Folding in vitro and transport in vivo of pre-β-lactamase are SecB independent

The rate of folding of the precursor of beta-lactamase is not influenced by the presence of SecB under conditions in which GroEL/ES retards the folding. Wild-type beta-lactamase and several mutants in the signal or the mature protein, affecting either transport or enzyme kinetics and probably folding, were examined for total expression, total enzymatic activity, and transported beta-lactamase (in vivo resistance) in secB- and secB+ strains. We conclude that there is no indication of any relevant interaction between SecB and pre-beta-lactamase in vitro, nor did the secB- mutation affect the transport of wild-type beta-lactamase or any of the mutant in vivo. Thus, putative Escherichia coli "folding modulators' must be of limited specificity.     

A Pro/Ser substitution in nucleoside diphosphate kinase of Drosophila melanogaster (mutation killer of prune) affects stability but not catalytic efficiency of the enzyme.

Nucleoside diphosphate kinase of Drosophila, recently identified as the product of the awd gene, is essential for larval development. The conditional lethal mutation Killer of prune maps to the same gene. We purified the nucleoside diphosphate kinases from wild-type and mutant larvae by a simple procedure involving affinity chromatography on blue Sepharose. Both proteins are purified as hexamers in their native state. The mutant protein, which carries a serine instead of proline at position 97, has structural properties and catalytic efficiency that are very similar to the wild-type protein. However, the mutant protein has a much lower stability to denaturation by heat and urea. Following dilution of urea with buffer the urea-denaturated mutant nucleoside diphosphate kinase accumulates as folded monomers and cannot recover its quaternary structure and enzymatic activity. In contrast, the wild-type enzyme recovers hexameric structure and activity. This suggests that the mutation affects the folding/assembly pathway without affecting the function of the mature protein once folded and assembled into the mature hexameric structure.     

Deletions in the N-terminal segment of the plasma membrane Ca(2+) pump impair the expression of a correctly folded functional enzyme

Mutant cDNAs encoding h4 plasma membrane Ca(2+) pumps with deletions in the N-terminal segment have been constructed and expressed in COS cells. As judged by immunoblotting, each construct was expressed at a high level similar to that of the wild-type enzyme. The removal of the first six amino acids had no effect on the Ca(2+) transport activity, but deletions in the segment 15-75 reduced the activity to undetectable levels. The d(43-56)h4 mutant, lacking amino acids 43-56, was also efficiently expressed in stable form in CHO cells. The Ca(2+) transport activity of d(43-56)h4 in this system was about 40% of that of the wild type. The d(43-56)h4 enzyme exhibited a similar affinity for Ca(2+), a slightly increased apparent affinity for ATP, and a slightly lower sensitivity to inhibition by vanadate than the wild-type enzyme. Analysis of the phosphoenzyme intermediate formed in the presence of lanthanum showed that the phosphorylation reaction was not affected, but the maximum amount of phosphoenzyme was reduced to the same extent as the Ca(2+) transport activity. These results suggest that the expressed d(43-56)h4 was a mixture of fully active and inactive enzyme. The d(43-56)h4 enzyme was more easily degraded by proteases and had a higher sensitivity to heat inactivation than the wild type suggesting that the loss of function was due to the improper folding and instability of the mutant protein. On the basis of these findings, it appears that the N-terminal segment of the plasma membrane Ca(2+) pump is neither essential for synthesis nor for catalytic activity but is critical for the expression of a correctly folded functional enzyme.     

