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At least 1,531 records · Page 85Linked to original sources
PubMed2003

Localization of Mycoplasma pneumoniae cytadherence-associated protein HMW2 by fusion with green fluorescent protein: implications for attachment organelle structure.

The terminal organelle of the cell wall-less pathogenic bacterium Mycoplasma pneumoniae is a complex structure involved in adherence, gliding motility and cell division. This membrane-bound extension of the mycoplasma cell possesses a characteristic electron-dense core. A number of proteins having direct or indirect roles in M. pneumoniae cytadherence have been previously localized to the terminal organelle. However, the cytadherence-accessory protein HMW2, which is required for the stabilization of several terminal organelle components, has been refractory to antibody-based approaches to subcellular localization. In the current study, we constructed a sandwich fusion of HMW2 and enhanced green fluorescent protein (EGFP) and expressed this fusion in wild-type M. pneumoniae and the hmw2- mutant I-2. The fusion protein was produced in both backgrounds at wild-type levels and supported stabilization of proteins HMW1, HMW3 and P65, and haemadsorption function in mutant I-2. Furthermore, the fusion protein was fluorescent and localized specifically to the terminal organelle. However, the EGFP moiety appeared to interfere partially with processes related to cell division, as transformant cells exhibited an increased incidence of bifurcated attachment organelles. These data together with structural predictions suggest that HMW2 is the defining component of the electron-dense core of the terminal organelle.

Bacterial Adhesion↗
PubMed2004

Psychological structure and neural correlates of event knowledge.

Humans are capable of storing and retrieving sequences of complex structured events. Here we report a study in which we establish the psychological structure of event knowledge and then use parametric event-related functional magnetic resonance imaging to identify its neural correlates. We demonstrate that event knowledge is organized along dissociable dimensions that are reflected in distinctive patterns of neural activation: social valence (amygdala and right orbitofrontal cortex), experience (medial prefrontal cortex) and engagement (left orbitofrontal cortex). Our study affirms the importance and uniqueness of the human prefrontal cortex in representing event knowledge.

Adaptation, Psychological↗
PubMed1998

A tetrameric subunit stoichiometry for a glutamate receptor-channel complex.

The structure of glutamate receptor-channel (GluR) subunits has recently been shown to differ from that of other ligand-gated channels and to contain a voltage-gated channel-like pore-forming motif. The view that the structure of GluR complexes is similar to the pentameric structure of other ligand-gated channels was questioned here. Studies of the response properties of the GluR1 subunit of the AMPA subtype of GluRs, co-expressed in Xenopus oocytes with its L646A mutant, which differs only by a greatly reduced sensitivity to quisqualate, provide new evidence suggesting that the GluR1 homomeric receptor channel has a tetrameric structure.

Animals↗
PubMed1982

The structure of the EF-Tu . GDP . Me2+ complex.

The structure of the MgGDP complex at the active site of elongation factor (EF-Tu) has been investigated by using phosphorothioate analogs of GDP in the absence and presence of various metal ions, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) measurements. The high stereoselectivity of EF-Tu for the diastereomers of guanosine 5'-O-(1-thiodiphosphate) (GDP[alpha S]) is independent of the nature of the metal ion and is caused by the interaction of the protein with the alpha-phosphate of GDP. By using GDP analogs where the oxygens at either the alpha-phosphate or the beta-phosphate have been selectively labelled with 17O and measuring their effect on the EPR spectrum of EF-Tu-bound manganese we are able to show that only the beta-phosphate of GDP is coordinated to the metal ion in the EF-Tu . Me2+ . GDP complex. 31P-NMR studies on GDP and guanosine 5'-O-(2-thiodiphosphate) (GDP[beta S]) bound to EF-Tu indicate that in the EF-Tu . Me2+ . GDP complex Mg2+ interacts more strongly with the beta-phosphate than with the alpha-phosphate. Together with binding studies using GDP[beta S] our NMR results also indicate that the protein is complexed to the beta-phosphorous of GDP via two oxygens.

