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Find video protocols related to scientific articles indexed in Pubmed.
Cytotoxicity of recombinant tamapin and related toxin-like peptides on model cell lines.
Chem. Res. Toxicol.
PUBLISHED: 05-12-2014
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The scorpion toxin tamapin displays the most potent and selective blockage against KCa2.2 channels known to date. In this work, we report the biosynthesis, three-dimensional structure, and cytotoxicity on cancer cell lines (Jurkat E6-1 and human mammary breast cancer MDA-MB-231) of recombinant tamapin and five related peptides bearing mutations on residues (R6A,R7A, R13A, R6A-R7A, and GS-tamapin) that were previously suggested to be important for tamapin's activity. The indicated cell lines were used as they constitutively express KCa2.2 channels. The studied toxin-like peptides displayed lethal responses on Jurkat T cells and breast cancer cells; their effect is dose- and time-dependent with IC50 values in the nanomolar range. The order of potency is r-tamapin>GS-tamapin>R6A>R13A>R6A-R7A>R7A for Jurkat T cells and r-tamapin>R7A for MDA-MB-231 breast cancer cells. Our structural determination by NMR demonstrated that r-tamapin preserves the folding of the ?KTx5 subfamily and that neither single nor double alanine mutations affect the three-dimensional structure of the wild-type peptide. In contrast, our activity assays show that changes in cytotoxicity are related to the chemical nature of certain residues. Our results suggest that the toxic activity of r-tamapin on Jurkat and breast cancer cells could be mediated by the interaction of charged residues in tamapin with KCa2.2 channels via the apoptotic cell death pathway.
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The Trichoderma reesei Cry1 protein is a member of the cryptochrome/photolyase family with 6-4 photoproduct repair activity.
PLoS ONE
PUBLISHED: 01-01-2014
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DNA-photolyases use UV-visible light to repair DNA damage caused by UV radiation. The two major types of DNA damage are cyclobutane pyrimidine dimers (CPD) and 6-4 photoproducts (6-4PP), which are repaired under illumination by CPD and 6-4 photolyases, respectively. Cryptochromes are proteins related to DNA photolyases with strongly reduced or lost DNA repair activity, and have been shown to function as blue-light photoreceptors and to play important roles in circadian rhythms in plants and animals. Both photolyases and cryptochromes belong to the cryptochrome/photolyase family, and are widely distributed in all organisms. Here we describe the characterization of cry1, a member of the cryptochrome/photolyase protein family of the filamentous fungus Trichoderma reesei. We determined that cry1 transcript accumulates when the fungus is exposed to light, and that such accumulation depends on the photoreceptor Blr1 and is modulated by Envoy. Conidia of cry1 mutants show decreased photorepair capacity of DNA damage caused by UV light. In contrast, strains over-expressing Cry1 show increased repair, as compared to the parental strain even in the dark. These observations suggest that Cry1 may be stimulating other systems involved in DNA repair, such as the nucleotide excision repair system. We show that Cry1, heterologously expressed and purified from E. coli, is capable of binding to undamaged and 6-4PP damaged DNA. Photorepair assays in vitro clearly show that Cry1 repairs 6-4PP, but not CPD and Dewar DNA lesions.
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Crystal structure of the shrimp proliferating cell nuclear antigen: structural complementarity with WSSV DNA polymerase PIP-box.
PLoS ONE
PUBLISHED: 01-01-2014
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DNA replication requires processivity factors that allow replicative DNA polymerases to extend long stretches of DNA. Some DNA viruses encode their own replicative DNA polymerase, such as the white spot syndrome virus (WSSV) that infects decapod crustaceans but still require host replication accessory factors. We have determined by X-ray diffraction the three-dimensional structure of the Pacific white leg shrimp Litopenaeus vannamei Proliferating Cell Nuclear Antigen (LvPCNA). This protein is a member of the sliding clamp family of proteins, that binds DNA replication and DNA repair proteins through a motif called PIP-box (PCNA-Interacting Protein). The crystal structure of LvPCNA was refined to a resolution of 3 Å, and allowed us to determine the trimeric protein assembly and details of the interactions between PCNA and the DNA. To address the possible interaction between LvPCNA and the viral DNA polymerase, we docked a theoretical model of a PIP-box peptide from the WSSV DNA polymerase within LvPCNA crystal structure. The theoretical model depicts a feasible model of interaction between both proteins. The crystal structure of shrimp PCNA allows us to further understand the mechanisms of DNA replication processivity factors in non-model systems.
