Characterization of stress and methylglyoxal inducible triose phosphate isomerase (OscTPI) from rice

As compared with plant system, triose phosphate isomerase (TPI), a crucial enzyme of glycolysis, has been well studied in animals. In order to characterize TPI in plants, a full-length cDNA encoding OscTPI was cloned from rice and expressed in E. coli. The recombinant OscTPI was purified to homogeneity and it showed Km value of 0.1281 ± 0.025 µM, and the Vmax value of 138.7 ± 16 µmol min (-1) mg (-1) which is comparable to the kinetic values studied in other plants. The OscTPI was found to be exclusively present in the cytoplasm when checked with the various methods. Functional assay showed that OscTPI could complement a TPI mutation in yeast. Real time PCR analysis revealed that OscTPI transcript level was regulated in response to various abiotic stresses. Interestingly, it was highly induced under different concentration of methylglyoxal (MG) stress in a concentration dependent manner. There was also a corresponding increase in the protein and the enzyme activity of OscTPI both in shoot and root tissues under MG stress. Our result shows that increases in MG leads to the increase in TPI which results in decrease of DHAP and consequently decrease in the level of toxic MG.

Triose phosphate isomerase, a novel enzyme-crystallin, and tau-crystallin in crocodile cornea

Several enzymes are known to accumulate in the cornea in unusually high concentrations. Based on the analogy with lens crystallins, these enzymes are called corneal crystallins, which are diverse and species-specific. Examining crystallins in lens and cornea in multiple species provides great insight into their evolution. We report data on major proteins present in the crocodile cornea, an evolutionarily distant taxon. We demonstrate that tau-crystallin/alpha-enolase and triose phosphate isomerase (TIM) are among the major proteins expressed in the crocodile cornea as resolved by 2D gel electrophoresis and identified by MALDI-TOF. These proteins might be classified as putative corneal crystallins. tau-Crystallin, known to be present in turtle and crocodile lens, has earlier been identified in chicken and bovine cornea, whereas TIM has not been identified in the cornea of any species. Immunostaining showed that tau-crystallin and TIM are concentrated largely in the corneal epithelium. Using western blot, immunofluorescence and enzymatic activity, we demonstrate that high accumulation of tau-crystallin and TIM starts in the late embryonic development (after the 24th stage of embryonic development) with maximum expression in a two-week posthatched animal. The crocodile corneal extract exhibits significant alpha-enolase and TIM activities, which increases in the corneal extract with development. Our results establishing the presence of tau-crystallin in crocodile, in conjunction with similar reports for other species, suggest that it is a widely prevalent corneal crystallin. Identification of TIM in the crocodile cornea reported here adds to the growing list of corneal crystallins.

Hypoxia up-regulates triosephosphate isomerase expression via a HIF-dependent pathway

The glycolytic enzyme triosephosphate isomerase (TPI) catalyses the reversible conversion of dihydroxyacetone phosphate into glyceraldehyde-3-phosphate. We report here that the expression of TPI at both the mRNA and protein levels is increased by hypoxia in vivo and in vitro. The temporal pattern of hypoxic TPI induction is very similar to that of genes triggered by the hypoxia-inducible transcription factor (HIF) and is mimicked characteristically by cobalt and by deferoxamine, but is absent in cells with a defective aryl hydrocarbon receptor nuclear translocator (ARNT, here HIF-1beta) and in cells lacking HIF-1alpha protein. We conclude from these findings that the expression of TPI is regulated via the HIF pathway and thus belongs to the family of classic oxygen-regulated genes. The physiological meaning of an increased expression of TPI in hypoxygenated tissues is probably to increase the flow of triosephosphates through the glycolytic cascade thus leading to an increase of anaerobic energy generation.