Binding Sites↗
PubMed2000

The three-dimensional structure of a complex of a murine Fab (NC10. 14) with a potent sweetener (NC174): an illustration of structural diversity in antigen recognition by immunoglobulins.

The three-dimensional structure of a complex of an Fab from a murine IgG2b(lambda) antibody (NC10.14) with a high potency sweet tasting hap- ten, N-(p-cyanophenyl)-N'-(diphenylmethyl)-N"-(carboxymethyl)guan idine (NC174), has been determined to 2.6 A resolution by X-ray crystallography. This complex crystallized in the triclinic space group P1, with two molecules in the asymmetric unit. In contrast to a companion monoclonal antibody (NC6.8) with a kappa-type light chain and similar high affinity for the NC174 ligand, the NC10.14 antibody possessed a large and deep antigen combining site bounded primarily by the third complementarity-determining regions (CDR3s) of the light and heavy chains. CDR3 of the heavy chain dominated the site and its crown protruded into the external solvent as a type 1' beta-turn. NC174 was nested against HCDR3 and was held in place by two tryptophan side-chains (L91 and L96) from LCDR3. The diphenyl rings were accommodated on an upper tier of the binding pocket that is largely hydrophobic. At the floor of the site, a positively charged arginine side-chain (H95) stabilized the orientation of the electronegative cyano group of the hapten. The negative charge on the acetate group was partially neutralized by a hydrogen bond with the phenolic hydroxyl group of tyrosine H58. Comparisons of the modes of binding of NC174 to the NC6.8 and NC10.14 antibodies illustrate the enormous structural and mechanistic diversity manifest by immune responses.

Amino Acid Sequence↗
PubMed2006

Table detection in online ink notes.

In documents, tables are important structured objects that present statistical and relational information. In this paper, we present a robust system which is capable of detecting tables from free style online ink notes and extracting their structure so that they can be further edited in multiple ways. First, the primative structure of tables, i.e., candidates for ruling lines and table bounding boxes, are detected among drawing strokes. Second, the logical structure of tables is determined by normalizing the table skeletons, identifying the skeleton structure, and extracting the cell contents. The detection process is similar to a decision tree so that invalid candidates can be ruled out quickly. Experimental results suggest that our system is robust and accurate in dealing with tables having complex structure or drawn under complex situations.

Algorithms↗
PubMed1994

Quantification of tight junction complexity by means of fractal analysis.

The concept of fractal geometry provides an elegant tool for the quantitative and objective structural description of various objects, the fractal analysis. Fractal analysis quantifies the structural complexity of objects by a characteristic singular value, the fractal dimension (FD). It can be estimated, e.g. by the box-counting method and provides a highly integrated measure in the range 1 < FD < 2 for curves extending within a plane. In this study, fractal analysis is used for the first time to evaluate the complexity of the tight junction network between adjoining cells. Bovine brain endothelial cells were cultured under various experimental conditions and the tight junctions were drawn to scale as visualized by the freeze fracture technique. These drawings were analyzed by fractal analysis, and by two other methods commonly used in this field, viz. the strand counting (SC) and complexity index (CI) methods. In contrast to the latter methods, the FD shows no directional preference and therefore no assumptions on the dynamic properties of the network's complexity are required. Thus, FD is demonstrated to provide the most sensitive, reliable and complete measure of tight junction complexity. In combination with SC and CI, additional information can be achieved concerning the directionality of the altered arrangement of tight junctional strands. Our analysis allows for the following conclusions. (1) Defined experimental influences can modify the complexity of tight junctions that are formed between endothelial cells in vitro, and (2) these structural modifications of the tight junctions are mainly due to an altered strand branching pattern.

Animals↗
PubMed1996

Structure of Dicopper Complexes of N,N,N',N'-Tetrakis[(2-benzimidazolyl)methyl]-2-hydroxy-1,3-diaminopropane with Coordinated Thiocyanate Counterions.

