Biomedical subjects

C Li

Publications and source records attributed to C Li.

At least 37 records · Page 2Linked to original sources

PubMed2000

Structural identification of two metabolites of catechins and their kinetics in human urine and blood after tea ingestion.

Tea is a popular beverage consumed worldwide. The metabolic fate of its major constituents, catechins, however, is not well-known. In this study, two catechin metabolites were detected in the urine and plasma of human volunteers after ingestion of green tea. These metabolites were identified by LC/ESI-MS and NMR as (-)-5-(3',4', 5'-trihydroxyphenyl)-gamma-valerolactone (M4) and (-)-5-(3', 4'-dihydroxyphenyl)-gamma-valerolactone (M6). The renal excretion of M4 and M6 had a 3 h lag time and peaked 7.5-13.5 h after ingestion of a single dose of green tea, while (-)-epigallocatechin (EGC) and (-)-epicatechin peaked at 2 h. M4 and M6 were two major tea metabolites with urinary cumulative excretions as high as 8-25 times the levels of EGC and (-)-epicatechin in some of our subjects, and accounted for 6-39% of the amounts of ingested EGC and (-)-epicatechin. Both the metabolites appeared to be produced by intestinal microorganisms, with EGC and (-)-epicatechin as the precursors of M4 and M6, respectively. Repeated ingestion of green tea produced a slight accumulative effect of the metabolites. They were also detected in the plasma, exhibiting kinetics similar to those of the urinary metabolites, and in the feces. Study on these metabolites may help us further understand the cancer chemopreventive actions and other beneficial effects of tea.

Adult↗

PubMed2000

Adenosine prevents activation of transcription factor NF-kappa B and enhances activator protein-1 binding activity in ischemic rat heart.

BACKGROUND: Adenosine prevents myocardial TNF-alpha production induced by ischemia/reperfusion, but the mechanisms are poorly understood. Transcription factors NF-kappa B and AP-1 have been implicated in the regulation of a variety of inducible gene expressions in response to oxidative stress and cellular defense. The effects of adenosine on NF-kappa B and AP-1 activation have not been clearly defined. This study demonstrated differential effects of adenosine on NF-kappa B and AP-1 nuclear binding activity in ischemic myocardium. METHODS: Isolated working rat hearts were subjected to 0, 1, 2, 3, 4, 5, 7.5, 10, 15, and 30 minutes of ischemia, with 4 to 6 hearts for each time point with and without adenosine (100 mumol/L). NF-kappa B and AP-1 binding activity in the nucleus were analyzed by electrophoretic mobility shift assay (EMSA). I kappa B alpha levels in the cytoplasm were measured by Western blot analysis. TNF-alpha mRNA levels were determined by RT-PCR. RESULTS: NF-kappa B binding activity in the nucleus significantly increased after 4 minutes of ischemia and remained to 30 minutes. The levels of I kappa B alpha protein in the cytoplasm markedly decreased after 4, 5, 7.5, and 10 minutes of ischemia. TNF-alpha mRNA levels peaked after 10 minutes of ischemia. AP-1 DNA binding activity was induced and persisted during all ischemic periods. Adenosine significantly inhibited NK-kappa B binding activity in the nucleus, markedly prevented the loss of I kappa B alpha proteins from the cytoplasm, and concomitantly down-regulated TNF-alpha mRNA expression, but enhanced AP-1 binding activity in the nucleus of ischemic myocardium. CONCLUSIONS: Adenosine modulation of NF-kappa B activation may be the cellular molecular mechanism of down-regulation of TNF-alpha mRNA expression. The cardioprotective properties of adenosine may be involved in the differential modulation of NF-kappa B and AP-1 activation during myocardial ischemia.

Adenosine↗

PubMed2000

CD105 antagonizes the inhibitory signaling of transforming growth factor beta1 on human vascular endothelial cells.

CD105 (endoglin), a receptor for transforming growth factor beta (TGFbeta), is highly expressed in tissue-cultured, activated endothelial cells in vitro and in tissues undergoing angiogenesis in vivo. The absence of CD105 in knockout mice leads to their death from defective vascular development, but the role of CD105 in the modulation of angiogenesis has not been elucidated. TGFbeta1 is a well-recognized regulator of angiogenesis. Using an antisense approach, we have shown that inhibition of CD105 protein translation in cultured human endothelial cells enhances the ability of TGFbeta1 to suppress growth and migration in these cells. The ability of endothelial cells to form capillary tubes was evaluated by the use of a 3-dimensional collagen matrix system where TGFbeta1 not only reduced the length of capillary-like structures, but also caused massive mortality in CD105-deficient cells compared to control cultures. These results provide direct evidence that CD105 antagonizes the inhibitory effects of TGFbeta1 on human vascular endothelial cells and that normal cellular levels of CD105 are required for the formation of new blood vessels.