Conserved Cysteine 126 in Triosephosphate Isomerase Is Required Not for Enzymatic Activity but for Proper Folding and Stability†

In triosephosphate isomerase, Cys126 is a conserved residue located close to the catalytic glutamate, Glu165. Although it has been mentioned that Cys126 and other nearby residues are required to maintain the active site geometry optimal for catalysis, no evidence supporting this idea has been reported to date. In this work, we studied the catalytic and stability properties of mutants C126A and C126S of Saccharomyces cerevisiae TIM (wtTIM). None of these amino acid replacements induced significant changes in the folding of wtTIM, as indicated by spectroscopic studies. C126S and C126A have K(M) and k(cat) values that are concomitantly reduced by only 4-fold and 1.5-fold, respectively, compared to those of wtTIM; in either case, however, the catalytic efficiency (k(cat)/K(M)) of the enzyme is barely affected. The affinity of mutated TIMs for the competitive inhibitor 2-phosphoglycolate augmented also slightly. In contrast, greater susceptibility to thermal denaturation resulted from mutation of Cys126, especially when it was changed to Ser. By using values of the rate constants for unfolding and refolding, we estimated that, at 25 degrees C, C126A and C126S are less stable than wtTIM by about 5.0 and 9.0 kcal mol(-)(1), respectively. Moreover, either of these mutations slows down the folding rate by a factor of 10 and decreases the recovery of the active enzyme after thermal unfolding. Thus, Cys126 is required for proper stability and efficient folding of TIM rather than for enzymatic catalysis.

Hypoxic up-regulation of triosephosphate isomerase expression in mouse brain capillary endothelial cells

A protein with a molecular mass of 27kDa was induced by hypoxia in a mouse brain capillary endothelial cell line and identified as triosephosphate isomerase (TPI) by amino-terminal sequencing. Hypoxia caused an elevation of the TPI protein level, concomitant with an increase of the TPI mRNA level. However, hypoxia resulted in an insufficient elevation of TPI activity level, compared to an increase of TPI protein level. When cells expressing the recombinant TPI protein with histidine tag were exposed to hypoxia and the TPI protein was affinity-purified, the catalytic activity (specific activity) of the TPI protein purified from hypoxic cells was substantially lower than that obtained from normoxic cells. In addition, three TPI isoforms with an electrophoretic multiplicity were found; two of the three isoforms were substantially increased in response to the hypoxia, but the level of the most acidic isoform was barely changed. The induction of TPI gene expression by hypoxia was suppressed by (1) a chelator of intracellular Ca(2+), (2) a blocker of non-selective cation channels, (3) a blocker of Na(+)/Ca(2+) exchangers, (4) an inhibitor of Ca(2+)/calmodulin-dependent protein kinases, and (5) an inhibitor of c-jun/AP-1 activation.

Identification of metabolic enzymes in renal cell carcinoma utilizing PROTEOMEX analyses

PROTEOMEX, an approach which combines conventional proteome analysis with serological screening, is a powerful tool to separate proteins and identify immunogenic components in malignant diseases. By applying this approach, we characterized nine metabolic enzymes which were differentially expressed in renal cell carcinoma (RCC) cell lines and compared their expression profiles to that of normal kidney epithelium cells. Four of these proteins, superoxide dismutase (SODC), triosephosphatase isomerase (TPIS), thioredoxin (THIO) and ubiquitin carboxyl-terminal hydrolase (UBL1) were further analysed for both their constitutive and interferon (IFN)-gamma inducible protein expression pattern in cell lines or tissue specimens derived from RCC or normal kidney epithelium using Western blot analysis and immunohistochemistry, respectively. With the exception of the RCC cell line MZ1940RC, which completely lacks the expression of UBL1, a heterogeneous and variable expression pattern of the different metabolic enzymes was detected in RCC and normal renal epithelium. The highest differences in the expression levels were found for THIO in the RCC cell lines, which was 2-fold upregulated when compared to autologous normal kidney epithelium. Moreover, IFN-gamma treatment did not influence the constitutive expression of these metabolic enzymes. Thus, PROTEOMEX represents a valuable approach for the identification of metabolic enzymes which might be used as markers for the diagnosis of RCC.