The structures of five dicopper complexes of binucleating ligand HL-H (N,N,N',N'-tetrakis[(2-benzimidazolyl)methyl]-2-hydroxy-1,3-diaminopropane) with thiocyanate and some other counterions were investigated by the X-ray diffraction method. In Cu(2)(HL-H)(NCS)(2)Cl(2).6H(2)O.CH(3)OH, 1 (a = 12.524(5) Å, b = 14.429(4) Å, c = 16.343(3) Å, alpha = 109.01(2) degrees, beta = 92.62(2) degrees, gamma = 115.27(3) degrees, Z = 2, triclinic, P&onemacr;), one chloride is not coordinated. Distorted square pyramidal (SP) geometry is found for both CuN(3)ClN and CuN(3)ON coordination sites in which the N(3) tripodal coordination sites come from the two symmetric halves of HL-H and the other nitrogen atoms come from thiocyanate ions. In Cu(2)(HL-H)(NCS)(2)(ClO(4))(2).6H(2)O.2EtOH, 2 (a = 10.955(2) Å, b = 15.366(5) Å, c = 18.465(9) Å, alpha = 65.57(4) degrees, beta = 89.73(3) degrees, gamma = 79.81(2) degrees, Z = 2, triclinic, P&onemacr;), the coordination environments for the two copper ions are both CuN(3)ON. However, their geometries are different: one is distorted SP and the other is distorted trigonal bipyramid (TBP). In Cu(2)(HL-H)(NCS)(2)(ClO(4))(2)Cl.H(3)O.3.5H(2)O, 3 (a = 11.986(6) Å, b = 12.778(5) Å, c = 17.81(1) Å, alpha = 82.41(4) degrees, beta = 75.44(5) degrees, gamma = 78.46(4) degrees, Z = 2, triclinic, P&onemacr;), the chloride ion does not coordinate to copper ion, but it is hydrogen bonded to the hydroxy hydrogen. The coordination environments for the two copper ions are both CuN(3)ON with distorted SP geometries. In Cu(2)(HL-H)(NCS)Cl(3).6H(2)O, 4 (a = 12.026(5) Å, b = 14.369(6) Å, c = 16.430(6) Å, alpha = 111.64(3) degrees, beta = 90.51(4) degrees, gamma = 113.90(3) degrees, Z = 2, triclinic, P&onemacr;), one chloride does not coordinate. The coordination environments for the two copper ions are CuN(3)ON in severely distorted TBP geometry and CuN(3)Cl(2) in SP geometry. In Cu(2)(HL-H)(NCS)(3)OH.2H(2)O.3CH(3)OH.Et(2)O, 5 (a = 18.322(5) Å, b = 15.543(6) Å, c = 19.428(7) Å, beta = 102.78(3) degrees, Z = 4, monoclinic, P2(1)/c), the hydroxide ion does not coordinate. The coordination environments for the two copper ions are CuN(3)N(2) with a geometry inbetween SP and TBP but slightly closer to SP and CuN(3)ON in distorted SP geometry. The distances between the copper ions are in the range 4.45-7.99 Å, indicating negligible interaction between the copper ions. The hydroxy groups of HL-H in 1-5all coordinate to copper ions either in a terminal mode (in complexes 1, 4, and 5, denoted as OHR(t)) or in a bridging mode (in complexes 2 and 3, denoted as OHR(b)). These hydroxy groups do not lose their protons in all cases. All thiocyanate anions coordinate to copper ions through nitrogen atoms. All copper ions in 1-5 are pentacoordinated. The fact that the CuN(3) geometries of the tripodal coordination sites in HL-H do not allow the formation of a square planar complex, may be the driving force for the formation of pentacoordinated complexes. From the structurally known dicopper complexes of the HL-H type ligands, the relative coordinating abilities of ligands to CuN(3) are OHR(t) > NCS(-) > Cl(-)(t) > OHR(b) approximately Cl(-)(b), where the letters b and t in parentheses denote bridging and terminal coordination modes respectively.

