Crystal structure of beta-amylase from Bacillus cereus var. mycoides at 2.2 A resolution.
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The crystal structure of beta-amylase from Bacillus cereus var. mycoides was determined by the multiple isomorphous replacement method. The structure was refined to a final R-factor of 0.186 for 102,807 independent reflections with F/sigma(F) > or = 2.0 at 2.2 A resolution with root-mean-square deviations from ideality in bond lengths, and bond angles of 0.014 A and 3.00 degrees, respectively. The asymmetric unit comprises four molecules exhibiting a dimer-of-dimers structure. The enzyme, however, acts as a monomer in solution. The beta-amylase molecule folds into three domains; the first one is the N-terminal catalytic domain with a (beta/alpha)8 barrel, the second one is the excursion part from the first one, and the third one is the C-terminal domain with two almost anti-parallel beta-sheets. The active site cleft, including two putative catalytic residues (Glu172 and Glu367), is located on the carboxyl side of the central beta-sheet in the (beta/alpha)8 barrel, as in most amylases. The active site structure of the enzyme resembles that of soybean beta-amylase with slight differences. One calcium ion is bound per molecule far from the active site. The C-terminal domain has a fold similar to the raw starch binding domains of cyclodextrin glycosyltransferase and glucoamylase. (+info)
Expression and characterisation of the heavy chain of tetanus toxin: reconstitution of the fully-recombinant dichain protein in active form.
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Tetanus toxin, composed of a disulphide-linked heavy (HC) and light (LC) chain, preferentially blocks the release of inhibitory neurotransmitters in the spinal cord by Zn2+-dependent proteolytic cleavage of synaptobrevin. This intoxication involves binding via HC to ecto-acceptors on peripheral nerve endings, followed by internalisation and retrograde transportation to its prime site of action in central neurons. To facilitate exploitation of the toxin's unique activities, HC was expressed at a high level in Escherichia coli as a fusion with maltose binding protein; after cleavage by thrombin, free HC was isolated and its identity confirmed by Western blotting and N-terminal microsequencing. The expressed and native HC gave very similar circular dichroism spectra, excluding any gross differences in their folded structures. Recombinant HC antagonised the neuromuscular paralysing activity of the native toxin, by competing for binding to neuronal ecto-acceptors. The HC was reconstituted with bacterially-expressed LC to create disulphide-bridged dichain toxin that blocked neuromuscular transmission. The fully-recombinant toxin produced spastic paralysis in mice characteristic of the blockade of central inhibitory synapses, revealing that it undergoes axonal transport to the spinal cord, like the native toxin but with a reduced efficacy. This first report of the large-scale production of recombinant tetanus toxin in active form should facilitate studies on the use of engineered innocuous forms of the toxin as neuronal transport vehicles. (+info)
The spineless-aristapedia and tango bHLH-PAS proteins interact to control antennal and tarsal development in Drosophila.
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The Drosophila spineless (ss) gene encodes a basic-helix-loop-helix-PAS transcription factor that is required for proper specification of distal antennal identity, establishment of the tarsal regions of the legs, and normal bristle growth. ss is the closest known homolog of the mammalian aryl hydrocarbon receptor (Ahr), also known as the dioxin receptor. Dioxin and other aryl hydrocarbons bind to the PAS domain of Ahr, causing Ahr to translocate to the nucleus, where it dimerizes with another bHLH-PAS protein, the aryl hydrocarbon receptor nuclear translocator (Arnt). Ahr:Arnt heterodimers then activate transcription of target genes that encode enzymes involved in metabolizing aryl hydrocarbons. In this report, we present evidence that Ss functions as a heterodimer with the Drosophila ortholog of Arnt, Tango (Tgo). We show that the ss and tgo genes have a close functional relationship: loss-of-function alleles of tgo were recovered as dominant enhancers of a ss mutation, and tgo-mutant somatic clones show antennal, leg, and bristle defects almost identical to those caused by ss(-) mutations. The results of yeast two-hybrid assays indicate that the Ss and Tgo proteins interact directly, presumably by forming heterodimers. Coexpression of Ss and Tgo in Drosophila SL2 cells causes transcriptional activation of reporters containing mammalian Ahr:Arnt response elements, indicating that Ss:Tgo heterodimers are very similar to Ahr:Arnt heterodimers in DNA-binding specificity and transcriptional activation ability. During embryogenesis, Tgo is localized to the nucleus at sites of ss expression. This localization is lost in a ss null mutant, suggesting that Tgo requires heterodimerization for translocation to the nucleus. Ectopic expression of ss causes coincident ectopic nuclear localization of Tgo, independent of cell type or developmental stage. This suggests that the interaction of Ss and Tgo does not require additional signals, unlike the ligand-dependent interaction of Ahr and Arnt. Despite the very different biological roles of Ahr and Arnt in insects and mammals, the molecular mechanisms by which these proteins function appear to be largely conserved. (+info)
Influence of the mu-chain C-terminal sequence on polymerization of immunoglobulin M.