"Early" protein synthesis of Lactobacillus delbrueckii ssp. bulgaricus in milk revealed by [35S] methionine labeling and two-dimensional gel electrophoresis

The proteomes of exponentially growing and stationary cells of Lactobacillus delbrueckii ssp. bulgaricus grown in rich medium (MRS) were separated by two-dimensional polyacrylamide gel electrophoresis (2-DE) and quantified after Coomassie staining. Stationary cells grown in MRS were inoculated in reconstituted skim milk, and "early" protein synthesis during the first 30 min of fermentation in milk was monitored by [35S]methionine labeling and 2-DE. In contrast to exponentially growing or stationary cells, the predominant "early" proteins were small (< 15 kDa) and of low pI (< 5.3). Quantification of the proteome of the "early" lag phase based on 47 "spots" revealed that only three "early" proteins accounted for more than 80% of the total label. They were identified as pI 4.7 and 4.9 isoforms of the heat-stable phosphoryl carrier protein (HPr) with 45.2 and 9.4% of total label, respectively, and an unknown protein called EPr1 ("early" protein 1) with 26.6% of total label. Although an N-terminal sequence of 19 amino acids was obtained, no homologs to EPr1 could be found. De novo synthesis of the 10 and 60 kDa heat shock proteins (GroES and GroEL) was considerably lower (0.04 and 0.9% of total label, respectively), indicating only low levels of stress. Synthesis of triosephosphate isomerase (Tpi) as marker for glycolytic enzymes reached only 0.08% of total label. Our results demonstrate that inoculation in milk, resulting in a change from glucose to lactose as carbon source, imposes only little need for synthesis of stress or glycolytic enzymes, as sufficient proteins are present in the stationary, MRS-grown cells. The high level of expression of the pI 4.7 isoform of HPr suggests a regulatory function of the presumed Ser-46 phosphorylated form of HPr.

Understanding protein lids: kinetic analysis of active hinge mutants in triosephosphate isomerase

In previous work we tested what three amino acid sequences could serve as a protein hinge in triosephosphate isomerase [Sun, J., and Sampson, N. S. (1998) Protein Sci. 7, 1495-1505]. We generated a genetic library encoding all 8000 possible 3 amino acid combinations at the C-terminal hinge and selected for those combinations of amino acids that formed active mutants. These mutants were classified into six phylogenetic families. Two families resembled wild-type hinges, and four families represented new types of hinges. In this work, the kinetic characteristics and thermal stabilities of mutants representing each of these families were determined in order to understand what properties make an efficient protein hinge, and why all of the families are not observed in nature. From a steady-state kinetic analysis of our mutants, it is clear that the partitioning between protonation of intermediate to form product and intermediate release from the enzyme surface to form methylglyoxal (a decomposition product) is not affected. The two most impaired mutants undergo a change in rate-limiting step from enediol formation to dihydroxyacetone phosphate binding. Thus, it appears that k(cat)/K(m)'s are reduced relative to wild type as a result of slower Michaelis complex formation and dissociation, rather than increased loop opening speed.

At least one intron is required for the nonsense-mediated decay of triosephosphate isomerase mRNA: a possible link between nuclear splicing and cytoplasmic translation

Mammalian cells have established mechanisms to reduce the abundance of mRNAs that harbor a nonsense codon and prematurely terminate translation. In the case of the human triosephosphate isomerase (TPI gene), nonsense codons located less than 50 to 55 bp upstream of intron 6, the 3'-most intron, fail to mediate mRNA decay. With the aim of understanding the feature(s) of TPI intron 6 that confer function in positioning the boundary between nonsense codons that do and do not mediate decay, the effects of deleting or duplicating introns have been assessed. The results demonstrate that TPI intron 6 functions to position the boundary because it is the 3'-most intron. Since decay takes place after pre-mRNA splicing, it is conceivable that removal of the 3'-most intron from pre-mRNA "marks" the 3'-most exon-exon junction of product mRNA so that only nonsense codons located more than 50 to 55 nucleotides upstream of the "mark" mediate mRNA decay. Decay may be elicited by the failure of translating ribosomes to translate sufficiently close to the mark or, more likely, the scanning or looping out of some component(s) of the translation termination complex to the mark. In support of scanning, a nonsense codon does not elicit decay if some of the introns that normally reside downstream of the nonsense codon are deleted so the nonsense codon is located (i) too far away from a downstream intron, suggesting that all exon-exon junctions may be marked, and (ii) too far away from a downstream failsafe sequence that appears to function on behalf of intron 6, i.e., when intron 6 fails to leave a mark. Notably, the proposed scanning complex may have a greater unwinding capability than the complex that scans for a translation initiation codon since a hairpin structure strong enough to block translation initiation when inserted into the 5' untranslated region does not block nonsense-mediated decay when inserted into exon 6 between a nonsense codon residing in exon 6 and intron 6.