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Structural and thermodynamic folding characterization of triosephosphate isomerases from Trichomonas vaginalis reveals the role of destabilizing mutations following gene duplication.
Proteins
PUBLISHED: 05-01-2013
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We report the structures and thermodynamic analysis of the unfolding of two triosephosphate isomerases (TvTIM1 and TvTIM2) from Trichomonas vaginalis. Both isoforms differ by the character of four amino acids: E/Q 18, I/V 24, I/V 45, and P/A 239. Despite the high sequence and structural similarities between both isoforms, they display substantial differences in their stabilities. TvTIM1 (E18, I24, I45, and P239) is more stable and less dissociable than TvTIM2 (Q18, V24, V45, and A239). We postulate that the identities of residues 24 and 45 are responsible for the differences in monomer stability and dimer dissociability, respectively. The structural difference between both amino acids is one methyl group. In TvTIMs, residue 24 is involved in packing ?-helix 1 against ?-helix 2 of each monomer and residue 45 is located at the center of the dimer interface forming a "ball and socket" interplay with a hydrophobic cavity. The mutation of valine at position 45 for an alanine in TvTIM2 produces a protein that migrates as a monomer by gel filtration. A comparison with known TIM structures indicates that this kind of interplay is a conserved feature that stabilizes dimeric TIM structures. In addition, TvTIMs are located in the cytoplasm and in the membrane. As TvTIM2 is an easily dissociable dimer, the dual localization of TvTIMs may be related to the acquisition of a moonlighting activity of monomeric TvTIM2. To our knowledge, this is the simplest example of how a single amino acid substitution can provide alternative function to a TIM barrel protein. Proteins 2014; 82:22-33. © 2013 Wiley Periodicals, Inc.
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Crystal structure of shrimp arginine kinase in binary complex with arginine-a molecular view of the phosphagen precursor binding to the enzyme.
J. Bioenerg. Biomembr.
PUBLISHED: 03-13-2013
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Arginine kinase (AK) is a key enzyme for energetic balance in invertebrates. Although AK is a well-studied system that provides fast energy to invertebrates using the phosphagen phospho-arginine, the structural details on the AK-arginine binary complex interaction remain unclear. Herein, we determined two crystal structures of the Pacific whiteleg shrimp (Litopenaeus vannamei) arginine kinase, one in binary complex with arginine (LvAK-Arg) and a ternary transition state analog complex (TSAC). We found that the arginine guanidinium group makes ionic contacts with Glu225, Cys271 and a network of ordered water molecules. On the zwitterionic side of the amino acid, the backbone amide nitrogens of Gly64 and Val65 coordinate the arginine carboxylate. Glu314, one of proposed acid-base catalytic residues, did not interact with arginine in the binary complex. This residue is located in the flexible loop 310-320 that covers the active site and only stabilizes in the LvAK-TSAC. This is the first binary complex crystal structure of a guanidine kinase in complex with the guanidine substrate and could give insights into the nature of the early steps of phosphagen biosynthesis.
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Alternative splice variants in TIM barrel proteins from human genome correlate with the structural and evolutionary modularity of this versatile protein fold.