Journal Article↗
PubMed2005

Structure, function and interfacial allosterism in phospholipase A2: insight from the anion-assisted dimer.

Enzymes that function on membrane surfaces offer many challenges to understanding structural and functional details due to the difficulties of obtaining relevant information of the protein in a physiological environment. Focusing on this aspect of structural biology, it is important to develop conditions that mimic the interaction of membrane proteins with their binding surface and ultimately the mechanisms of action. This approach has been used to characterize the allosteric nature of secreted phospholipase A2 (PLA2) to its substrate interface. The breakthrough here was to crystallize the pancreatic group-IB PLA2 in an anion-assisted dimer with five coplanar phosphate anions bound. In the anion-assisted dimer structure one molecule of a tetrahedral mimic inhibitor and five anions are shared between the two subunits of the dimer. The sn-2-phosphate of the inhibitor, which mimics the tetrahedral intermediate of the esterolysis reaction, is bound in the active site of one subunit, and the alkyl chain extends into the active site slot of the second subunit across the subunit-subunit interface. This interface-bound structural mimic provided insight into the active site environment and specific anionic interactions to the i-face of the protein. The presence or absence of a single critical active site water, corresponds to the difference between the activated or inactivated form of the enzyme. The anion-assisted dimer structure supports a calcium coordinated nucleophilic water mechanism, with its pK(a) modulated by this assisting water. This working model has been further strengthened with an enzyme-product complex structure solved with the hydrolysis products of the substrate PAF also bound to the anion-assisted dimer form of PLA2. Additional confirmation of the assisting-water mechanism comes from a structure of the inactive zymogen proPLA2 also crystallized in an anion-assisted dimer. Remarkably, the assisting water present in the activated complex is absent in this proPLA2 structure.

Allosteric Site↗
PubMed2005

Atomic scale modelling of the cores of dislocations in complex materials part 2: applications.

In an accompanying article, we have described a methodology for the simulation of dislocations in structurally complex materials. We illustrate the applicability of this method through studies of screw dislocations in a structurally simple ionic ceramic (MgO), a molecular ionic mineral (forsterite, Mg2SiO4), a semi-ionic zeolite (siliceous zeolite A) and a covalent molecular crystalline material (the pharmaceutical, orthorhombic paracetamol-II). We focus on the extent of relaxation and the structure of the dislocation cores and comment on similarities and points of disparity between these materials. It is found that the magnitude of the relaxation varies from material to material and does not simply correlate with the magnitude of the principal elastic constants in an easily predictable fashion, or with the size of the cohesive lattice energy or length of the Burgers vector, which emphasises the need to model the non-linear forces and atomic structure of the core.

Acetaminophen↗
PubMed2000

Crystal structure and site-specific mutagenesis of pterin-bound human phenylalanine hydroxylase.