Sulfhydryl reagent susceptibility in proteins with high sequence similarity--triosephosphate isomerase from Trypanosoma brucei, Trypanosoma cruzi and Leishmania mexicana

The amino acid sequence of triosephosphate isomerase from Trypanosoma brucei, Trypanosoma cruzi, and Leishmania mexicana have an identity of 68%. Using the numbering system for the T. brucei enzyme, in their aligned sequences, the T. cruzi and leishmanial enzymes have cysteine residues at positions 14, 40, 117 and 126. T. brucei triosephosphate isomerase has cysteine residues at positions 14, 40 and 126, and a valine residue at position 117. Dithionitrobenzoic acid and methylmethane thiosulfonate inhibited the three enzymes, but T. cruzi triosephosphate isomerase was more than 100-fold more sensitive. The sensitivity of wild type triosephosphate isomerase from T. cruzi and T. brucei to the reagents was equal to that of the Cys117Val and Val117Cys mutant enzymes, respectively. Triosephosphate isomerases that have cysteine residues at positions 40 and 126, but lack a cysteine residue at position 14 are insensitive to methylmethane thiosulfonate. Thus, sulfhydryl reagents act on Cys14. At stoichiometric concentrations, the reagents inhibited the three enzymes as a consequence of structural alterations as measured by binding of 8-anilino-1-napthalenesulfonic acid to previously buried hydrophobic regions. However, the times for half-maximal alterations were 10 min, 15 hours and over 30 hours for T. cruzi, T. brucei and L. mexicana triosephosphate isomerase, respectively. The effect of pH on the action of the sulfhydryl reagents and molecular modeling showed no differences in the solvent accessibility of Cys14. As Cys14 forms part of the dimer interface, the data indicate that, in the three enzymes, barriers of different magnitude hinder the interaction between the sulfhydryl reagents and Cys14. The barrier is lower in T. cruzi triosephosphate isomerase which makes its dimer interface more susceptible for perturbation.

Intron position affects expression from the tpi promoter in rice

A series of promoter-GUS fusion constructs containing a portion of the rice triosephosphate isomerase (tpi) promoter, the first tpi intron, and the gene encoding bacterial beta-glucuronidase (GUS) were made. These constructs were electroporated into rice protoplasts and transient expression was monitored. Inclusion of the first intron from the rice tpi gene enhanced expression of the GUS gene from the tpi promoter when it was placed 5' of the GUS gene. When the tpi intron was placed in the 3'-untranslated region no enhancement of GUS gene expression was observed, indicating the importance of position in intron-mediated enhancement of gene expression.

The use of antibodies for characterization and quantification of a recombinant protein

In this study, we characterized proteinase A secreted by recombinant Saccharomyces cerevisiae bearing a multicopy plasmid containing the encoding gene (PEP4). Polyclonal and monoclonal antibodies were raised to study the product heterogeneity. Characterization of proteinase A was performed by immunoelectrophoresis and immunoblotting techniques. None of the monoclonal antibodies raised against proteinase A was found to react with the glycosyl side chains; thus cross-reaction with other glycosylated proteins (e.g. carboxypeptidase Y) was very low. This study allowed us to develop an ELISA method for the quantification of proteinase A in culture supernatants as well as the evaluation of monoclonal antibodies for their use in immunoaffinity chromatography.

Stabilization of human triosephosphate isomerase by improvement of the stability of individual alpha-helices in dimeric as well as monomeric forms of the protein