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Immunoglobulin (IgM) is found in various states of covalent polymerization (microL)n, where n is typically 8, 10, or 12. The usual form of IgM of bony fish is tetrameric (8 microL units) as compared to the pentameric form (10 microL units) observed in cartilaginous fish and mammals. Two hypotheses were tested in this study. First, that the length of the mu-chain C terminus following Cys575 determines whether an IgM polymerizes as a tetramer or as a pentamer. This was tested by examining the covalent polymerization state of mouse IgM mutated to contain a series of mu-chain C-termini from bony and cartilaginous fish. The results proved this hypothesis wrong: mouse IgM bearing the C-terminal sequence of shark, salmon and cod mu-chain behaved identically to native mouse IgM, forming predominantly (microL)10 and (microL)12 forms. The second hypothesis was that an additional Cys residue near the C terminus of the mu-chain is responsible for the multiple covalent structures seen in IgM of the channel catfish. The addition of a catfish C terminus to the mouse mu-chain resulted, as predicted, in the production of a series of covalently bonded forms, with the major species being (microL)4. When a Ser-Cys unit was removed from the catfish C terminus added to the mouse mu-chain, this resulted in production of IgM indistinguishable in structure from that of wild-type mouse IgM. (+info)
Structural requirement of the calcium-channel subunit alpha2delta for gabapentin binding.
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Gabapentin [Neurontin, 1-(aminomethyl)cyclohexaneacetic acid] is a novel anticonvulsant drug with a high binding affinity for the Ca(2+)-channel subunit alpha(2)delta. In this study, the gabapentin-binding properties of wild-type and mutated porcine brain alpha(2)delta proteins were investigated. Removal of the disulphide bonds between the alpha(2) and the delta subunits did not result in a significant loss of gabapentin binding, suggesting that the disulphide linkage between the two subunits is not required for binding. Singly expressed alpha(2) protein remained membrane associated. However, alpha(2) alone was unable to bind gabapentin, unless the cells were concurrently transfected with the expression vector for delta, suggesting that both alpha(2) and delta are required for gabapentin binding. Using internal deletion mutagenesis, we mapped two regions [amino acid residues 339-365 (DeltaF) and 875-905 (DeltaJ)] within the alpha(2) subunit that are not required for gabapentin binding. Further, deletion of three other individual regions [amino acid residues 206-222 (DeltaD), 516-537 (DeltaH) and 583-603 (DeltaI)] within the alpha(2) subunit disrupted gabapentin binding, suggesting the structural importance of these regions. Using alanine to replace four to six amino acid residues in each of these regions abolished gabapentin binding. These results demonstrate that region D, between the N-terminal end and the first putative transmembrane domain of alpha(2), and regions H and I, between the putative splicing acceptor sites (Gln(511) and Ser(601)), may play important roles in maintaining the structural integrity for gabapentin binding. Further single amino acid replacement mutagenesis within these regions identified Arg(217) as critical for gabapentin binding. (+info)
Evidence that platelet and tumour heparanases are similar enzymes.