PLoS ONE
PUBLISHED: 01-01-2013
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After the surprisingly low number of genes identified in the human genome, alternative splicing emerged as a major mechanism to generate protein diversity in higher eukaryotes. However, it is still not known if its prevalence along the genome evolution has contributed to the overall functional protein diversity or if it simply reflects splicing noise. The (??)8 barrel or TIM barrel is one of the most frequent, versatile, and ancient fold encountered among enzymes. Here, we analyze the structural modifications present in TIM barrel proteins from the human genome product of alternative splicing events. We found that 87% of all splicing events involved deletions; most of these events resulted in protein fragments that corresponded to the (??)2, (??)4, (??)5, (??)6, and (??)7 subdomains of TIM barrels. Because approximately 7% of all the splicing events involved internal ?-strand substitutions, we decided, based on the genomic data, to design ?-strand and ?-helix substitutions in a well-studied TIM barrel enzyme. The biochemical characterization of one of the chimeric variants suggests that some of the splice variants in the human genome with ?-strand substitutions may be evolving novel functions via either the oligomeric state or substrate specificity. We provide results of how the splice variants represent subdomains that correlate with the independently folding and evolving structural units previously reported. This work is the first to observe a link between the structural features of the barrel and a recurrent genetic mechanism. Our results suggest that it is reasonable to expect that a sizeable fraction of splice variants found in the human genome represent structurally viable functional proteins. Our data provide additional support for the hypothesis of the origin of the TIM barrel fold through the assembly of smaller subdomains. We suggest a model of how nature explores new proteins through alternative splicing as a mechanism to diversify the proteins encoded in the human genome.
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Molecular modeling and expression of the Litopenaeus vannamei proliferating cell nuclear antigen (PCNA) after white spot syndrome virus shrimp infection.
Results Immunol
PUBLISHED: 05-01-2011
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Proliferating cell nuclear antigen (PCNA) is the eukaryotic sliding clamp that tethers DNA polymerase to DNA during replication. The full-length cDNA of the Pacific white shrimp Litopenaeus vannamei PCNA (LvPCNA) was cloned and encoded a protein of 260 amino acids that is highly similar to other Crustacean PCNAs. The theoretical shrimp PCNA structure has all the domains that are necessary for its interaction with template DNA and DNA polymerase. RT-PCR analysis showed that LvPCNA is expressed mainly in muscle and hemocytes and much less in hepatopancreas and gills. LvPCNA mRNA levels are not statistically different in muscle from healthy and challenged shrimp with the white spot syndrome virus (WSSV). In contrast, the mRNA levels of the viral DNA polymerase show a biphasic pattern with expression at 6 h post-infection and later at 24 and 48 h. These results suggest that in shrimp muscle LvPCNA levels are steadily kept to allow viral replication and that WSSV DNA polymerase (WSSV-DNApol) is more responsive towards later stages of infection. More knowledge of the DNA replication machinery would result in a better understanding of the mechanism and components of viral replication, since the WSSV genome does not have all the components required for assembly of a fully functional replisome.
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Structure and biochemical characterization of proliferating cellular nuclear antigen from a parasitic protozoon.
Acta Crystallogr. D Biol. Crystallogr.
PUBLISHED: 02-14-2011
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Proliferating cellular nuclear antigen (PCNA) is a toroidal-shaped protein that is involved in cell-cycle control, DNA replication and DNA repair. Parasitic protozoa are early-diverged eukaryotes that are responsible for neglected diseases. In this work, a PCNA from a parasitic protozoon was identified, cloned and biochemically characterized and its crystal structure was determined. Structural and biochemical studies demonstrate that PCNA from Entamoeba histolytica assembles as a homotrimer that is able to interact with and stimulate the activity of a PCNA-interacting peptide-motif protein from E. histolytica, EhDNAligI. The data indicate a conservation of the biochemical mechanisms of PCNA-mediated interactions between metazoa, yeast and parasitic protozoa.
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Integration Host Factor (IHF) binds to the promoter region of the phtD operon involved in phaseolotoxin synthesis in P. syringae pv. phaseolicola NPS3121.
BMC Microbiol.