Human triosephosphate isomerase (hTIM) is a dimeric enzyme of identical subunits, adopting the alpha/beta-barrel fold. In a previous work, a monomeric mutant of hTIM was engineered in which Met14 and Arg98, two interface residues, were changed to glutamine. Analysis of equilibrium denaturation of this monomeric mutant, named M14Q/R98Q, revealed that its conformational stability, 2.5kcal/mol, is low as compared to the stability of dimeric hTIM (19.3 kcal/mol). The fact that this value is also lower than the conformational stabilities usually found for monomeric proteins suggests that the hTIM monomers are thermodynamically unstable. In the present work, we attempted to stabilize the M14Q/R98Q mutant by introducing stabilizing mutations in alpha-helices of the protein. Five mutations were proposed, designed to increase alpha-helix propensity by introducing alanines at solvent-exposed sites (Q179A, K193A), to introduce favorable interactions with helix dipoles (Q179D, S105D), or to reduce the conformational entropy of unfolding by introducing proline residues at the "N-cap" position of alpha-helices (A215P). Three replacements (Q179D, K193A, and A215P) were found to increase the stability of the native dimeric hTIM and the monomeric M14Q/R98Q. These results suggest that the monomeric hTIM mutant can be stabilized to a considerable extent by following well-established rules for protein stabilization. A comparison of the stabilizing effect performed by the mutations on the dimeric hTIM and the monomeric M14Q/R98Q allowed us to reinforce a model of equilibrium denaturation proposed for both proteins.

Cytoplasmic mRNA for human triosephosphate isomerase is immune to nonsense-mediated decay despite forming polysomes

Nonsense codons between position 14 within the first exon and position 193 within the penultimate exon of the human gene for triosephosphate isomerase reduce mRNA abundance to 25% of normal. The reduction in abundance is due to the decay of newly synthesized mRNA that copurifies with nuclei. TPI mRNA that copurifies with cytoplasm is immune to decay. We show here that immunity is not due to the failure of nonsense-containing mRNA to form polysomes. This finding indicates that cytoplasmic mRNA, in contrast to nucleus-associated mRNA, may have lost one or more factors that are required for nonsense-mediated decay or gained one or more factors that confer immunity to nonsense-mediated decay.

The structural basis for pseudoreversion of the E165D lesion by the secondary S96P mutation in triosephosphate isomerase depends on the positions of active site water molecules

The structural basis for the improvement in catalytic efficiency of the mutant E165D chicken triosephosphate isomerase by the secondary mutation, S96P, has been analyzed using a combination of X-ray crystallography and Fourier transform infrared spectroscopy. All X-ray structures were of the complex of phosphoglycolohydroxamate (PGH), an intermediate analog, with the isomerase, and each was solved to a resolution of 1.9 A. Comparison of the structure of the double mutant, E165D.S96P, with that of the single mutant, E165D, as well as with the wild-type isomerase shows only insignificant differences in the positions of the side chains in all of the mutants when compared with the wild-type isomerase, except that in both the E165D and E165D.S96P mutants, the aspartate side chain was approximately 0.7 A further away from the substrate analog than the glutamate side chain. Significant differences were observed in the crystal structure of the E165D.S96P double mutant in the positions of ordered water molecules bound at the active site. The loss of two water molecules located near the side chain at position 165 was observed in isomerases containing the S96P mutation. The resulting increase in hydrophobicity of the pocket probably causes an increase in the pKa of the catalytic base, D165, thereby improving its basicity. A new ordered water molecule was observed underneath the bound PGH in the E165D.S96P structure, which likely decreases the pKa's of the substrate protons, thereby increasing their acidity. An enzyme derived carbonyl stretch at 1746 cm-1 that is only observed in the IR spectrum of the E165D.S96P double mutant isomerase with bound substrates has been assigned to a stable ground state protonated D165-enediol(ate) intermediate complex. Thus, the gain in activity resulting from the S96P second site change probably results from a combination of improving the basicity of the enzyme, improving the acidity of the substrate protons, and stabilization of a reaction intermediate. All three of these effects seem to be caused by changes in bound water molecules.

Three new crystal structures of point mutation variants of monoTIM: conformational flexibility of loop-1, loop-4 and loop-8

BACKGROUND

Wild-type triosephosphate isomerase (TIM) is a very stable dimeric enzyme. This dimer can be converted into a stable monomeric protein (monoTIM) by replacing the 15-residue interface loop (loop-3) by a shorter, 8-residue, loop. The crystal structure of monoTIM shows that two active-site loops (loop-1 and loop-4), which are at the dimer interface in wild-type TIM, have acquired rather different structural properties. Nevertheless, monoTIM has residual catalytic activity.

