Regional Distribution of Human Trypsinogen 4 in Human Brain at mRNA and Protein Level

Gene PRSS3 on chromosome 9 of the human genome encodes, due to alternative splicing, both mesotrypsinogen and trypsinogen 4. Mesotrypsinogen has long been known as a minor component of trypsinogens expressed in human pancreas, while the mRNA for trypsinogen 4 has recently been identified in brain and other human tissues. We measured the amount of trypsinogen 4 mRNA and the quantity of the protein as well in 17 selected areas of the human brain. Our data suggest that human trypsinogen 4 is widely but unevenly distributed in the human brain. By immunohistochemistry, here we show that this protease is localized in neurons and glial cells, predominantly in astrocytes. In addition to cellular immunoreactivity, human trypsinogen 4 immunopositive dots were detected in the extracellular matrix, supporting the view that human trypsinogen 4 might be released from the cells under special conditions. Júlia Tóth and Erika Siklódi contributed equally to this work.     

Molecular cloning and expression of mouse brain sialidase.

Sialidase (EC 3.2.1.18) catalyzes the release of sialic acid from sialo-oligosaccharides, gangliosides, or sialo-glycoproteins. In this investigation, we cloned a novel cDNA for mouse brain sialidase and expressed the cDNA in COS-7 cells. This 1,699 bp cDNA codes for a 41.6 kDa protein consisting of 372 deduced amino acid residues. In COS-7 cells transiently transfected with the cDNA, a 250-fold increase was observed in specific activity toward 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid. Similarity searches of the nonredundant GenBank peptide sequence database by the PSI-BLAST program identified rat, hamster, human, and bacterial sialidases homologous to this mouse brain sialidase. Amino acid sequence identities to rat and hamster sialidases (84% and 77%, respectively) suggest that this form of sialidase is conserved in rodents. Sequence identities to human and mouse lysosomal sialidases (30% and 28%, respectively) indicate that the mouse brain sialidase is distinct from the lysosomal enzyme. Mouse brain sialidase has two amino acid sequence motifs common to bacterial sialidases: the 'F/YRIP' motif and the 'Asp-box' motif. The 'F/YRIP' motif is present near the N terminus while two 'Asp-box' motifs are present downstream.     

Transfer RNA structural change is a key element in the reassignment of the CUG codon in Candida albicans.

The human pathogenic yeast Candida albicans and a number of other Candida species translate the standard leucine CUG codon as serine. This is the latest addition to an increasing number of alterations to the standard genetic code which invalidate the theory that the code is frozen and universal. The unexpected finding that some organisms evolved alternative genetic codes raises two important questions: how have these alternative codes evolved and what evolutionary advantages could they create to allow for their selection? To address these questions in the context of serine CUG translation in C.albicans, we have searched for unique structural features in seryl-tRNA(CAG), which translates the leucine CUG codon as serine, and attempted to reconstruct the early stages of this genetic code switch in the closely related yeast species Saccharomyces cerevisiae. We show that a purine at position 33 (G33) in the C.albicans Ser-tRNA(CAG) anticodon loop, which replaces a conserved pyrimidine found in all other tRNAs, is a key structural element in the reassignment of the CUG codon from leucine to serine in that it decreases the decoding efficiency of the tRNA, thereby allowing cells to survive low level serine CUG translation. Expression of this tRNA in S.cerevisiae induces the stress response which allows cells to acquire thermotolerance. We argue that acquisition of thermotolerance may represent a positive selection for this genetic code change by allowing yeasts to adapt to sudden changes in environmental conditions and therefore colonize new ecological niches.     

A part of the transmembrane domain of pro-TNF can function as a cleavable signal sequence that generates a biologically active secretory form of TNF.

To determine the minimum requirement in the 76-residue leader sequence of pro-tumor necrosis factor (TNF) for membrane translocation across the endoplasmic reticulum (ER) and for the maturation of pro-TNF, we constructed pro-TNF mutants in which a part of the transmembrane domain of pro-TNF was directly linked to the N-terminus of the mature domain, and evaluated their translocational behavior across the ER-membrane and their secretion from the transfected cells. The in vitro translation/translocation assay involving a canine pancreatic microsomal membrane system including a mutant, Delta-75-47, -32-1, revealed that the N-terminal half of the transmembrane domain of pro-TNF consisting of 14 residues functioned as a cleavable signal sequence; it generated a cleaved form of TNF having a molecular mass similar to that of mature TNF. Analysis of the cleavage site by site-directed mutagenesis indicated that the site was inside the leader sequence of this mutant. When the mutant, Delta-75-47, -32-1, was expressed in COS-1 cells, efficient secretion of a biologically active soluble TNF was observed. Further deletion of the hydrophobic domain from this mutant inhibited the translocation, indicating that some extent of hydrophobicity is indispensable for the membrane translocation of the mature domain of TNF. Thus, the N-terminal half of the transmembrane domain of pro-TNF could function as a cleavable signal sequence when linked to the mature domain of TNF, and secretion of a biologically active secretory form of TNF could be achieved with this 14-residue hydrophobic segment. In intact pro-TNF, however, this 14-residue sequence could not function as a cleavable signal sequence during intracellular processing, indicating that the remainder of the 76-residue leader sequence of pro-TNF inhibits the signal peptide cleavage and thus enables the leader sequence to function as a type II signal-anchor sequence that generates a transmembrane form of TNF.     

Differential regulation of trypsinogen mRNA translation: full-length mRNA sequences encoding two oppositely charged trypsinogen isoenzymes in the dog pancreas.

