) with a dissociation constant of 14 microM and a Hill coefficient of 0.7. Dynamic light scattering and size exclusion chromatography show that the Ca(2+)-bound protein exists as a dimer at protein concentrations lower than 150 microM and forms a tetramer at concentrations above 200 microM. The Ca(2+)-free protein is a tetramer in the concentration range 20-450 microM. Isothermal titration calorimetry and dynamic light scattering indicate that the Ca(2+)-free protein tetramer binds endothermically (DeltaH = +25 kcal/mol) to four molecules of DNA derived from the downstream regulatory element (DRE) of either the prodynorphin or c-fos genes. One DRE molecule binds tightly to the protein with a dissociation constant (K(d)) of 75 nM, and the other three bind more weakly (K(d) = 640 nM). No significant DNA binding was observed for the Ca(2+)-bound protein. The N-terminal protein fragment (residues 1-70) binds nonspecifically to DRE in a Ca(2+)-independent manner, whereas a C-terminal fragment containing the four EF-hands (residues 65-256) binds DRE (K(d) = 200 nM) in a Ca(2+)-regulated and sequence-specific fashion. The C-terminal fragment is a tetramer in the Ca(2+)-free state and dissociates into dimers at saturating Ca(2+) levels.

Solution structure of the homodimeric core domain of Escherichia coli histidine kinase EnvZ.

Escherichia coli osmosensor EnvZ is a protein histidine kinase that plays a central role in osmoregulation, a cellular adaptation process involving the His-Asp phosphorelay signal transduction system. Dimerization of the transmembrane protein is essential for its autophosphorylation and phosphorelay signal transduction functions. Here we present the NMR-derived structure of the homodimeric core domain (residues 223-289) of EnvZ that includes His 243, the site of autophosphorylation and phosphate transfer reactions. The structure comprises a four-helix bundle formed by two identical helix-turn-helix subunits, revealing the molecular assembly of two active sites within the dimeric kinase.

NMR structure of the histidine kinase domain of the E. coli osmosensor EnvZ.

Bacteria live in capricious environments, in which they must continuously sense external conditions in order to adjust their shape, motility and physiology. The histidine-aspartate phosphorelay signal-transduction system (also known as the two-component system) is important in cellular adaptation to environmental changes in both prokaryotes and lower eukaryotes. In this system, protein histidine kinases function as sensors and signal transducers. The Escherichia coli osmosensor, EnvZ, is a transmembrane protein with histidine kinase activity in its cytoplasmic region. The cytoplasmic region contains two functional domains: domain A (residues 223-289) contains the conserved histidine residue (H243), a site of autophosphorylation as well as transphosphorylation to the conserved D55 residue of response regulator OmpR, whereas domain B (residues 290-450) encloses several highly conserved regions (G1, G2, F and N boxes) and is able to phosphorylate H243. Here we present the solution structure of domain B, the catalytic core of EnvZ. This core has a novel protein kinase structure, distinct from the serine/threonine/tyrosine kinase fold, with unanticipated similarities to both heatshock protein 90 and DNA gyrase B.