Animals↗

PubMed2000

Biomechanical stress-induced apoptosis in vein grafts involves p38 mitogen-activated protein kinases.

The present study was designed to investigate whether apoptosis occurs in early-stage vein grafts and to determine the mechanisms by which mechanical stress contributes to apoptosis in vascular smooth muscle cells (SMCs). Apoptosis in vessel walls of mouse vein grafts was confirmed by morphological changes and by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL). TUNEL(+) cells in vein grafts 1, 4, and 8 wk postoperatively was 13%, 29%, and 21%, respectively, and apoptosis occurred mainly in veins grafted to arteries, remaining unchanged in vein-to-vein grafts. When mouse, rat, and human arterial SMCs were cultured on a flexible membrane and subjected to cyclic strain stress, apoptosis was observed in a time- and strength-dependent manner. All three types of SMCs showed apoptotic death as confirmed by TUNEL, propidium iodide, and annexin V staining. To further study the signal pathways leading to apoptosis, activities of p38, a subfamily of mitogen-activated protein kinases (MAPKs), were determined. Mechanical stress resulted in p38 MAPK activation, reaching high levels within 8 min. SB 202190, a specific inhibitor for p38 MAPKs, prevented SMC apoptosis in response to mechanical stress. SMC lines stably transfected with a dominant negative rac, an upstream signal transducer, or overexpressing MAPK phosphatase-1, a negative regulator for MAPKs, completely inhibited mechanical stress stimulated p38 activation and abolished mechanical stress-induced apoptosis. Thus, we provide solid evidence that one of the earliest events in venous bypass grafts is apoptosis, in which mechanical stress-induced p38-MAPK activation is responsible for transducing signals leading to apoptosis.-Mayr, M., Li, C., Zou, Y., Huemer, U., Hu, Y., Xu, Q. Biomechanical stress-induced apoptosis in vein grafts involves p38 mitogen-activated protein kinases.

Animals↗

PubMed2000

Nucleotide sequence and genome organization of apple latent spherical virus: a new virus classified into the family Comoviridae.

A virus with isometric virus particles (ca. 25 nm) was isolated from an apple tree and named Apple latent spherical virus (ALSV). Virus particles purified from infected Chenopodium quinoa formed two bands with densities of 1.41 and 1.43 g/cm(3) in CsCl equilibrium density-gradient centrifugation, indicating that the virus is composed of two components. The virus had two ssRNA species (RNA1 and RNA2) and three capsid proteins (Vp25, Vp24 and Vp20). The complete nucleotide sequences of RNA1 and RNA2 were determined to be 6815 nt and 3384 nt excluding the 3' poly(A) tail, respectively. RNA1 contains two partially overlapping ORFs encoding polypeptides of molecular mass 23 kDa ('23K'; ORF1) and 235 kDa ('235K'; ORF2); RNA2 has a single ORF encoding a polypeptide of 108 kDa ('108K'). The 235K protein has, in order, consensus motifs of the protease cofactor, the NTP-binding helicase, the cysteine protease and the RNA polymerase, in good agreement with the gene arrangement of viruses in the COMOVIRIDAE: The 108K protein contains an LPL movement protein (MP) motif near the N terminus. Direct sequencing of the N-terminal amino acids of the three capsid proteins showed that Vp25, Vp20 and Vp24 are located in this order in the C-terminal region of the 108K protein. The cleavage sites of the 108K polyprotein were Q/G (MP/Vp25 and Vp25/Vp20) and E/G (Vp20/Vp24). Phylogenetic analysis of the ALSV RNA polymerase domain showed that ALSV falls into a cluster different from the nepo-, como- and fabavirus lineages.

Amino Acid Sequence↗

PubMed2000

Chromosomal numerical aberrations detected by fluorescence in situ hybridization on bladder washings from patients with bladder cancer.