PUBLISHED: 02-09-2011
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Pseudomonas syringae pv. phaseolicola, the causal agent of halo blight disease in beans, produces a toxin known as phaseolotoxin, in whose synthesis participate a group of genes organized within the genome in a region known as the "Pht cluster". This region, which is thought to have been acquired by horizontal gene transfer, includes 5 transcriptional units, two monocistronic (argK, phtL) and three polycistronic (phtA, phtD, phtM), whose expression is temperature dependent. So far, the regulatory mechanisms involved in phaseolotoxin synthesis have not been elucidated and the only well-established fact is the requirement of low temperatures for its synthesis. In this work, we searched for regulatory proteins that could be involved in phaseolotoxin synthesis, focusing on the regulation of the phtD operon.
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White spot syndrome virus Orf514 encodes a bona fide DNA polymerase.
Molecules
PUBLISHED: 01-11-2011
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White spot syndrome virus (WSSV) is the causative agent of white spot syndrome, one of the most devastating diseases in shrimp aquaculture. The genome of WSSV includes a gene that encodes a putative family B DNA polymerase (ORF514), which is 16% identical in amino acid sequence to the Herpes virus 1 DNA polymerase. The aim of this work was to demonstrate the activity of the WSSV ORF514-encoded protein as a DNA polymerase and hence a putative antiviral target. A 3.5 kbp fragment encoding the conserved polymerase and exonuclease domains of ORF514 was overexpressed in bacteria. The recombinant protein showed polymerase activity but with very low level of processivity. Molecular modeling of the catalytic protein core encoded in ORF514 revealed a canonical polymerase fold. Amino acid sequence alignments of ORF514 indicate the presence of a putative PIP box, suggesting that the encoded putative DNA polymerase may use a host processivity factor for optimal activity. We postulate that WSSV ORF514 encodes a bona fide DNA polymerase that requires accessory proteins for activity and maybe target for drugs or compounds that inhibit viral DNA replication.
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Biochemical characterization of the DNA ligase I from Entamoeba histolytica.
Mol. Biochem. Parasitol.
PUBLISHED: 06-19-2010
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DNA ligases play an essential role in DNA replication and repair. Herein, we report the cloning and biochemical characterization of DNA ligase I from the protozoan parasite Entamoeba histolytica (EhDNAligI). EhDNAligI is an ATP-dependent DNA ligase of 685 amino acids with 35% identity to human DNA ligase I. This report shows that heterologous expressed EhDNAligI is able to perform the three conserved steps of a DNA ligation reaction: adenylation, binding to a 5-phosphorylated nicked DNA substrate and sealing of the nick. EhDNAligI is strongly inhibited by NaCl and displays optimal activity at pH 7.5. EhDNAligI uses Mn2+ or Mg2+ as metal cofactors and ATP as nucleotide cofactor. EhDNAligI has a nicked DNA binding constant of 6.6microM and follows Michaelis-Menten steady-state kinetics with a K(m) ATP of 64nM and a k(cat) of 2.4min(-1). Accordingly to its properties as a family I DNA ligase, EhDNAligI is able to ligate a RNA strand upstream of a nucleic acid nick, but not in the downstream or the template position. We propose that EhDNAligI is involved in sealing DNA nicks during lagging strand synthesis and may have a role in base excision repair in E. histolytica.
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Crystal structure of the cysteine protease inhibitor 2 from Entamoeba histolytica: functional convergence of a common protein fold.
Gene
PUBLISHED: 06-09-2010
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Cysteine proteases (CP) are key pathogenesis and virulence determinants of protozoan parasites. Entamoeba histolytica contains at least 50 cysteine proteases; however, only three (EhCP1, EhCP2 and EhCP5) are responsible for approximately 90% of the cysteine protease activity in this parasite. CPs are expressed as inactive zymogens. Because the processed proteases are potentially cytotoxic, protozoan parasites have developed mechanisms to regulate their activity. Inhibitors of cysteine proteases (ICP) of the chagasin-like inhibitor family (MEROPS family I42) were recently identified in bacteria and protozoan parasites. E. histolytica contains two ICP-encoding genes of the chagasin-like inhibitor family. EhICP1 localizes to the cytosol, whereas EhICP2 is targeted to phagosomes. Herein, we report two crystal structures of EhICP2. The overall structure of EhICP2 consists of eight ?-strands and closely resembles the immunoglobulin fold. A comparison between the two crystal forms of EhICP2 indicates that the conserved BC, DE and FG loops form a flexible wedge that may block the active site of CPs. The positively charged surface of the wedge-forming loops in EhICP2 contrasts with the neutral surface of the wedge-forming loops in chagasin. We postulate that the flexibility and positive charge observed in the DE and FG loops of EhICP2 may be important to facilitate the initial binding of this inhibitor to the battery of CPs present in E. histolytica.