RESULTS

Three new structures of variants of monoTIM are presented, a double-point mutant crystallized in the presence and absence of bound inhibitor, and a single-point mutant in the presence of a different inhibitor. These new structures show large structural variability for the active-site loops, loop-1, loop-4 and loop-8. In the structures with inhibitor bound, the catalytic lysine (Lys13 in loop-1) and the catalytic histidine (His95 in loop-4) adopt conformations similar to those observed in wild-type TIM, but very different from the monoTIM structure.

CONCLUSIONS

The residual catalytic activity of monoTIM can now be rationalized. In the presence of substrate analogues the active-site loops, loop-1, loop-4 and loop-8, as well as the catalytic residues, adopt conformations similar to those seen in the wild-type protein. These loops lack conformational flexibility in wild-type TIM. The data suggest that the rigidity of these loops in wild-type TIM, resulting from subunit-subunit contacts at the dimer interface, is important for optimal catalysis.

The Lactococcus lactis triosephosphate isomerase gene, tpi, is monocistronic

Triosephosphate isomerase (EC 5.3.1.1) from Lactococcus lactis was purified to electrophoretic homogeneity. Approximately 3 mg purified enzyme (specific activity 3300 U mg-1) was obtained from 70 g (wet wt) cells. In solution, triosephosphate isomerase (pI 4.0-4.4) was observed to exist as a homodimer (M(r) 57,000) of noncovalently linked subunits. The sequence of the first 37 amino acid residues from the NH2-terminus were determined by step-wise Edman degradation. This sequence, and that of a region conserved in all known bacterial triosephosphate isomerases, was used to design oligonucleotide primers for the synthesis of a lactococcal tpi probe by PCR. The probe was used to isolate a molecular clone of tpi from a lambda GEM11 library of L. lactis LM0230 DNA. The nucleotide sequence of tpi predicted a protein of 252 amino acids with the same NH2-terminal sequence as that determined for the purified enzyme and a subunit M(r) of 26,802 after removal of the NH2-terminal methionine. Escherichia coli cells harbouring a plasmid containing tpi had 15-fold higher triosephosphate isomerase activity than isogenic plasmid-free cells, confirming the identity of the cloned gene. Northern analysis of L. lactis LM0230 RNA showed that a 900 base transcript hybridized with tpi. The 5' end of the transcript was determined by primer extension analysis to be a G located 65 bp upstream from the tpi start codon. These transcript analyses indicated that in L. lactis, tpi is expressed on a monocistronic transcript. Nucleotide sequencing indicated that the DNA adjacent to tpi did not encode another Embden-Meyerhoff-Parnas pathway enzyme. The location of tpi on the L. lactis DL11 chromosome map was determined to be between map coordinates 1.818 and 1.978.

Crystal structure of the mutant yeast triosephosphate isomerase in which the catalytic base glutamic acid 165 is changed to aspartic acid

The three-dimensional structure of the E165D mutant of the glycolytic enzyme yeast triosephosphate isomerase has been determined by X-ray diffraction at a nominal resolution of 2 A. For crystallization, the mutant enzyme was complexed with the tight-binding intermediate analog, phosphoglycolohydroxamate. Comparison with the structure of the wild-type enzyme reveals that, as originally intended, replacement of the catalytic base Glu-165 with the shorter side chain of aspartic acid has increased the distance between the base and the intermediate analog by 1 A. In addition, the catalytic base is oriented in the E165D structure so as to use the anti orbital of the carboxylate for proton abstraction; in the structure of the wild-type enzyme, the syn orbital is oriented for this purpose. It has been hypothesized that the 1000-fold loss in catalytic activity for this mutant triosephosphate isomerase is due either to the use of the less basic anti orbital for proton transfer or to the greater distance between the base and the substrate. The structure of yeast E165D triosephosphate isomerase suggests that both distance and orientation factors contribute to the loss of activity in the mutant enzyme and, therefore, that both factors contribute to the catalytic efficiency of the wild-type enzyme.