OBJECTIVE: Previous studies on touch biopsy specimens have determined numerical or structural changes involving many different chromosomes in bladder cancer. The aim of this study was to evaluate the use of fluorescence in situ hybridization (FISH) assay in bladder washings as an objective technique to detect chromosomal numerical aberrations in bladder cancer. The main advantages of bladder washings are that they can be easily collected during the clinical follow-up of patients with superficial bladder cancer and they do not contain so many degenerate cells as urine samples. METHODS: We collected specimens from 25 patients who underwent transurethral resection of bladder tumors. Double target FISH assays with centromeric labeled probes for chromosomes 7, 8, 9 and 11 were used on the bladder washings and on the touch biopsy slides. The results were compared to flow cytometry and tumor grade and stage. RESULTS: We found monosomy 9 and trisomy 7, 8, 9 and 11 in 28, 32, 36, 28 and 25% respectively of the patients. FISH analysis of bladder washing versus touch biopsy specimens were concordant in approximately 90% of the slides. Total DNA aneuploidy correlated well with numerical aberrations of chromosomes 7, 8 and 11, but not with chromosome 9. CONCLUSION: Although better hybridization efficiency was obtained on touch biopsy slides, the results in bladder washings were in high concordance. FISH analysis on bladder washing samples may become a simple tool to improve the accuracy of cytology.

Adult↗

PubMed2000

Ras/Rac-Dependent activation of p38 mitogen-activated protein kinases in smooth muscle cells stimulated by cyclic strain stress

p38, a subfamily of the mitogen-activated protein kinases (MAPKs), is a crucial signal transducer between a variety of extracellular stimuli and gene expression in mammalian cells. This kinase is activated in cultured cells stimulated by heat shock, osmotic stress, and proinflammatory cytokines, but a similar activation of p38 MAPKs in vascular smooth muscle cells (SMCs) stimulated by mechanical stress has yet to be studied. We studied signal pathways leading to time- and strength-dependent p38 activation in rat SMCs in response to cyclic strain stress. p38 phosphorylation in stressed SMCs showed maximal activation at 10 minutes. This activation was significantly inhibited by pretreatment of the SMCs with pertussis toxin, a G-protein antagonist, and enhanced by treatment with suramin, a growth factor receptor antagonist, but opposite effects in the activation of extracellular signal-regulated kinases stimulated by mechanical forces were found. p38 activation was markedly reduced in stressed SMCs after protein kinase C depletion. Interestingly, SMC lines stably expressing dominant-negative ras (ras N17) or rac1 (rac1 N17) almost abolished p38 phosphorylation induced by cyclic strain stress. When p38 activation was inhibited by the specific inhibitor SB 202190, SMC migration, determined in a Boyden chamber in response to stimulation with platelet-derived growth factor-BB, and SMC proliferation, stimulated by cyclic strain stress, were abrogated. Thus, we provide the first evidence that cyclic strain stress rapidly activates p38 MAPKs via activation of protein kinase C ras/rac signal pathways, suggesting that p38 MAPKs are important signal transducers mediating the mechanical stress-induced cell responses essential for SMC migration and proliferation.

Journal Article↗

PubMed2000

Immediate post-abortal insertion of the frameless GyneFix IUD: initial experience in China.

The promising results obtained with the frameless GyneFix IUD for interval insertion led to the belief that the technology could be useful for immediate post-abortal application to reduce the number of induced and repeat abortions. IUDs have many advantages and the method is acceptable and safe. However, side effects and expulsion of conventional IUDs remain a problem. In an attempt to minimise these problems, the frameless GyneFix IUD was developed. Clinical studies conducted over the past 14 years have shown that the principles on which the device is based are valid. An increasing bulk of clinical evidence shows that the design characteristics of the device (fixed, frameless and flexible) are responsible for the low expulsion, high effectiveness, and high tolerance and continuation rates.1 Experience with the frameless IUD in China since 1989 confirms the international experience with the device. The present communication is a second report from China with the GyneFix IUD. In the present study the GyneFix IUD is inserted immediately following termination of pregnancy of less than 10 weeks amenorrhea. It is concluded that immediate post-abortal insertion of the frameless IUD is easy and safe, and appears to be as reliable and as effective as when it is inserted at interval. The GyneFix IUD could, therefore, constitute an important new option in the prevention of abortion.

Abortion, Induced↗

PubMed2000

Comparison of action of paclitaxel and poly(L-glutamic acid)-paclitaxel conjugate in human breast cancer cells.