Human lysosomal protective protein has cathepsin A-like activity distinct from its protective function

The protective protein was first discovered because of its deficiency in the metabolic storage disorder galactosialidosis. It associates with lysosomal beta-galactosidase and neuraminidase, toward which it exerts a protective function necessary for their stability and activity. Human and mouse protective proteins are homologous to yeast and plant serine carboxypeptidases. Here, we provide evidence that this protein has enzymatic activity similar to that of lysosomal cathepsin A: 1) overexpression of human and mouse protective proteins in COS-1 cells induces a 3-4-fold increase of cathepsin A-like activity; 2) this activity is reduced to approximately 1% in three galactosialidosis patients with different clinical phenotypes; 3) monospecific antibodies raised against human protective protein precipitate virtually all cathepsin A-like activity in normal human fibroblast extracts. Mutagenesis of the serine and histidine active site residues abolishes the enzymatic activity of the respective mutant protective proteins. These mutants, however, behave as the wild-type protein with regard to intracellular routing, processing, and secretion. In contrast, modification of the very conserved Cys60 residue interferes with the correct folding of the precursor polypeptide and, hence, its intracellular transport and processing. The secreted active site mutant precursors, endocytosed by galactosialidosis fibroblasts, restore beta-galactosidase and neuraminidase activities as effectively as wild-type protective protein. These findings indicate that the catalytic activity and protective function of the protective protein are distinct.     

Proteolytic activity of novel human immunodeficiency virus type 1 proteinase proteins from a precursor with a blocking mutation at the N terminus of the PR domain.

The mature human immunodeficiency virus type 1 proteinase (PR; 11 kDa) can cleave all interdomain junctions in the Gag and Gag-Pol polyprotein precursors. To determine the activity of the enzyme in its precursor form, we blocked release of mature PR from a truncated Gag-Pol polyprotein by introducing mutations into the N-terminal Phe-Pro cleavage site of the PR domain. The mutant precursor autoprocessed efficiently upon expression in Escherichia coli. No detectable mature PR was released; however, several PR-related products ranging in size from approximately 14 to 18 kDa accumulated. Products of the same size were generated when mutant precursors were digested with wild-type PR. Thus, PR can utilize cleavage sites in the region upstream of the PR domain, resulting in the formation of extended PR species. On the basis of active-site titration, the PR species generated from mutated precursor exhibited wild-type activity on peptide substrates. However, the proteolytic activity of these extended enzymes on polyprotein substrates provided exogenously was low when equimolar amounts of extended and wild-type PR proteins were compared. Mammalian cells expressing the mutated precursor produced predominantly precursor and considerably reduced amounts of mature products. Released particles consisted mostly of uncleaved or partially cleaved polyproteins. Our results suggest that precursor forms of PR can autoprocess but are less efficient in processing of the Gag precursor for formation of mature virus particles.     

Folding regulates autoprocessing of HIV-1 protease precursor

Autoprocessing of HIV-1 protease (PR) precursors is a crucial step in the generation of the mature protease. Very little is known regarding the molecular mechanism and regulation of this important process in the viral life cycle. In this context we report here the first and complete residue level investigations on the structural and folding characteristics of the 17-kDa precursor TFR-PR-C(nn) (161 residues) of HIV-1 protease. The precursor shows autoprocessing activity indicating that the solution has a certain population of the folded active dimer. Removal of the 5-residue extension, C(nn) at the C-terminal of PR enhanced the activity to some extent. However, NMR structural characterization of the precursor containing a mutation, D25N in the PR at pH 5.2 and 32 degrees C under different conditions of partial and complete denaturation by urea, indicate that the precursor has a high tendency to be unfolded. The major population in the ensemble displays some weak folding propensities in both the TFR and the PR regions, and many of these in the PR region are the non-native type. As both D25N mutant and wild-type PR are known to fold efficiently to the same native dimeric form, we infer that TFR cleavage enables removal of the non-native type of preferences in the PR domain to cause constructive folding of the protein. These results indicate that intrinsic structural and folding preferences in the precursor would have important regulatory roles in the autoprocessing reaction and generation of the mature enzyme.     

