A specific targeting domain in mature exotoxin A is required for its extracellular secretion from Pseudomonas aeruginosa.

A number of Gram-negative bacteria, including Pseudomonas aeruginosa, actively secrete a subset of periplasmic proteins into their surrounding medium. The presence of a putative extracellular targeting signal within one such protein, exotoxin A, was investigated. A series of exotoxin A truncates, fused to beta-lactamase, was constructed. Hybrid proteins, which carry at their N- termini 120, 255, 355 or the entire 613 residues of the mature exotoxin A, were stable and were secreted into the extracellular medium. Hybrid proteins which carry residues 1-30 and 1-60 of the mature exotoxin A were unstable; however, they could be detected entirely within the cells after a short labeling period. A hybrid with beta-lactamase was constructed which carried only the N-terminal residues 1-3 and region 60-120 of exotoxin A. It was also secreted into the culture medium, suggesting that a specific 60 amino acid domain contains the necessary targeting information for translocation of exotoxin A across the outer membrane. The secretion of the hybrid proteins is independent of the passenger protein, since a similar exotoxin A-murine interleukin 4 hybrid protein was also secreted. The extracellular targeting signal between amino acids 60 and 120 is rich in anti-parallel beta-sheets. It has been shown previously to be involved in the interaction of the exotoxin A with the receptors of the eukaryotic cells. In the three- dimensional view, the targeting region is on the toxin surface where it is easily accessible to the components of the extracellular secretion machinery.     

Influence of deletions within domain II of exotoxin A on its extracellular secretion from Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative bacterium that secretes many proteins into the extracellular medium via the Xcp machinery. This pathway, conserved in gram-negative bacteria, is called the type II pathway. The exoproteins contain information in their amino acid sequence to allow targeting to their secretion machinery. This information may be present within a conformational motif. The nature of this signal has been examined for P. aeruginosa exotoxin A (PE). Previous studies failed to identify a common minimal motif required for Xcp-dependent recognition and secretion of PE. One study identified a motif at the N terminus of the protein, whereas another one found additional information at the C terminus. In this study, we assess the role of the central PE domain II composed of six alpha-helices (A to F). The secretion behavior of PE derivatives, individually deleted for each helix, was analyzed. Helix E deletion has a drastic effect on secretion of PE, which accumulates within the periplasm. The conformational rearrangement induced in this variant is predicted from the three-dimensional PE structure, and the molecular modification is confirmed by gel filtration experiments. Helix E is in the core of the molecule and creates close contact with other domains (I and III). Deletion of the surface-exposed helix F has no effect on secretion, indicating that no secretion information is contained in this helix. Finally, we concluded that disruption of a structured domain II yields an extended form of the molecule and prevents formation of the conformational secretion motif.     

Amino acids at the N- and C-termini of human glutamate carboxypeptidase II are required for enzymatic activity and proper folding.

Human glutamate carboxypeptidase II (GCPII) is a co-catalytic metallopeptidase and its putative catalytic domain is homologous to the aminopeptidases from Vibrio proteolyticus and Streptomyces griseus. In humans, the enzyme is expressed predominantly in the nervous system and the prostate. The prostate form, termed prostate-specific membrane antigen, is overexpressed in prostate cancer and is used as a diagnostic marker of the disease. Inhibition of the form of GCPII expressed in the central nervous system has been shown to protect against ischemic injury in experimental animal models. Human GCPII consists of 750 amino acids, and six individual domains were predicted to constitute the protein structure. Here, we report the analysis of the contribution of these putative domains to the structure/function of recombinant human GCPII. We cloned 13 mutants of human GCPII that are truncated or extended at one or both the N- and C-termini of the GCPII sequence. The clones were used to generate stably transfected Drosophila Schneider's cells, and the expression and carboxypeptidase activities of the individual protein products were determined. The extreme C-terminal region of human GCPII was found to be critical for the hydrolytic activity of the enzyme. The deletion of as few as 15 amino acids from the C-terminus was shown to completely abolish the enzymatic activity of GCPII. Furthermore, the GCPII carboxypeptidase activity was abrogated upon removal of more than 60 amino acid residues from the N-terminus of the protein. Overall, these results clearly show that amino acid segments at the N- and C-termini of the ectodomain of GCPII are essential for its carboxypeptidase activity and/or proper folding.     

Translocation mediated by domain II of Pseudomonas exotoxin A: transport of barnase into the cytosol.