Cloning of the triosephosphate isomerase gene of Plasmodium falciparum and expression in Escherichia coli

A major supply of energy in the rapidly multiplying intraerythrocytic Plasmodium falciparum is from the glycolytic pathway. We have isolated the cDNA and genomic clones of the glycolytic enzyme, triosephosphate isomerase (TPI) by polymerase chain reaction (PCR). Degenerate oligonucleotides obtained by reverse translation of conserved polypeptide sequences derived from TPIs of other organisms, were used to prime PCR on P. falciparum DNA. The P. falciparum TPI gene is interrupted by a single intron which divides the coding region into two exons. The coding region encodes a protein of 248 amino acids which is of the same size as TPIs from other organisms and shares 42-45% homology with other known eukaryotic TPIs. On comparison with human TPI the catalytic domain was found to be highly conserved, while significant variations occurred at the other regions in the protein sequence. The P. falciparum TPI gene was cloned into the expression vector pTrc99A and hyperexpressed as an unfused protein in Escherichia coli. The 28-kDa protein was shown to be catalytically active.

Evidence to implicate translation by ribosomes in the mechanism by which nonsense codons reduce the nuclear level of human triosephosphate isomerase mRNA

The abundance of the mRNA for human triosephosphate isomerase (TPI) is decreased to 20-30% of normal by frameshift and nonsense mutations that prematurely terminate translation within the first three-quarters of the reading frame. The decrease has been shown to be attributable to a reduced level of TPI mRNA that copurifies with nuclei. Given that the translational reading frame of an mRNA is assessed in the cytoplasm during protein synthesis, cytoplasmic and nuclear RNA processes may be linked. Alternatively, a nuclear mechanism may exist whereby in-frame nonsense codons can be identified. To differentiate between these two possibilities, two distinct modulators of protein synthesis have been tested for the ability to influence the nonsense-codon-mediated reduction in the mRNA level. (i) A suppressor tRNA, which acts in trans to suppress an amber nonsense codon within TPI mRNA, and (ii) a hairpin structure in the 5' untranslated region of TPI mRNA, which acts exclusively in cis to inhibit initiation of TPI mRNA translation, were found, individually, and to a greater extent, together, to abrogate the decrease in mRNA. These results show that tRNA and ribosomes coordinately mediate the effect of a nonsense codon on the level of newly synthesized TPI mRNA. We suggest that the premature termination of TPI mRNA translation in the cytoplasm can reduce the level of TPI mRNA that fractionates with nuclei.

Identification, sequence analysis, and expression of a Corynebacterium glutamicum gene cluster encoding the three glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase, and triosephosphate isomerase

To investigate a possible chromosomal clustering of glycolytic enzyme genes in Corynebacterium glutamicum, a 6.4-kb DNA fragment located 5' adjacent to the structural phosphoenolpyruvate carboxylase (PEPCx) gene ppc was isolated. Sequence analysis of the ppc-proximal part of this fragment identified a cluster of three glycolytic genes, namely, the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene gap, the 3-phosphoglycerate kinase (PGK) gene pgk, and the triosephosphate isomerase (TPI) gene tpi. The four genes are organized in the order gap-pgk-tpi-ppc and are separated by 215 bp (gap and pgk), 78 bp (pgk and tpi), and 185 bp (tpi and ppc). The predicted gene product of gap consists of 336 amino acids (M(r) of 36,204), that of pgk consists of 403 amino acids (M(r) of 42,654), and that of tpi consists of 259 amino acids (M(r) of 27,198). The amino acid sequences of the three enzymes show up to 62% (GAPDH), 48% (PGK), and 44% (TPI) identity in comparison with respective enzymes from other organisms. The gap, pgk, tpi, and ppc genes were cloned into the C. glutamicum-Escherichia coli shuttle vector pEK0 and introduced into C. glutamicum. Relative to the wild type, the recombinant strains showed up to 20-fold-higher specific activities of the respective enzymes. On the basis of codon usage analysis of gap, pgk, tpi, and previously sequenced genes from C. glutamicum, a codon preference profile for this organism which differs significantly from those of E. coli and Bacillus subtilis is presented.