Precursor of beta-lactamase is enzymatically inactive. Accumulation of the preprotein in Saccharomyces cerevisiae

Synthesis and properties of the bacterial precursor of beta-lactamase (E.C.3.5.2.6) were studied in Saccharomyces cerevisiae transformants. A protease-deficient yeast mutant was transformed with the plasmid pADH040-2 conferring high expression of the bla gene. Besides precisely processed beta-lactamase, transformed yeast cells contained mainly bla precursor up to the amount of 2% of total cellular protein. The precursor was shown to be synthesized on free polysomes in vivo but could be processed with rough microsomal membranes in a cell-free translation system. By applying an isolation procedure using high-salt conditions, the labile precursor could be separated in a native form from the mature beta-lactamase. Thereby it could be shown that the pre-beta-lactamase had virtually no enzymatic activity in contrast to the mature enzyme, which was indistinguishable from bacterial beta-lactamase. Furthermore, the precursor was highly susceptible to proteolytic degradation by trypsin under conditions which did not affect the mature enzyme. Accordingly, the protein conformation of the precursor must be substantially different from that of the authentic beta-lactamase, demonstrating that specific processing and transport of beta-lactamase is associated with directing the protein to a distinct conformation.     

The folding properties of the Escherichia coli maltose-binding protein influence its interaction with SecB in vitro.

It has been proposed that the cytoplasmic SecB protein functions as a component of the Escherichia coli protein export machinery by serving as an antifolding factor that retards folding of the precursor maltose-binding protein (preMBP) into a translocation-incompetent form. In this study, it was found that SecB directly interacts with wild-type preMBP and various mutationally altered MBP species synthesized in vitro to form a SecB-MBP complex that can be precipitated with anti-SecB serum. The association of SecB with wild-type preMBP was relatively unstable; such a complex was formed only when SecB was present cotranslationally or after denaturation of previously synthesized preMBP and was detected with only low efficiency. In marked contrast, MBP species that were defective in the ability to assume the stable conformation of wild-type preMBP or that exhibited significantly slower folding kinetics formed much more stable complexes with SecB. In one case, we demonstrated that SecB did not need to be present cotranslationally for complex formation to occur. Formation of a complex between SecB and MBP was clearly not dependent on the MBP signal peptide. However, we were unable to detect complex formation between SecB and MBP lacking virtually the entire signal peptide but having a completely intact mature moiety. This MBP species folded at a rate considerably faster than that of wild-type preMBP. The propensity of this mutant protein to assume the native conformation of mature MBP apparently precludes a stable association with SecB, whereas an MBP species lacking a signal peptide but exhibiting altered folding properties did form a complex with SecB that could be precipitated with anti-SecB serum.     

Active-site mutants of beta-lactamase: use of an inactive double mutant to study requirements for catalysis

We have studied the catalytic activity and some other properties of mutants of Escherichia coli plasmid-encoded RTEM beta-lactamase (EC 3.5.2.6) with all combinations of serine and threonine residues at the active-site positions 70 and 71. (All natural beta-lactamases have conserved serine-70 and threonine-71.) From the inactive double mutant Ser-70----Thr, Thr-71----Ser [Dalbadie-McFarland, G., Cohen, L. W., Riggs, A. D., Morin, C., Itakura, K., & Richards, J. H. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 6409-6413], an active revertant, Thr-71----Ser (i.e., residue 70 in the double mutant had changed from threonine to the serine conserved at position 70 in the wild-type enzyme), was isolated by an approach that allows identification of active revertants in the absence of a background of wild-type enzyme. This mutant (Thr-71----Ser) has about 15% of the catalytic activity of wild-type beta-lactamase. The other possible mutant involving serine and threonine residues at positions 70 and 71 (Ser-70----Thr) shows no catalytic activity. The primary nucleophiles of a serine or a cysteine residue [Sigal, I. S., Harwood, B. G., & Arentzen, R. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 7157-7160] at position 70 thus seem essential for enzymatic activity. Compared to wild-type enzyme, all three mutants show significantly reduced resistance to proteolysis; for the active revertant (Thr-71----Ser), we have also observed reduced thermal stability and reduced resistance to denaturation by urea.     