The crystal structure of the dimeric catalytic domain (residues 118-424) of human PheOH (hPheOH), cocrystallized with the oxidized form of the cofactor (7,8-dihydro-L-biopterin, BH(2)), has been determined at 2.0 A resolution. The pterin binds in the second coordination sphere of the catalytic iron (the C4a atom is 6.1 A away), and interacts through several hydrogen bonds to two water molecules coordinated to the iron, as well as to the main chain carbonyl oxygens of Ala322, Gly247, and Leu249 and the main chain amide of Leu249. Some important conformational changes are seen in the active site upon pterin binding. The loop between residues 245 and 250 moves in the direction of the iron, and thus allows for several important hydrogen bonds to the pterin ring to be formed. The pterin cofactor is in an ideal orientation for dioxygen to bind in a bridging position between the iron and the pterin. The pterin ring forms an aromatic pi-stacking interaction with Phe254, and Tyr325 contributes to the positioning of the pterin ring and its dihydroxypropyl side chain by hydrophobic interactions. Of particular interest in the hPheOH x BH(2) binary complex structure is the finding that Glu286 hydrogen bonds to one of the water molecules coordinated to the iron as well as to a water molecule which hydrogen bonds to N3 of the pterin ring. Site-specific mutations of Glu286 (E286A and E286Q), Phe254 (F254A and F254L), and Tyr325 (Y325F) have confirmed the important contribution of Glu286 and Phe254 to the normal positioning of the pterin cofactor and catalytic activity of hPheOH. Tyr325 also contributes to the correct positioning of the pterin, but has no direct function in the catalytic reaction, in agreement with the results obtained with rat TyrOH [Daubner, S. C., and Fitzpatrick, P. F. (1998) Biochemistry 37, 16440-16444]. Superposition of the binary hPheOH.BH(2) complex onto the crystal structure of the ligand-free rat PheOH (which contains the regulatory and catalytic domains) [Kobe, B., Jennings, I. G., House, C. M., Michell, B. J., Goodwill, K. E., Santarsiero, B. D., Stevens, R. C., Cotton, R. G. H., and Kemp, B. E. (1999) Nat. Struct. Biol. 6, 442-448] reveals that the C2'-hydroxyl group of BH(2) is sufficiently close to form hydrogen bonds to Ser23 in the regulatory domain. Similar interactions are seen with the hPheOH.adrenaline complex and Ser23. These interactions suggest a structural explanation for the specific regulatory properties of the dihydroxypropyl side chain of BH(4) (negative effector) in the full-length enzyme in terms of phosphorylation of Ser16 and activation by L-Phe.

Binding Sites↗
PubMedDate not supplied

Capillary electrophoresis of poly(amidoamine) dendrimers: from simple derivatives to complex multifunctional medical nanodevices.

Multifunctional poly(amidoamine) (PAMAM) dendrimer-based nanodevices provide novel nanoplatforms for targeting, imaging, and treatment of cancers in vitro and in vivo. Generational, skeletal, and substitutional dispersities are always present in dendrimer-based medical nanodevices. Molecular distribution plays a central role for one to evaluate the quality of PAMAM materials for medical applications. Capillary electrophoresis (CE) has been extensively used as a characterization technique to analyze a range of PAMAM dendrimers, from simple PAMAM derivatives to complex multifunctional PAMAM nanodevices. This review reports the recent advances in the analysis and characterization of a variety of PAMAM dendrimer-based nanoparticles ranging from polycationic and polyanionic PAMAM derivatives to PAMAMs of different generations and defined substitutions, and to complex multifunctional PAMAM nanodevices containing targeting ligands, dyes, and drugs. Understanding the structural complexity of dendrimer nanodevices is crucial for their use as multifunctional imaging, targeting, and cancer therapeutic devices, as well as for their use in biosensing, diagnostics, and control of biological systems.

Amides↗
PubMed2004

The low-density lipoprotein receptor-related protein associates with calnexin, calreticulin, and protein disulfide isomerase in receptor-associated-protein-deficient fibroblasts.

The low-density lipoprotein receptor-related protein (LRP) is a large (>600 kDa) multi-ligand-binding cell surface receptor that is now known to participate in a diverse range of cellular events. To accomplish this diverse role, LRP is composed of repetitive amino acid motifs consisting of complement-type and EGF precursor-type repeats. Within these repeats are six conserved cysteine residues that form the core disulfide bond structure of each repeat. To accommodate the intricate folding that such a complex structure dictates, a specialized chaperone is present in the endoplasmic reticulum (ER) called the receptor-associated protein (RAP) that binds to LRP immediately following its biosynthesis and assists in its exocytic transport. Interestingly, RAP -/- mice show reduced LRP expression in certain cell types, but not a more global affect on LRP expression that was expected. Such a tissue-restricted effect by RAP prompted an investigation if other ER chaperones associate with LRP to assist in its complex folding requirements and compensate for the absence of RAP in RAP -/- cells. Fibroblasts obtained from RAP -/- mice demonstrate similar LRP expression levels and subcellular distribution as RAP +/+ fibroblasts. Moreover, RAP -/- cells show an identical exocytic trafficking rate for LRP as RAP +/+ cells and comparable cell surface internalization kinetics. In RAP -/- cells, three well-known ER chaperones, calnexin, calreticulin, and protein disulfide isomerase (PDI), associate with LRP and likely compensate for the absence of RAP.