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In order to enter tissues, blood-borne metastatic tumour cells and leucocytes need to extravasate through the vascular basal lamina (BL), a process which involves a battery of degradative enzymes. A key degradative enzyme is the endoglycosidase heparanase, which cleaves heparan sulphate (HS), an important structural component of the vascular BL. Previously, tumour-derived heparanase activity (which has been shown to be related to the metastatic potential of murine and human melanoma cell lines) was reported to cleave HS and be inhibited by heparin, as distinct from human platelet heparanase, which cleaved both substrates [Nakajima, Irimura and Nicolson (1988) J. Cell Biochem. 36, 157-167]. We recently reported the purification of human platelet heparanase and showed that the enzyme is a 50-kDa endoglucuronidase [Freeman and Parish (1998) Biochem. J. 330, 1341-1350]. We now report the purification and characterization of heparanase activity from highly metastatic rat 13762 MAT mammary adenocarcinoma and human HCT 116 colonic carcinoma cells and from rat liver using essentially the same procedure that was reported for purification of the human platelet enzyme. The rat 13762 MAT tumour enzyme, which has a native M(r) of 45 kDa when analysed by gel-filtration chromatography and by SDS/PAGE, was observed to be an endoglucuronidase that degraded heparin and HS to fragments of the same sizes as the human platelet enzyme does. N-deglycosylation of both the human platelet and rat 13762 MAT tumour enzymes gave, in each case, a 41-kDa band by SDS/PAGE analysis, demonstrating that the observed difference in M(r) between the platelet and tumour enzymes may have been due largely to differences in the relative amounts of N-glycosylation. Two peptides were isolated following Endoproteinase Lys-C digestion of both the human platelet and rat 13762 MAT tumour heparanases and were shown to be highly similar. Both the rat liver and human colonic carcinoma heparanases also degraded both heparin and HS to fragments of the same sizes as the human platelet enzyme does. Western-blot analysis of an SDS/PAGE gel using antibodies raised against human platelet heparanase demonstrated that human platelet, human tumour and rat tumour heparanases were immunochemically cross-reactive. In conclusion, because of the similarities in their sizes, substrate specificities, peptide sequences and immunoreactivities, we propose that heparanase activities present in human platelets, rat liver and in rat and human tumour cells are, in fact, mediated by a similar enzyme. (+info)
Processing and functional display of the 86 kDa heterodimeric penicillin G acylase on the surface of phage fd.
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The large heterodimeric penicillin G acylase from Alcaligenes faecalis was displayed on the surface of phage fd. We fused the coding sequence (alpha subunit-internal peptide-beta subunit) to the gene of a phage coat protein. A modified g3p signal sequence was used to direct the polypeptide to the periplasm. Here we show that a heterodimeric enzyme can be expressed as a fusion protein that matures to an active biocatalyst connected to the coat protein of phage fd, resulting in a phage to which the beta-subunit is covalently linked and the alpha-subunit is non-covalently attached. The enzyme can be displayed either fused to the minor coat protein g3p or fused to the major coat protein g8p. In both cases the penicillin G acylase on the phage has the same Michaelis constant as its freely soluble counterpart, indicating a proper folding and catalytic activity of the displayed enzyme. The display of the heterodimer on phage not only allows its further use in protein engineering but also offers the possibility of applying this technology for the excretion of the enzyme into the extracellular medium, facilitating purification of the protein. With the example of penicillin acylase the upper limit for a protein to become functionally displayed by phage fd has been further explored. Polyvalent display was not observed despite the use of genetic constructs designed for this aim. These results are discussed in relation to the pore size being formed by the g4p multimer. (+info)
Receptor signaling: when dimerization is not enough.
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Activation of receptors that signal via tyrosine kinase domains has been thought to involve receptor dimerization and transphosphorylation of juxtaposed catalytic domains. Recent results suggest things might be more complex - specific intersubunit conformational changes within a dimer can also be important. (+info)