Biogenesis of lysosomal enzymes in the alpha-glucosidase II-deficient modA mutant of Dictyostelium discoideum: retention of alpha-1,3-linked glucose on N-linked oligosaccharides delays intracellular transport but does not alter sorting of alpha-mannosidase or beta-glucosidase

The endoplasmic reticulum-localized enzyme alpha-glucosidase II is responsible for removing the two alpha-1,3-linked glucose residues from N-linked oligosaccharides of glycoproteins. This activity is missing in the modA mutant strain, M31, of Dictyostelium discoideum. Results from both radiolabeled pulse-chase and subcellular fractionation experiments indicate that this deficiency did not prevent intracellular transport and proteolytic processing of the lysosomal enzymes, alpha-mannosidase and beta-glucosidase. However, the rate at which the glucosylated precursors left the rough endoplasmic reticulum was several-fold slower than the rate at which the wild-type precursors left this compartment. Retention of glucose residues did not disrupt the binding of the precursor forms of the enzymes with intracellular membranes, indicating that the delay in movement of proteins from the ER did not result from lack of association with membranes. However, the mutant alpha-mannosidase precursor contained more trypsin-sensitive sites than did the wild-type precursor, suggesting that improper folding of precursor molecules might account for the slow rate of transport to the Golgi complex. Percoll density gradient fractionation of extracts prepared from M31 cells indicated that the proteolytically processed mature forms of alpha-mannosidase and beta-glucosidase were localized to lysosomes. Finally, the mutation in M31 may have other, more dramatic, effects on the lysosomal system since two enzymes, N-acetylglucosaminidase and acid phosphatase, were secreted much less efficiently from lysosomal compartments by the mutant strain.     

Differential effects of mutations in three domains on folding, quaternary structure, and intracellular transport of vesicular stomatitis virus G protein

The vesicular stomatitis virus glycoprotein (G protein) is an integral membrane protein which assembles into noncovalently associated trimers before transport from the endoplasmic reticulum. In this study we have examined the folding and oligomeric assembly of twelve mutant G proteins with alterations in the cytoplasmic, transmembrane, or ectodomains. Through the use of conformation-specific antibodies, we found that newly synthesized G protein folded into a conformation similar to the mature form within 1-3 min of synthesis and before trimer formation. Mutant proteins not capable of undergoing correct initial folding did not trimerize, were not transported, and were found in large aggregates. They had, as a rule, mutations in the ectodomain, including several with altered glycosylation patterns. In contrast, mutations in the cytoplasmic domain generally had little effect on folding and trimerization. These mutant proteins, whose ectodomains were identical to the wild-type by several assays, were either transported to the cell surface slowly or not at all. We concluded that while correct ectodomain folding and trimer formation are prerequisites for transport, they alone are not sufficient. The results suggest that the cytoplasmic domain of the wild-type protein may facilitate rapid, efficient transport from the ER, which can be easily affected or eliminated by tail mutations that do not detectably affect the ectodomain.     

Analysis of the effect of potato carboxypeptidase inhibitor pro-sequence on the folding of the mature protein.

Protein folding can be modulated in vivo by many factors. While chaperones act as folding catalysts and show broad substrate specificity, some pro-peptides specifically facilitate the folding of the mature protein to which they are bound. Potato carboxypeptidase inhibitor (PCI), a 39-residue protein carboxypeptidase inhibitor, is synthesized in vivo as a precursor protein that includes a 27-residue N-terminal and a seven-residue C-terminal pro-regions. In this work the disulfide-coupled folding of mature PCI in vitro has been compared with that of the same protein extended with either the N-terminal pro-sequence (ProNtPCI) or both N- and C-terminal pro-sequences (ProPCI), and also with the N-terminal pro-sequence in trans (ProNt + PCI). No significant differences can be observed in the folding kinetics or efficiencies of all these molecules. In addition, in vivo folding studies in Escherichia coli have been performed using wild-type PCI and three PCI mutant forms with and without the N-terminal pro-sequence, the mutations had been previously reported to affect folding of the PCI mature form. The extent to which the 'native-like' form was secreted to the media by each construction was not affected by the presence of the N-terminal pro-sequence. These results indicate that PCI does not depend on the N-terminal pro-sequence for its folding in both, in vitro and in vivo in E. coli. However, structural analysis by spectroscopy, hydrogen exchange and limited proteolysis by mass spectrometry, indicate the capability of such N-terminal pro-sequence to fold within the precursor form.     