Animals↗
PubMed2005

A case of selfish nucleolar segregation.

Mitotic segregation of nucleolus in fission and budding yeast proceeds without disassembling its complex structure, creating challenging problems for transmission of nucleolus-organizing regions during nuclear division. The SMC complex called condensin, which plays a leading role in organizing mitotic structure of chromosomes in all eukaryotes, is essential for nucleolar segregation in budding yeast, where rDNA chromatin is the main target of mitotic condensin activity. Mitosis-specific condensin targeting to the nucleolus presents an attractive model to study mechanisms controlling condensin binding to specific chromatin domains. Recent reports suggest that the early-anaphase release of Cdc14 from the nucleolus (FEAR pathway) controls the proficiency of nucleolar segregation by promoting the mitotic condensin function in rDNA. This finding uncovers an essential function for the FEAR pathway and postulates the unique nucleolar self-regulatory mechanism, which evolved to recruit two essential enzymatic activities, Cdc14 phosphatase and condensin ATP-dependent supercoiling, for the specific task of segregating nucleoli without their disassembly.

Adenosine Triphosphatases↗
PubMed2001

Structures of trihydroxynaphthalene reductase-fungicide complexes: implications for structure-based design and catalysis.

BACKGROUND: Trihydroxynaphthalene reductase catalyzes two intermediate steps in the fungal melanin biosynthetic pathway. The enzyme, a typical short-chain dehydrogenase, is the biochemical target of three commercial fungicides. The fungicides bind preferentially to the NADPH form of the enzyme. RESULTS: Three X-ray structures of the Magnaporthe grisea enzyme complexed with NADPH and two commercial and one experimental fungicide were determined at 1.7 A (pyroquilon), 2.0 A (2,3-dihydro-4-nitro-1H-inden-1-one, 1), and 2.1 A (phthalide) resolutions. The chemically distinct inhibitors occupy similar space within the enzyme's active site. The three inhibitors share hydrogen bonds with the side chain hydroxyls of Ser-164 and Tyr-178 via a carbonyl oxygen (pyroquilon and 1) or via a carbonyl oxygen and a ring oxygen (phthalide). Active site residues occupy similar positions among the three structures. A buried water molecule that is hydrogen bonded to the NZ nitrogen of Lys-182 in each of the three structures likely serves to stabilize the cationic form of the residue for participation in catalysis. CONCLUSIONS: The pro S hydrogen of NADPH (which is transferred as a hydride to the enzyme's naphthol substrates) is directed toward the carbonyl carbon of the inhibitors that mimic an intermediate along the reaction coordinate. Modeling tetrahydroxynaphthalene and trihydroxynaphthalene in the active site shows steric and electrostatic repulsion between the extra hydroxyl oxygen of the former substrate and the sulfur atom of Met-283 (the C-terminal residue), which accounts, in part, for the 4-fold greater substrate specificity for trihydroxynaphthalene over tetrahydroxynaphthalene.

Antifungal Agents↗
PubMed2003

Towards an understanding of the poliovirus replication complex: the solution structure of the soluble domain of the poliovirus 3A protein.