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The R2R3 Myb protein family in Entamoeba histolytica.
Gene
PUBLISHED: 02-04-2010
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The MYB DNA-binding domain is conserved in vertebrates, plants, and fungi. This domain mediates the DNA-binding activity of proteins (that have transcription factor activity) in a sequence-specific manner and is also used for the protection of telomeric regions. The MYB DNA-binding domain contains three imperfect conserved repeats of 52 amino acids (R1, R2, and R3). Within each repeat, there are three tryptophans that are separated by 18 or 19 amino acids. In order to understand the role of Myb transcription factors in Entamoeba histolytica, we searched for MYB DNA-binding domain containing proteins using the amino acid sequence of human c-Myb as the query. We found 34 putative MYB DNA-binding domain containing proteins, which clustered into three monophyletic groups. Family I members conserve only the R2 and R3 repeats in their MYB DNA-binding domain and were dubbed in this report as EhMybR2R3. Family II includes single-repeat proteins related to human telomeric binding proteins. Family III is predicted to comprise proteins with one single repeat where the region corresponding to the conserved tryptophan of the third alpha helix is replaced by a (S)/(T)HAQK(Y)/(F)F motif; this family was named EhMybSHAQKYF. In this work, we focused on proteins that belong to the EhMybR2R3 family. RT-PCR analysis showed that EhMybR2R3 genes were differentially expressed in trophozoites grown in basal culture conditions. Purified rEhMyb10 protein, belonging to the EhMybR2R3 family, was able to bind a consensus Myb recognition element in vitro. In addition, using nuclear extracts from trophozoites of E. histolytica, we were able to detect Myb DNA-binding activity to this sequence. Our in silico surveys demonstrated that this consensus sequence is present in E. histolytica gene promoters. Interestingly, these promoters include different families of genes that are related to signal transduction, vesicular transport, heat shock response, and virulence. Thus, Myb putative transcription factors in E. histolytica could be involved in the transcriptional regulation of genes participating in several different pathways.
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A nuclear family A DNA polymerase from Entamoeba histolytica bypasses thymine glycol.
PLoS Negl Trop Dis
PUBLISHED: 01-19-2010
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Eukaryotic family A DNA polymerases are involved in mitochondrial DNA replication or translesion DNA synthesis. Here, we present evidence that the sole family A DNA polymerase from the parasite protozoan E. histolytica (EhDNApolA) localizes to the nucleus and that its biochemical properties indicate that this DNA polymerase may be involved in translesion DNA synthesis.
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Purification, refolding and autoactivation of the recombinant cysteine proteinase EhCP112 from Entamoeba histolytica.
Protein Expr. Purif.
PUBLISHED: 05-12-2009
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The cysteine proteinase EhCP112 and the adhesin EhADH112 assemble to form the EhCPADH complex involved in Entamoeba histolytica virulence. To further characterize this cysteine proteinase, the recombinant full-length EhCP112 enzyme was expressed and purified under denaturing conditions. After a refolding step under reductive conditions, the inactive precursor (ppEhCP112) was processed to a 35.5 kDa mature and active enzyme (EhCP112). The thiol specific inhibitor E-64, but not serine or aspartic proteinase inhibitors arrested this activation process. The activation step of the proenzyme followed by the mature enzyme suggests an autocatalytic process during EhCP112 maturation. The experimentally determined processing sites observed during EhCP112 activation lie close to processing sites of other cysteine proteinases from parasites. The kinetic parameters of the mature EhCP112 were determined using hemoglobin and azocasein as substrates. The proteinase activity of EhCP112 was completely inhibited by thiol inhibitors, E-64, TLCK, and chymostatin, but not by general proteinase inhibitors. Since EhCP112 is a proteinase involved in the virulence of E. histolytica, a reliable source of active EhCP112 is a key step for its biochemical characterization and to carry out future protein structure-function studies.