Pseudomonas exotoxin A (PE) is a protein toxin composed of three structural domains. Functional analysis of PE has revealed that domain I is the cell-binding domain and that domain III functions in ADP ribosylation. Domain II was originally designated as the translocation domain, mediating the transfer of domain III to the cytosol, because mutations in this domain result in toxin molecules with normal cell-binding and ADP-ribosylation activities but which are not cytotoxic. However, the results do not rule out the possibility that regions of PE outside of domain II also participate in the translocation process. To investigate this problem, we have now constructed a toxin in which domain III of PE is replaced with barnase, the extracellular ribonuclease of Bacillus amyloliquefaciens. This chimeric toxin, termed PE1-412-Bar, is cytotoxic to a murine fibroblast cell line and to a murine hybridoma resistant to the ADP-ribosylation activity of PE. A mutant form of PE1-412-Bar with an inactivating mutation in domain II at position 276 was significantly less toxic. Because the cytotoxic effect of PE1-412-Bar was due to the ribonuclease-activity of barnase molecules which had been translocated to the cytosol, we conclude that domain II of PE is not only essential but also probably sufficient to carry out the translocation process.     

Alanine scanning mutagenesis and functional analysis of the fibronectin-like collagen-binding domain from human 92-kDa type IV collagenase.

The human 72-kDa (CLG4A) and 92-kDa (CLG4B) type IV collagenases contain a domain consisting of three contiguous copies of the fibronectin (FN)-derived type II homology unit (T2HU), T2HU-1, T2HU-2, and T2HU-3. To investigate the functional role of this domain, we have constructed plasmids expressing beta-galactosidase fusion proteins with one or more of the CLG4B-derived T2HU. The gelatin binding assays demonstrate that a single copy of T2HU-2 renders beta-galactosidase capable of binding gelatin. The three repeats, however, differ dramatically in their capacity to bind gelatin, with T2HU-1 and T2HU-3 having significantly less binding activity than T2HU-2. Using alanine scanning mutagenesis we have defined the amino acid residues (Arg307, Asp309, Asn319, Tyr320, Asp323) that are critical for gelatin binding of T2HU-2. The low gelatin binding of T2HU-1 compared to T2HU-2 was traced to the non-conserved residues Ala228-Ala and Leu253-Pro. The results suggest that the gelatin binding of the type IV collagenase proenzyme is mediated by the FN-like domain, although the presence of another gelatin-binding site cannot be excluded. The FN domain-mediated binding, however, is not a rate-limiting step in the hydrolysis of gelatin by the enzyme.     

Structural and mutational analysis of archaeal ATP-dependent RNA ligase identifies amino acids required for RNA binding and catalysis

An ATP-dependent RNA ligase from Methanobacterium thermoautotrophicum (MthRnl) catalyzes intramolecular ligation of single-stranded RNA to form a closed circular RNA via covalent ligase-AMP and RNA-adenylylate intermediate. Here, we report the X-ray crystal structures of an MthRnl•ATP complex as well as the covalent MthRnl–AMP intermediate. We also performed structure-guided mutational analysis to survey the functions of 36 residues in three component steps of the ligation pathway including ligase-adenylylation (step 1), RNA adenylylation (step 2) and phosphodiester bond synthesis (step 3). Kinetic analysis underscored the importance of motif 1a loop structure in promoting phosphodiester bond synthesis. Alanine substitutions of Thr117 or Arg118 favor the reverse step 2 reaction to deadenylate the 5′-AMP from the RNA-adenylate, thereby inhibiting step 3 reaction. Tyr159, Phe281 and Glu285, which are conserved among archaeal ATP-dependent RNA ligases and are situated on the surface of the enzyme, are required for RNA binding. We propose an RNA binding interface of the MthRnl based on the mutational studies and two sulfate ions that co-crystallized at the active site cleft in the MthRnl–AMP complex.     

Alanine and glutamine synthesis and release from skeletal muscle. II. The precursor role of amino acids in alanine and glutamine synthesis.