Poliovirus is a positive-strand RNA virus and the prototypical member of the Picornaviridae family. Upon infection, the viral RNA genome is translated from a single open reading frame into a polypeptide which undergoes a series of cleavages to ultimately form four structural and seven non-structural proteins. A replication complex is then formed which replicates the viral genome into negative and positive strands for further translation, replication, and packaging into viral progeny. Poliovirus 3A protein (3A) is a critical component of the viral replication complex and is the putative target of enviroxime, an antiviral drug shown to block viral replication. 3A also inhibits host cell endoplasmic reticulum-to-Golgi apparatus transport, a function which may play a key role in viral evasion from the host immune response. 3A, an 87-residue protein consisting of a soluble N terminus and a hydrophobic C terminus, is formed by the cleavage of the precursor protein 3AB into 3A and 3B (VPg). Although they differ by only 22 residues, the precursor protein 3AB and its cleavage product 3A have distinct functions in viral replication. We have determined the structure of the soluble, N-terminal domain of 3A (3A-N) using NMR spectroscopy. We show that 3A-N exists as a symmetric dimer, and each monomer consists of an alpha-helical hairpin with unstructured, yet functional, N- and C termini. We also show that the 3A-N structure contains a negatively charged surface patch and provides a context for interpreting the biochemical characteristics of a number of previously reported 3A and 3AB mutants.

Amino Acid Sequence↗
PubMed1997

Recognition of bacteriophage Qbeta plus strand RNA as a template by Qbeta replicase: role of RNA interactions mediated by ribosomal proteins S1 and host factor.

RNA-protein interactions between bacteriophage Qbeta plus strand RNA and the components of the Qbeta replicase system were studied by deletion analysis. Internal, 5'-terminal and 3'-terminal deletions were assayed for template activity with replicase in vitro. Of the two internal binding sites previously described for replicase, we found that the S-site (map position 1247 to 1346) could be deleted without any significant effect on template activity, whereas deletion of the M-site (map position 2545 to 2867) resulted in a strong inactivation and a high salt sensitivity of the residual activity. Binding complexes of the deletion mutant RNAs with the different proteins involved in Qbeta RNA replication were analysed by electron microscopy. The formation of looped complex structures, previously reported and explained as simultaneous interactions with replicase at the S and the M-site, was abolished by deleting the S-site but, surprisingly, not by deleting the M-site. The same types of complexes observed with replicase were also formed with purified protein S1 (the alpha subunit of replicase), suggesting that these internal interactions with Qbeta RNA are mediated by the S1 protein. The Qbeta host factor, a protein required for the template activity of the Qbeta plus strand, was reported earlier to form similar complexes by binding to the S and M-sites (or adjacent sites) and in addition to the 3'-end, resulting in double-looped structures. The patterns of looped complexes observed with the deletion mutant RNAs suggest that the binding of host factor might not involve the S and M-sites themselves but adjacent downstream sites. An additional internal host factor interaction near map position 2300 was detected with several mutant RNAs. Qbeta RNA molecules with 3'-truncations formed 3'-terminal loops with similar efficiency as wild-type RNA, indicating that recognition of the 3'-end by host factor is not dependent on a specific 3'-terminal base sequence.

Allolevivirus↗
PubMed2002

Novel Mediator proteins of the small Mediator complex in Drosophila SL2 cells.

The Mediator complex is generally required for transcriptional regulation in species ranging from yeast to human. Throughout evolution, the functional diversity of the Mediator complex has been enhanced to meet the increasing requirements for sophisticated gene regulation. It is likely that greater structural complexity is thus required to accomplish these new, complex regulatory functions. In this study, we took systematic steps to examine various types of Mediator complexes in Drosophila melanogaster. Such efforts led to the identification of three distinct forms of Mediator complexes. In exploring their compositional and functional heterogeneity, we found that the smallest complex (C1) is highly enriched in a certain type of Drosophila cells and possesses novel Mediator proteins. The subunits shared among the three Mediator complexes (C1, C2, and C3) appear to form a stable modular structure that serves as a binding surface for transcriptional activator proteins. However, only C2 and C3 were able to support activated transcription in vitro. These findings suggest that different cell types may require distinct Mediator complexes, some of which may participate in nuclear processes other than the previously identified functions.

Amino Acid Sequence↗