Biogenesis of outer membrane protein PhoE of Escherichia coli. Evidence for multiple SecB-binding sites in the mature portion of the PhoE protein.

Efficient in vivo translocation of the precursor of Escherichia coli outer membrane protein PhoE across the inner membrane is shown to depend on SecB protein. A set of mutants, carrying internal deletions in the phoE gene, was used to locate a possible SecB-binding site and/or a site that makes the protein dependent on SecB for export. Except for two small mutant PhoE proteins, the in vivo and in vitro translocation of all mutant proteins was more efficient in the presence of SecB. The interaction of SecB protein with wild-type and mutant PhoE proteins, synthesized in vitro, was further studied in co-immunoprecipitation experiments with anti-SecB protein serum. The efficiencies of co-immunoprecipitation of precursor and mature PhoE were very similar, indicating the absence of a SecB-binding site in the signal sequence. Moreover, all mutant proteins with deletions in the mature moiety of the PhoE protein were co-immunoprecipitated in these assays, albeit mostly with reduced efficiency. Taken together, these results indicate the existence of multiple SecB-binding sites in the mature portion of the PhoE protein.     

Identification of a glutamic acid and an aspartic acid residue essential for catalytic activity of aspergillopepsin II, a non-pepsin type acid proteinase

Aspergillopepsin II from Aspergillus niger var. macrosporus is a non-pepsin type or pepstatin-insensitive acid proteinase. To identify the catalytic residues of the enzyme, all acidic residues that are conserved in the homologous proteinases of family A4 were replaced with Asn, Gln, or Ala using site-directed mutagenesis. The wild-type and mutant pro-enzymes were heterologously expressed in Escherichia coli and refolded in vitro. The wild-type pro-enzyme was shown to be processed into a two-chain active enzyme under acidic conditions. Most of the recombinant mutant pro-enzymes showed significant activity under acidic conditions because of autocatalytic activation except for the D123N, D123A, E219Q, and E219A mutants. The D123A, E219Q, and E219A mutants showed neither enzymatic activity nor autoprocessing activity under acidic conditions. The circular dichroism spectra of the mutant pro- and mature enzymes were essentially the same as those of the wild-type pro- and mature enzyme, respectively, indicating that the mutant pro-enzymes were correctly folded. In addition, two single and one double mutant pro-enzyme, D123E, E219D, and D123E/E219D, did not show enzymatic activity under acidic conditions. Taken together, Glu-219 and Asp-123 are deduced to be the catalytic residues of aspergillopepsin II.     

Ornithine transcarbamylase in liver mitochondria

Summary Ornithine transcarbamylase (ornithine carbamoyltransferase, EC 2.1.3.3), the second enzyme of urea synthesis, is localized in the matrix of liver mitochondria of ureotelic animals. The enzyme is encoded by a nuclear gene, synthesized outside the mitochondria, and must then be transported into the organelle. The rat liver enzyme is initially synthesized on membrane-free polysomes in the form of a larger precursor with an amino-terminal extension of 3 400–4 000 daltons. In rat liver slices and isolated rat hepatocytes, the pulse-labeled precursor is first released into the cytosol and is then transported with a half life of 1 2 min into the mitochondria where it is proteolytically processed to the mature form of the enzyme. The precursor synthesized in vitro exists in a highly aggregated form and has a conformation different from that of the mature enzyme. The precursor has an isoelectric point (pI = 7.9) higher than that of the mature enzyme (pI = 7.2).The precursor synthesized in vitro can be taken up and processed to the mature enzyme by isolated rat liver mitochondria. The mitochondrial transport and processing system requires membrane potential and a high integrity of the mitochondria. The transport and processing activities are conserved between mammals and birds or amphibians and is presumably common to more than one precursor. Potassium ion, magnesium ion, and probably a cytosolic protein(s), in addition to the transcarbamylase precursor and the mitochondria, are required for the maximal transport and processing of the precursor.A mitochondrial matrix protease which converts the precursor to a product intermediate in size between the precursor and the mature subunit has been highly purified. The protease has an estimated molecular weight of 108 000 and an optimal pH of 7.5–8.0, and appears to be a metal protease. The protease does not cleave several of the protein and peptide substrates tested. The role of this protease in the precursor processing remains to be elucidated.Rats subjected to different levels of protein intake and to fasting show significant changes in the level of enzyme protein and activity of ornithine transcarbamylase. The dietary-dependent changes in the enzyme level are due mainly to an altered level of functional mRNA for the enzyme. In contrast, during fasting, the increase in the enzyme level is associated with a decreased level of translatable mRNA forthe enzyme.Pathological aspects of ornithine transcarbamylase including the enzyme deficiency and reduced activities of the enzyme in Reye's syndrome are also described. A possibility that impaired transport of the enzyme precursor into the mitochondria leads to a reduced enzyme activity, is proposed.Abbreviation pOTC precursor of ornithine transcarbamylase     