Illicit secretion of a cytoplasmic protein into the periplasm of Escherichia coli requires a signal peptide plus a portion of the cognate secreted protein. Demarcation of the critical region of the mature protein

The beta-lactamase signal peptide alone is not sufficient to direct secretion of chicken muscle triosephosphate isomerase, a normally cytoplasmic protein, into the periplasm of Escherichia coli. The signal peptide and at least the first 3 residues of the mature beta-lactamase are required before any secretion of the isomerase can be observed. At this point the level of secretion is very low, but the addition of further residues of the mature beta-lactamase enhances the secretion of the hybrid protein. The maximum level of secretion is achieved when 12 or more residues of the mature beta-lactamase intervene between the signal peptide and the isomerase. It is the proximity of an arginine residue at position 3 of the isomerase that is responsible for the blockade to secretion of these hybrid proteins (see Summers, R.G., Harris, C.R., and Knowles, J.R. (1989) J. Biol. Chem. 264, 20082-20088). With 12 residues of the mature beta-lactamase between the signal peptide and the isomerase, the offending arginine now lies at position 15 of the hybrid. The 14 residues that immediately follow the signal peptide therefore define a region of constrained properties that is critical to the secretability of proteins from E. coli.

A conservative amino acid substitution, arginine for lysine, abolishes export of a hybrid protein in Escherichia coli. Implications for the mechanism of protein secretion

A hybrid protein that comprises the beta-lactamase signal peptide fused precisely to chicken muscle triosephosphate isomerase is not secreted into the periplasm of Escherichia coli. The protein can be secreted, however, if an arginine residue at position 3 of the isomerase is replaced by either a serine or a proline residue. In contrast, replacement of a neighboring lysine residue has no effect on secretion of the protein. Furthermore, if the arginine is removed from position 3 to generate a secreted protein, but is then reintroduced in place of the neighboring lysine, the blockade to secretion is re-established. The singular effect of the arginine residue on secretion does not result from the role this residue plays in the formation or stabilization of the native isomerase structure: mutational alterations remote from the N terminus of the isomerase that prevent the proper folding of the protein do not relieve the block to secretion. The finding that an arginine residue prevents secretion while a lysine residue does not, suggests that basic residues near the mature N terminus of a secreted protein must be deprotonated if orderly export is to occur. This implies that the signal peptide along with the N-terminal portion of the mature protein partitions directly into the lipid bilayer in the course of the secretory process.

Hominoid triosephosphate isomerase: characterization of the major cell proliferation specific isozyme

Proliferating cells derived from hominoid species contain electrophoretically separable forms of triosephosphate isomerase (TPI), including a constitutive isozyme and major and minor cell proliferation specific isozymes. Genetic studies have shown that the constitutive and inducible isozymes are products of the same structural gene. A procedure has been developed for the rapid isolation of the constitutive and major proliferation specific TPI isozymes from human lymphoblastoid B cells. [35S]methionine labeled isozymes were purified through several steps of polyacrylamide gel electrophoresis in sufficient quantities for turnover studies and preliminary structural analysis. The intact isozymes were subjected to 23 steps of automated Edman degradation; both preparations yield a [35S] PTH-methionine only at cycle 14, as expected if the protein is TPI. Neither isozyme contains a blocked NH2-terminus and length heterogeneity at the amino terminal does not exist. A comparison of the two purified isozymes on 2-D PAGE confirms that the constitutive isozyme consists of only type 1 subunits while the major proliferation specific isozyme is composed of a type 1 subunit and a unique type 2 subunit. The type 1 and type 2 subunits differ by at least four charge units under native, nondenaturing conditions of electrophoresis but do not differ in molecular mass. The difference between the type 1 and type 2 subunits is covalent, as the difference in isoelectric point between the two subunits is stable to both 2% SDS and 8 M urea. The expression of TPI-2 does not correlate with the existence of the labile asparagine residues. Turnover studies indicate that the level of each subunit is regulated by differences in rates of synthesis rather than degradation but a precursor-product relationship between the subunits was not observed. Thus the mechanism for synthesis of TPI-2 must operate either during mRNA processing or nascent peptide synthesis and then only in cells from hominoid species.

Human glyceraldehyde-3-phosphate dehydrogenase in man-rodent somatic cell hybrids

The human enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) forms heteropolymers with the rodent enzyme in man-rodent somatic cell hybrids. A gene specifying GAPDH is syntenic with the genes specifying the glycolytic enzymes triosephosphate isomerase (TPI) and lactate dehydrogenase B (LDH-B). The synteny of GAPDH, TPI, and LDH-B is the first evidence for the syntenic association of human genes that specify enzymes of the Embden-Meyerhof glycolytic pathway.