The new anticancer agent poly(L-glutamic acid)-paclitaxel (PG-TXL) is a conjugate of paclitaxel and the water-soluble polyglutamate carrier. The observation that PG-TXL appears to possess antitumor activity superior to free paclitaxel in preclinical studies suggests that PG-TXL might possess favorable pharmacokinetic properties and/or have a mechanism of action different from that of paclitaxel. The purpose of this study was to compare the pharmacological action of PG-TXL and free paclitaxel in a panel of breast cancer cell lines with emphasis on their ability to induce apoptosis, their effects on cell cycle progression, and their cellular uptake. Morphological analysis and biochemical characterizations demonstrated that both compounds have similar abilities to induce apoptosis in cells expressing wild-type p53 (MCF-7) or mutant p53 (MDA-MB435 and MDA-MB453). Although MCF-7 cells were less sensitive to each compound than MDA-MB435 and MDA-MB453 cells, transfection experiments demonstrated that p53 did not appear to play a significant role in drug-induced cell death with either agent. Flow cytometry analysis further revealed that both free paclitaxel and PG-TXL induced a characteristic G2/M arrest in the cell cycle, consistent with the disturbance of microtubule polymerization as their mechanism of action. Western blot analysis showed that paclitaxel and PG-TXL downregulated HER2/neu expression in a similar fashion. HPLC analysis revealed that paclitaxel was released from the PG-TXL conjugate in vitro. The released paclitaxel, not the glutamic acid polymer, was subsequently transported into the cells. These results suggest that PG-TXL exerts its anticancer activity by continuous release of free paclitaxel, and that the favorable pharmacokinetics and drug distribution of the PG-TXL conjugate in vivo are likely the main factors contributing to its superior anticancer activity.

Antineoplastic Agents, Phytogenic↗

PubMed2000

Cimetidine absorption and elimination in rat small intestine.

The purpose of this study was to determine the characteristics of intestinal absorption and metabolism of cimetidine. The initial finding of the appearance of cimetidine sulfoxide in rat and human jejunum from cimetidine perfusions had prompted an isolation of mucosal membrane transport and enterocyte metabolism contributions in earlier membrane vesicle and microsomal studies, respectively. In this report, perfusion studies in rat small intestine detail regional differences in intestinal elimination. Cimetidine S-oxide appears to a significantly greater extent in the jejunum compared with the ileum. Jejunal metabolite appearance is shown to be a function of the pH-dependent intracellular uptake of cimetidine. Cimetidine permeability decreases with increasing perfusion concentration in both jejunum and ileum. Similar permeability magnitudes and concentration dependence are observed in both regions. Perfusion studies with inhibitors of cimetidine mucosal transport and inhibitors of microsomal S-oxidation provide an inhibition profile suggesting that jejunal cimetidine permeability decreases with increasing intracellular cimetidine concentration. The data support a reduction in paracellular cimetidine absorption as controlled by intracellular cimetidine. This inference is drawn on the basis of mass balance. Because significant appearance of cimetidine S-oxide was previously found in human jejunal perfusions, this region-dependent intestinal elimination process detailed in rats may be relevant to drug plasma-level double peaks observed in clinical studies. Saturation of jejunal metabolism at typical oral doses may limit paracellular absorption of cimetidine in the jejunum and contribute to the double peak phenomenon and to absorption variability.

Administration, Oral↗

PubMed2000

[IL-4 gene transfer induces the differentiation of cells and inhibits the activity of telomerase in hepatoblastoma cells].

OBJECTIVE: To investigate the effects of human interleukin-4(hIL-4) gene transfer on the differentiation and the activities of telomerase in hepatoblastoma cells. METHODS: Retroviral vector (PL-IL-4-SN) was employed to introduce hIL-4 gene into human hepatoblastoma cell line(Hep G2)cells. Trypan blue and Wright's stain, radioimmunoassay, in situ hybridization, flow cytometry, PCR-ELISA were used to determine the change in morphology and cell cycle, the expression of proto-oncogenes, the synthesis of AFP and the activities of telomerase in IL-4 gene transferred tumor cells. RESULTS: The shape of the hepatoblastoma cells tended to become relatively mature; the growth of the cells was significantly suppressed(P<0.05); the synthesis of AFP was reduced from 15.36+/-0. 67 ng x10(6) cells(-1); x24h(-1); to 3.26 +/- 1.43ng x10(6) cells(-1); x 24h(-1); the proliferation of the cells was significantly suppressed(P<0.05); the cell cycle was arrested at G(0)/ G(1) stage; the expression of c-fos, c-jun, c-myc and the activities of telomerase were remarkably decreased in IL-4 gene-modified Hep G2 cells. CONCLUSION: hIL-4 gene transfer could induce the differentiation of heptoblastoma cells and down-regulate the activity of tolemarase in Hep G2 cells.