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A transposon-derived DNA polymerase from Entamoeba histolytica displays intrinsic strand displacement, processivity and lesion bypass.
PLoS ONE
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Entamoeba histolytica encodes four family B2 DNA polymerases that vary in amino acid length from 813 to 1279. These DNA polymerases contain a N-terminal domain with no homology to other proteins and a C-terminal domain with high amino acid identity to archetypical family B2 DNA polymerases. A phylogenetic analysis indicates that these family B2 DNA polymerases are grouped with DNA polymerases from transposable elements dubbed Polintons or Mavericks. In this work, we report the cloning and biochemical characterization of the smallest family B2 DNA polymerase from E. histolytica. To facilitate its characterization we subcloned its 660 amino acids C-terminal region that comprises the complete exonuclease and DNA polymerization domains, dubbed throughout this work as EhDNApolB2. We found that EhDNApolB2 displays remarkable strand displacement, processivity and efficiently bypasses the DNA lesions: 8-oxo guanosine and abasic site.Family B2 DNA polymerases from T. vaginalis, G. lambia and E. histolytica contain a Terminal Region Protein 2 (TPR2) motif twice the length of the TPR2 from ?29 DNA polymerase. Deletion studies demonstrate that as in ?29 DNA polymerase, the TPR2 motif of EhDNApolB2 is solely responsible of strand displacement and processivity. Interestingly the TPR2 of EhDNApolB2 is also responsible for efficient abasic site bypass. These data suggests that the 21 extra amino acids of the TPR2 motif may shape the active site of EhDNApolB2 to efficiently incorporate and extended opposite an abasic site. Herein we demonstrate that an open reading frame derived from Politons-Mavericks in parasitic protozoa encode a functional enzyme and our findings support the notion that the introduction of novel motifs in DNA polymerases can confer specialized properties to a conserved scaffold.
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Crystallization and X-ray diffraction studies of crustacean proliferating cell nuclear antigen.
Acta Crystallogr. Sect. F Struct. Biol. Cryst. Commun.
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Proliferating cell nuclear antigen (PCNA), a member of the sliding clamp family of proteins, interacts specifically with DNA replication and repair proteins through a small peptide motif called the PCNA-interacting protein or PIP box. PCNA is recognized as one of the key proteins involved in DNA metabolism. In the present study, the recombinant PCNA from Litopenaeus vannamei (LvPCNA) was heterologously overexpressed and purified using metal ion-affinity chromatography. Crystals suitable for diffraction grew overnight using the hanging-drop vapour-diffusion method. LvPCNA crystals belong to space group C2 with unit-cell parameters a=144.6, b=83.4, c=74.3?Å, ?=117.6°. One data set was processed to 3?Å resolution, with an overall Rmeas of 0.09 and a completeness of 93.3%. Initial phases were obtained by molecular replacement using a homology model of LvPCNA as the search model. Refinement and structural analysis are underway. This report is the first successful crystallographic analysis of a marine crustacean decapod shrimp (L. vannamei) proliferating cell nuclear antigen.
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Cellular and biochemical characterization of two closely related triosephosphate isomerases from Trichomonas vaginalis.
Parasitology
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The glycolytic enzyme triosephosphate isomerase catalyses the isomerization between glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Here we report that Trichomonas vaginalis contains 2 fully functional tpi genes. Both genes are located in separated chromosomal context with different promoter regulatory elements and encode ORFs of 254 amino acids; the only differences between them are the character of 4 amino acids located in ?-helices 1, 2 and 8. Semi-quantitative RT-PCR assays showed that tpi2 transcript is approximately 3·3-fold more abundant than tpi1. Using an anti-TvTIM2 polyclonal antibody it was demonstrated that TIM proteins have a cytoplasmic localization and both enzymes are able to complement an Escherichia coli strain carrying a deletion of its endogenous tpi gene. Both TIM proteins assemble as dimers and their secondary structure assessment is essentially identical to TIM from Saccharomyces cerevisiae. The kinetic catalytic constants of the recombinant enzymes using glyceraldehyde-3-phosphate as substrate are similar to the catalytic constants of TIMs from other organisms including parasitic protozoa. As T. vaginalis depends on glycolysis for ATP production, we speculate 2 possible reasons to maintain a duplicated tpi copy on its genome: an increase in gene dosage or an early event of neofunctionalization of TIM as a moonlighting protein.