The synthesis and release of alanine and glutamine have been studied in the intact rat epitrochlaris skeletal muscle preparation. Aspartate, cysteine, leucine, valine, methionine, isoleucine, serine, theronine, and glycine increased significantly the formation and release of alanine from muscle. Cysteine, leucine, valine, methionine, isoleucine, tyrosine, lysine, and phenylalanine increased the rate of glutamine synthesis. Only ornithine, arginine, and tryptophan were without effect on the synthesis of either alanine or glutamine. Half-maximal stimulation of alanine and glutamine formation by added amino acids was observed with concentrations ranging between 0.5 and 1.0 mM. Increases in alanine and glutamine formation were not accompanied by changes in pyruvate production or glucose uptake. The progressive decline in alanine and glutamine synthesis noted on prolonged incubation was prevented by the addition of amino acids to the incubation medium. Stimulation of alanine synthesis by added amino acids was unaffected by inhibition of glycolysis with iodoacetate. Inhibition of alanine aminotransferase with aminooxyacetate significantly decreased alanine formation. Pyruvate and ammonium chloride did not increase further the rate of either alanine or glutamine formation above that produced by added amino acids. These data indicate that most amino acids are precursors for alanine and glutamine synthesis in skeletal muscle. A general mechanism is presented for the de novo formation of alanine from amino acids in skeletal muscle, and the importance of proteolysis for the supply of amino acid precursors for alanine and glutamine synthesis is discussed.     

Random insertion and deletion mutagenesis for construction of protein library containing nonnatural amino acids.

A new method was developed for the generation of a library of mutant proteins that contained nonnatural amino acids. The method, "random insertion and deletion (RID) mutagenesis", is based on the deletion of an arbitrary number of bases at random positions and, at the same time, the insertion of an arbitrary sequence into the same position. By using this method, randomly selected three consecutive bases in the gene of green fluorescence protein (GFP) were replaced by a CGGT 4-base codon. When this DNA library was expressed in E. coli, about 80% of colonies lost the fluorescence. The non-fluorescent colonies were picked up and the genes were sequenced. Replacement of three consecutive bases by CGGT 4-base codon was found in two of the four mutated genes.     

The C-terminal domain of aminopeptidase A is an intramolecular chaperone required for the correct folding, cell surface expression, and activity of this monozinc aminopeptidase

Aminopeptidase A (APA, EC 3.4.11.7) is a type II integral membrane glycoprotein responsible for the conversion of angiotensin II to angiotensin III in the brain. Previous site-directed mutagenesis studies and the recent molecular modeling of the APA zinc metallopeptidase domain have shown that all the amino acids involved in catalysis are located between residues 200 and 500. The APA ectodomain is cleaved in the kidney into an N-terminal fragment corresponding to the zinc metallopeptidase domain, and a C-terminal fragment of unknown function. We investigated the function of this C-terminal domain, by expressing truncated APAs in Chinese hamster ovary and AtT-20 cells. Deletion of the C-terminal domain abolished the maturation and enzymatic activity of the N-terminal domain, which was retained in the endoplasmic reticulum as an unfolded protein bound to calnexin. Expression in trans of the C-terminal domain resulted in association of the N- and C-terminal domains soon after biosynthesis, allowing folding rescue, maturation, cell surface expression, and activity of the N-terminal zinc metallopeptidase domain. We also show that the C-terminal domain is not required for the catalytic activity of APA but is essential for its activation. Moreover, we show that the C-terminal domain of aminopeptidase N (EC 3.4.11.2, APN) also promotes maturation and cell surface expression of the N-terminal domain of APN, suggesting a common role of the C-terminal domain in the monozinc aminopeptidase family. Our data provide the first demonstration that the C-terminal domain of an eukaryotic exopeptidase acts as an intramolecular chaperone.     

Synthesis, spectroscopic, mutagenic, and cytotoxicity studies of some mixed-ligand platinum(II) complexes of 2,2'-bipyridine and amino acids

Seven platinum(II) complexes of the type [Pt(bipy)(AA)]n+ (where n = 1 or 0 and AA is anion of L-valine, L-isoleucine, L-aspartic acid (dianion), L-glutamic acid (dianion), L-glutamine, L-proline, or S-methyl-L-cysteine) have been prepared and characterized. The modes of binding of amino acids in these complexes have been ascertained particularly by infrared and 1H NMR spectral studies. The L-glutamine complex shows a ID50 value (50% inhibitory dose) in the range of greater than 20 micrograms/ml to 100 micrograms/ml of the complex. However, the complexes of L-valine, L-isoleucine, L-aspartic acid, L-glutamic acid, L-proline, and S-methyl-L-cysteine show ID50 values greater than 100 micrograms/ml of the complex. The above complexes also show inferior growth inhibition of P-388 cells than platinum(II) complexes of 2,2'-bipyridine with L-alanine, L-leucine, L-methionine, and L-aspargine as reported earlier. The platinum(II) complexes of 2,2'-bipyridine with glycine (Gly), L-alanine (Ala), L-leucine (leu), L-valine (Val), L-methionine (Met), L-phenylalanine (Phe), L-serine (Ser), L-tyrosine (Tyr) and L-tryptophan (Trp) have been tested for mutagenesis using TA 100 and TA 98 strains. They show nonmutagenicity. This is in contrast to the cis-[Pt(NH3)2Cl2] showing a base pair substitution mutagenesis.     