Improvement of expression level of keratinase Sfp2 from Streptomyces fradiae by site-directed mutagenesis of its N-terminal pro-sequence

The keratinase Sfp2, produced by Streptomyces fradiae var. k11, is a serine alkaline protease first synthesized as pre-pro-mature precursor, of which the N-terminal propeptide must be autocatalytically cleaved on the C-terminal of P1 amino acid to produce mature enzyme. Single amino acid substitutions were introduced at positions ?1 and ?2 to improve the expression level of mature Sfp2. The specific activity of L(?1)F mutant (48935 U/mg) was nine times that of wild-type Sfp2, whereas the mutants L(?1)D, L(?1)G, L(?1)H, K(?2)E, and K(?2)L had 2–52 % of the specific activity of wild-type. The yield of mature Sfp2 of L(?1)F mutant was estimated to be 800 μg/mg total protein and 112 mg/l culture supernatant, nine and twice that of wild-type, respectively. The L(?1)F mutant exhibited similar enzymatic properties to wild-type.     

Protein phosphorylation corrects the folding defect of the neuroblastoma (S120G) mutant of human nucleoside diphosphate kinase A/Nm23-H1

Human nucleoside diphosphate (NDP) kinase A is a 'house-keeping' enzyme essential for the synthesis of nonadenine nucleoside (and deoxynucleoside) 5'-triphosphate. It is involved in complex cellular regulatory functions including the control of metastatic tumour dissemination. The mutation S120G has been identified in high-grade neuroblastomas. We have shown previously that this mutant has a folding defect: the urea-denatured protein could not refold in vitro. A molten globule folding intermediate accumulated, whereas the wild-type protein folded and associated into active hexamers. In the present study, we report that autophosphorylation of the protein corrected the folding defect. The phosphorylated S120G mutant NDP kinase, either autophosphorylated with ATP as donor, or chemically prosphorylated by phosphoramidate, refolded and associated quickly with high yield. Nucleotide binding had only a small effect. ADP and the non-hydrolysable ATP analogue 5'-adenyly-limido-diphosphate did not promote refolding. ATP-promoted refolding was strongly inhibited by ADP, indicating protein dephosphorylation. Our findings explain why the mutant enzyme is produced in mammalian cells and in Escherichia coli in a soluble form and is active, despite the folding defect of the S120G mutant observed in vitro. We generated an inactive mutant kinase by replacing the essential active-site histidine residue at position 118 with an asparagine residue, which abrogates the autophosphorylation. The double mutant H118N/S120G was expressed in inclusion bodies in E. coli. Its renaturation stops at a folding intermediate and cannot be reactivated by ATP in vitro. The transfection of cells with this double mutant might be a good model to study the cellular effects of folding intermediates.     