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Crystallization and X-ray diffraction studies of arginine kinase from the white Pacific shrimp Litopenaeus vannamei.
Acta Crystallogr. Sect. F Struct. Biol. Cryst. Commun.
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Crystals of an unligated monomeric arginine kinase from the Pacific whiteleg shrimp Litopenaeus vannamei (LvAK) were successfully obtained using the microbatch method. Crystallization conditions and preliminary X-ray diffraction analysis to 1.25?Å resolution are reported. Data were collected at 100?K on NSLS beamline X6A. The crystals belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 56.5, b = 70.2, c = 81.7?Å. One monomer per asymmetric unit was found, with a Matthews coefficient (V(M)) of 2.05?Å(3)?Da(-1) and 40% solvent content. Initial phases were determined by molecular replacement using a homology model of LvAK as the search model. Refinement was performed with PHENIX, with final R(work) and R(free) values of 0.15 and 0.19, respectively. Biological analysis of the structure is currently in progress.
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Conservation of promoter melting mechanisms in divergent regions of the single-subunit RNA polymerases.
Biochemistry
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The single-subunit RNA polymerases make up a widespread family of proteins found in phage, mitochondria, and chloroplasts. Unlike the phage RNAPs, the eukaryotic RNAPs require accessory factors to melt their promoters and diverge from the phage RNAPs in the regions where functions associated with promoter melting in the latter have been mapped, suggesting that promoter melting mechanisms in the eukaryotic RNAPs diverge from those in the phage enzymes. However, here we show that an element in the yeast mitochondrial RNAP, identified by sequence alignment with the T7 phage RNAP, fulfills a role in promoter melting similar to that filled by the T7RNAP "intercalating hairpin". The yeast mitochondrial RNAP intercalating hairpin appears to be as important in promoter melting as the mitochondrial transcription factor, MTF1, and both a structurally integral hairpin and MTF1 are required to achieve high levels of transcription on a duplex promoter. Deletions from the hairpin also relieve MTF1 inhibition of promoter escape on premelted promoters, likely because such deletions disrupt interactions with the upstream edge of the transcription bubble. These results are consistent with recent structural and functional studies of human mitochondrial RNAP and further reveal the surprising extent of mechanistic conservation between the eukaryotic and phage-encoded members of the single-subunit RNAP family.
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New tricks of an old pattern: structural versatility of scorpion toxins with common cysteine spacing.
J. Biol. Chem.
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Scorpion venoms are a rich source of K(+) channel-blocking peptides. For the most part, they are structurally related small disulfide-rich proteins containing a conserved pattern of six cysteines that is assumed to dictate their common three-dimensional folding. In the conventional pattern, two disulfide bridges connect an ?-helical segment to the C-terminal strand of a double- or triple-stranded ?-sheet, conforming a cystine-stabilized ?/? scaffold (CS?/?). Here we show that two K(+) channel-blocking peptides from Tityus scorpions conserve the cysteine spacing of common scorpion venom peptides but display an unconventional disulfide pattern, accompanied by a complete rearrangement of the secondary structure topology into a CS helix-loop-helix fold. Sequence and structural comparisons of the peptides adopting this novel fold suggest that it would be a new elaboration of the widespread CS?/? scaffold, thus revealing an unexpected structural versatility of these small disulfide-rich proteins. Acknowledgment of such versatility is important to understand how venom structural complexity emerged on a limited number of molecular scaffolds.
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