Cell Differentiation↗

PubMed2000

[A molecular cytogenetic study on chromosome anomalies of chronic myeloid leukemia].

OBJECTIVE: To study the bcr/abl fusion gene in chronic myeloid leukemia (CML) cases with Philadelphia chromosome(Ph) negative, and variant Ph translocations and other chromosome anomalies. METHODS: The status of bcr/abl fusion gene in 8 kinds of chromosome anomalies was tested with dual-color fluorescence in situ hybridization (D-FISH) technique. RESULTS: The positive hybridization was demonstrated in 3 cases of variant Ph CML and other 7 cases of standard Ph, respectively. In 2 cases of Ph-negative CML, one case that had karyotype such as 46,XY,t(9;18) showed positive hybridization, the other case with 46,XY,inv(2) showed negative result. CONCLUSION: The result demonstrates that D-FISH technique is reliable for detecting bcr/abl fusion gene. The existence of negative bcr/abl fusion gene CML implies that some other initial events inducing CML might have occurred in such cases.

Adult↗

PubMed1999

Molecular cloning and mapping of a novel ADAM gene (ADAM29) to human chromosome 4.

Members of the ADAM family (type I integral membrane protein with a disintegrin and metalloprotease domain) have been implicated in many important biological processes involving cell-cell and cell-matrix interactions, such as fertilization and myoblast fusion. We report here the cDNA sequence of a novel human ADAM gene (ADAM29) that contains a putative fusion peptide. Northern blot analysis revealed that the mRNA of ADAM29 is highly expressed in the testis. By radiation hybrid panel mapping, the ADAM29 gene was assigned to human chromosome 4q34.2-qter.

ADAM Proteins↗

PubMed1999

Mechanism of corepressor binding and release from nuclear hormone receptors.

The association of transcription corepressors SMRT and N-CoR with retinoid and thyroid receptors results in suppression of basal transcriptional activity. A key event in nuclear receptor signaling is the hormone-dependent release of corepressor and the recruitment of coactivator. Biochemical and structural studies have identified a universal motif in coactivator proteins that mediates association with receptor LBDs. We report here the identity of complementary acting signature motifs in SMRT and N-CoR that are sufficient for receptor binding and ligand-induced release. Interestingly, the motif contains a hydrophobic core (PhixxPhiPhi) similar to that found in NR coactivators. Surprisingly, mutations in the amino acids that directly participate in coactivator binding disrupt the corepressor association. These results indicate a direct mechanistic link between activation and repression via competition for a common or at least partially overlapping binding site.

Amino Acid Sequence↗

PubMed1999

Crystal structure of a thermophilic alcohol dehydrogenase substrate complex suggests determinants of substrate specificity and thermostability.

The crystal structure of a thermophilic alcohol dehydrogenase (TBAD) from Thermoanaerobacter brockii has been determined in a binary complex with sec-butanol as substrate to a resolution of 3.0 A. Van der Waals interactions of the carbon C1 atom of sec-butanol with atoms in His59, Ala85, Trp110, Asp150, and Leu294 account for the substrate preference of this enzyme for secondary over primary alcohols. A crevice from the surface to the active site provides access for substrates and products. This opening is lined with the hydrophobic residues Ile49, Leu107, Trp110, Tyr267, Leu294 as well as Cys283 and Met285 from another molecule within the tetrameric assembly. This might explain the tolerance of this enzyme toward organic solvents. The zinc ion occupies a position in the active site, which is too remote for direct interaction with the alcohol group. A mechanism is suggested whereby the introduction of NADP would trigger a displacement of the zinc ion to its catalytic site. Features important for the unusually high melting temperature of 98 degrees C are suggested by comparison to the crystal structure of a highly homologous mesophilic alcohol dehydrogenase from Clostridium beijerinckii (CBAD). The thermophilic enzyme has a more hydrophilic exterior, a more hydrophobic interior, a smaller surface area, more prolines, alanines, and fewer serines than CBAD. Furthermore, in the thermophilic enzyme the number of all types of intersubunit interactions in these tetrameric enzymes is increased: more salt bridges, hydrogen bonds, and hydrophobic interactions. All these effects combined can account for the higher melting temperature of the thermophilic enzyme.

Alcohol Dehydrogenase↗

PubMed1999

Placental membrane inflammation and risks of maternal-to-child transmission of HIV-1 in Uganda.