Glycosidase active site mutations in human alpha-L-iduronidase

Mucopolysaccharidosis type I (MPS I; McKusick 25280) results from a deficiency in alpha-L-iduronidase activity. Using a bioinformatics approach, we have previously predicted the putative acid/base catalyst and nucleophile residues in the active site of this human lysosomal glycosidase to be Glu182 and Glu299, respectively. To obtain experimental evidence supporting these predictions, wild-type alpha-L-iduronidase and site-directed mutants E182A and E299A were individually expressed in Chinese hamster ovary-K1 cell lines. We have compared the synthesis, processing, and catalytic properties of the two mutant proteins with wild-type human alpha-L-iduronidase. Both E182A and E299A transfected cells produced catalytically inactive human alpha-L-iduronidase protein at levels comparable to the wild-type control. The E182A protein was synthesized, processed, targeted to the lysosome, and secreted in a similar fashion to wild-type alpha-L-iduronidase. The E299A mutant protein was also synthesized and secreted similarly to the wild-type enzyme, but there were alterations in its rate of traffic and proteolytic processing. These data indicate that the enzymatic inactivity of the E182A and E299A mutants is not due to problems of synthesis/folding, but to the removal of key catalytic residues. In addition, we have identified a MPS I patient with an E182K mutant allele. The E182K mutant protein was expressed in CHO-K1 cells and also found to be enzymatically inactive. Together, these results support the predicted role of E182 and E299 in the catalytic mechanism of alpha-L-iduronidase and we propose that the mutation of either of these residues would contribute to a very severe clinical phenotype in a MPS I patient.     

Spectral and enzymatic properties of human recombinant myeloperoxidase: comparison with the mature enzyme.

Human recombinant myeloperoxidase (recMPO), purified from an engineered Chinese hamster ovary (CHO) cell line, has been characterized and compared to the mature enzyme isolated from polymorphonuclear leukocytes. Both molecules appear essentially similar in physicochemical enzymatic terms according to the following observations. 1. The unprocessed recombinant protein displays the characteristic light absorption spectra of ferric mature MPO and exhibits its typical spectral changes in the presence of dithionite or hydrogen peroxide. 2. The addition of 14C-labeled 5-aminolevulinic acid, a heme precursor, to the culture medium of recombinant CHO cells yields labeled recMPO, indicating the presence of a heme-like structure in the molecule. 3. Like mature MPO, recMPO has a peroxidatic activity and catalyzes the oxidation of chloride ions in the presence of hydrogen peroxide, producing hypochlorous acid as measured by the monochlorodimedon assay. For both enzymes, the chlorinating activity optimally occurs around pH 5.0 at about 100 microM of hydrogen peroxide and is strongly inhibited by methimazole. 4. Diethylpyrocarbonate significantly reduces the enzymatic activity of both molecules, suggesting that histidine residues may be of prime importance in the active site of the enzymes. 5. According to infrared spectroscopy data, both enzymes present a very similar secondary structure organization. In conclusion, the data suggest that the processing of the precursor enzyme (recMPO) into the mature form occurs without major structural and functional consequences.     

The disulfide bond in the Aeromonas hydrophila lipase/acyltransferase stabilizes the structure but is not required for secretion or activity.

Vibrio and Aeromonas spp. secrete an unusual 35-kDa lipase that shares several properties with mammalian lecithin-cholesterol acyltransferase. The Aeromonas hydrophila lipase contains two cysteine residues that form an intramolecular disulfide bridge. Here we show that changing either of the cysteines to serine does not reduce enzymatic activity, indicating that the disulfide bond is not required for correct folding. However, when either of the cysteines is replaced, the enzyme is more readily denatured by urea and more sensitive to degradation by trypsin than is the wild-type enzyme, evidence that the bridge has an important role in stabilizing the protein's structure. The two mutant proteins with serine-for-cysteine replacements were secreted by Aeromonas salmonicida containing the cloned genes, although the levels of both in the culture supernatants were lower than the level of the wild-type enzyme. When the general secretory pathway was blocked with carbonyl cyanide chlorophenylhydrazone, the cell-associated pools of the mutant enzymes appeared to be degraded, whereas the wild-type pool remained stable. We conclude that reduced extracellular levels of the mutant proteins are the result of their increased sensitivities to proteases encountered inside the cell during export.