Interaction of caerulein, glucose, and amino acids on insulin secretion from the perfused rat pancreas

The effect of caerulein on insulin response to graded amounts of glucose from the isolated perfused rat pancreas was investigated in the presence or absence of an amino acids mixture. Caerulein at a concentration of 0.1 ng/ml which is a submaximal concentration for an effect on exocrine pancreatic secretion potentiated insulin responses to glucose concentrations less than 200 mg/dl, but produced no further increase when added to a glucose stimulus over a 200 mg/dl. However, in the presence of amino acids the insulin response to 200 mg/dl glucose was significantly potentiated by the stimulation of 0.1 ng/ml caerulein. The effectiveness of caerulein as an insulinotropic agent depended on the glucose concentration only when amino acids were present. These results indicate that caerulein, at a concentration which stimulate pancreatic exocrine secretion, has a synergistic effect on insulin response to glucose and amino acids and therefore raises the possibility that endogenously released CCK may contribute to the entero-insular axis.     

In vitro mutagenesis of Caenorhabditis elegans cuticle collagens identifies a potential subtilisin-like protease cleavage site and demonstrates that carboxyl domain disulfide bonding is required for normal function but not assembly.

The importance of conserved amino acids in the amino and carboxyl non-Gly-X-Y domains of Caenorhabditis elegans cuticle collagens was examined by analyzing site-directed mutations of the sqt-1 and rol-6 collagen genes in transgenic animals. Altered collagen genes on transgenic arrays were shown to produce appropriate phenotypes by injecting in vivo cloned mutant alleles. Equivalent alterations in sqt-1 and rol-6 generally produced the same phenotypes, indicating that conserved amino acids in these two collagens have similar functions. Serine substitutions for either of two conserved carboxyl domain cysteines produced LRol phenotypes. Substitution for both cysteines in sqt-1 also resulted in an LRol phenotype, demonstrating that disulfide bonding is important for normal function but not required for assembly. Arg-1 or Arg-4 to Cys mutations in homology block A (HBA; consensus, 1-RXRRQ-5; in the amino non-Gly-X-Y domain) caused RRol phenotypes, while the same alteration at Arg-3 had no effect, indicating that Arg-3 is functionally different from Arg-1 and Arg-4. Substitutions of Arg-4 with Ser, Leu, or Glu also produced the RRol phenotype, while Lys substitutions for Arg-1 or Arg-4 did not generate any abnormal phenotypes. His substitutions for Arg-1 or Arg-4 caused somewhat less severe RRol phenotypes. Therefore, strong positively charged residues, Arg or Lys, are required at positions 1 and 4 for normal function. The conserved pattern of arginines in HBA matches the cleavage sites of the subtilisin-like endoproteinases. HBA may be a cleavage site for a subtilisin-like protease, and cleavage may be important for cuticle collagen processing.     

Cellular locations of Pseudomonas syringae pv. syringae HrcC and HrcJ proteins, required for harpin secretion via the type III pathway

The complete hrp-hrc-hrmA cluster of Pseudomonas syringae pv. syringae 61 encodes 28 polypeptides. A saprophytic bacterium carrying this cluster is capable of secreting HrpZ-a harpin encoded by hrpZ-in an hrp-dependent manner, which suggests that this cluster contains sufficient components to assemble functional type III secretion machinery. Sequence data show that HrcJ and HrcC are putative outer membrane proteins, and nonpolar mutagenesis demonstrates they are all required for HrpZ secretion. In this study, we investigated the cellular localization of the HrcC and HrcJ proteins by Triton solubilization, sucrose-gradient isopycnic centrifugation, and immunogold labeling of the bacterial cell surface. Our results indicate that HrcC is indeed an outer membrane protein and that HrcJ is located between both membranes. Their membrane localization suggests that they might be involved in the formation of a supramolecular structure for protein secretion.