UNLABELLED: Prospective follow-up of 172 HIV-infected pregnant women and their infants was conducted at Mulago Hospital, Kampala, Uganda during 1990 to 1992. Information was collected on maternal immune status (CD4 counts or clinical AIDS), and concurrent infections with sexually transmitted diseases. Infants were observed on a follow-up basis to determine HIV infection, using polymerase chain reaction (PCR) under 15 months of age and enzyme immunoassay/Western blot for those older than 15 months. Placental membrane inflammation (chorioamnionitis and funisitis), and placental villous inflammation (villitis, intervillitis, and deciduitis) were diagnosed by histopathology. Mother-to-child HIV transmission rates were assessed, and adjusted odds ratios (OR) and 95% confidence intervals (95% CI) of transmission were estimated using women with no placental pathology or evidence of immune suppression as a reference group. RESULTS: The overall mother-to-child HIV transmission rate was 23.3%. Women with no placental membrane inflammation or immune suppression had a transmission rate of 11.3%; compared with 25.5% in women with placental inflammation and no immunosuppression (adjusted OR, 2.87; 95% CI, 1.04-7.90), and 37.0% in immunosuppressed women (OR, 3.07; 95% CI, 1.42-6.67). We estimate that 34% of HIV transmission could be prevented by treatment of placental membrane inflammation in nonimmunocompromised women. Transmission rates were 40.9% with genital ulcer disease (OR, 3.57; 95% CI, 1.28-9.66). Placental villous inflammation and artificial rupture of membranes did not increase transmission rates and cesarean section was associated with a nonsignificant reduction of risk (OR, 0.70; 95% CI 0.24-2.06). CONCLUSION: Placental membrane inflammation increases the rate of mother-to-child HIV transmission.

Blotting, Western↗

PubMed1999

FMRFamide-related neuropeptide gene family in Caenorhabditis elegans.

Neuropeptides are used as signaling molecules in the nervous system of most organisms, including mammals. The family of FMRFamide (Phe-Met-Arg-Phe-NH2)-like neuropeptides (FaRPs) all share an RFamide sequence at their C-termini and have been shown to have diverse functions in the central and peripheral nervous systems. In the nematode Caenorhabditis elegans, FMRFamide-like peptides (FaRPs) are expressed in at least 10% of the neurons, including motor, sensory, and interneurons that are involved in movement, feeding, defecation, and reproduction. Twenty-two genes, designated flp-1 through flp-22, encode FaRPs in C. elegans, although there are likely to be additional flp genes to be identified. Each flp gene encodes a different set of FaRPs, yielding a predicted total of 59 distinct FaRPs; a few of the genes may also encode non-FaRPs. Inactivation of some of the flp genes indicates that at least one flp gene has unique functions, while at least two flp genes appear to have overlapping functions with other flp genes. These results suggest that a complex family of FaRPs have varied roles through all stages of development and in adulthood in C. elegans.

Animals↗

PubMed1999

Crystal structure of colicin E3 immunity protein: an inhibitor of a ribosome-inactivating RNase.

BACKGROUND: Colicins are antibiotic-like proteins of Escherichia coli that kill related strains. Colicin E3 acts as an RNase that specifically cleaves 16S rRNA, thereby inactivating the ribosomes in the infected cell. The producing organism is protected against colicin E3 by a specific inhibitor, the immunity protein Im3, which forms a tight 1:1 complex with colicin E3 and renders it inactive. Crystallographic studies on colicin E3 and Im3 have been undertaken to unravel the structural basis for the ribonucleolytic activity and its inhibition. RESULTS: The crystal structure of Im3 has been determined to a resolution of 1.8 A. The structure consists of a four-standard antiparallel beta sheet flanked by three alpha helices on one side of the sheet. Thr7, Phe9, Phe16 and Phe74 form a hydrophobic cluster on the surface of the protein in the vicinity of Cys47. This cluster is part of a putative binding pocket which also includes nine polar residues. CONCLUSIONS: The putative binding pocket of Im3 is the probable site of interaction with colicin E3. The six acidic residues in the pocket may interact with some of the numerous basic residues of colicin E3. The involvement of hydrophobic moieties in the binding is consistent with the observation that the tight complex can only be dissociated by denaturation. The structure of Im3 resembles those of certain nucleic acid binding proteins, in particular domain II of topoisomerase I and RNA-binding proteins that contain the ribonucleoprotein (RNP) sequence motif. This observation suggests that Im3 has a nucleic acid binding function in addition to binding colicin E3.

Bacterial Proteins↗