In the absence of changes in functional mRNA levels, stimulation of the pancreas with caerulein, a peptide analog of cholecystokinin, has been previously shown to increase the synthesis of anionic but not cationic trypsinogen. To look for structure-function correlations, a high-yield, full-length cDNA library has been constructed from canine pancreatic poly(A)+ mRNA. Full-length clones coding for the two major trypsinogen isoenzyme forms have been identified by colony hybridization and verified by in vitro translation of hybrid-selected mRNA in the presence of microsomal membranes and an optimal redox potential. Disulfide-bonded translation products were separated and identified by two-dimensional isoelectric focusing-sodium dodecyl sulfate-gel electrophoresis. Nucleotide sequence analysis allowed us to deduce the amino acid sequences for the anionic and cationic forms of canine trypsinogen, which contain 232 and 231 residues, respectively (77% amino acid identity), and the 15-residue amino terminal signal sequences (53% amino acid identity) associated with the two presecretory forms. Measurements of relative and absolute mRNA levels, when related to relative protein synthesis values, indicated that the translational efficiency of anionic trypsinogen mRNA exceeded that of cationic trypsinogen mRNA by 1.5- to 2.9-fold under basal conditions. Analysis of the 5' noncoding regions of trypsinogen mRNAs revealed a striking conservation of sequence (10 of 12 bases) between dog and rat anionic trypsinogen forms. This contrasted markedly with the divergence of the 5' noncoding regions observed between dog anionic and cationic trypsinogen mRNAs.     

Direct expression of recombinant activated human protein C, a serine protease

Human protein C, like other serine proteases, is normally secreted as an inactive zymogen. It is converted to its active form extracellularly by limited proteolysis with the thrombin-thrombomodulin complex. This activation results from the removal of a 12-residue activation peptide from the NH2 terminus of the heavy (COOH-terminal) chain. We report here a successful strategy for the activation of human protein C during post-translational cellular processing, resulting in the secretion of activated protein C from transfected mammalian cells. Deletion of the nucleotides encoding the activation peptide resulted in the expression of a protease with less than 5% of the expected activity. However, the replacement of the activation peptide with an 8-residue sequence (Pro-Arg-Pro-Ser-Arg-Lys-Arg-Arg) involved in the proteolytic processing of the human insulin receptor precursor resulted in the direct expression of fully activated protein C. The mutant protein was shown to be correctly processed by NH2-terminal sequence analysis. This strategy for successful expression of an activated form of protein C may apply to the expression of active forms of other proteases which are naturally expressed as zymogens.     

New vascular endothelial growth factor isoform generated by internal ribosome entry site-driven CUG translation initiation.

We recently demonstrated that the very long 5'-untranslated region (5'-UTR) of the vascular endothelial growth factor (VEGF) mRNA contains two independent internal ribosome entry sites (IRES A and B). In the human sequence, four potential CUG translation initiation codons are located in between these IRES and are in frame with the classical AUG start codon. By in vitro translation and COS-7 cell transfections, we demonstrate that a high mol wt VEGF isoform [called large VEGF (L-VEGF)] is generated by an alternative translation initiation process, which occurs at the first of these CUG codons. Using a bicistronic strategy, we show that the upstream IRES B controls the translation initiation of L-VEGF. This isoform is 206 amino acids longer than the classical AUG-initiated form. With a specific antibody raised against this NH2 extension, we show that the L-VEGF is present in different mouse tissues or in transfected COS-7 cells. We also demonstrate that L-VEGF is cleaved into two fragments: a 23-kDa NH2-specific fragment and a fragment with an apparent size similar to that of the classical AUG-initiated form. This cleavage requires the integrity of a hydrophobic sequence located in the central part of the L-VEGF molecule. This sequence actually plays the role of signal peptide in the classical AUG-initiated form. The AUG-initiated form and the COOH cleavage product of the L-VEGF are both secreted. In contrast, the large isoform and its NH2 fragment present an intracellular localization. These data unravel a further level of complexity in the regulation of VEGF expression.     

Human mesotrypsin is a unique digestive protease specialized for the degradation of trypsin inhibitors

Mesotrypsin is an enigmatic minor human trypsin isoform, which has been recognized for its peculiar resistance to natural trypsin inhibitors such as soybean trypsin inhibitor (SBTI) or human pancreatic secretory trypsin inhibitor (SPINK1). In search of a biological function, two conflicting theories proposed that due to its inhibitor-resistant activity mesotrypsin could prematurely activate or degrade pancreatic zymogens and thus play a pathogenic or protective role in human pancreatitis. In the present study we ruled out both theories by demonstrating that mesotrypsin was grossly defective not only in inhibitor binding, but also in the activation or degradation of pancreatic zymogens. We found that the restricted ability of mesotrypsin to bind inhibitors or to hydrolyze protein substrates was solely due to a single evolutionary mutation, which changed the serine-protease signature glycine 198 residue to arginine. Remarkably, the same mutation endowed mesotrypsin with a novel and unique function: mesotrypsin rapidly hydrolyzed the reactive-site peptide bond of the Kunitz-type trypsin inhibitor SBTI, and irreversibly degraded the Kazal-type temporary inhibitor SPINK1. The observations suggest that the biological function of human mesotrypsin is digestive degradation of trypsin inhibitors. This mechanism can facilitate the digestion of foods rich in natural trypsin inhibitors. Furthermore, the findings raise the possibility that inappropriate activation of mesotrypsinogen in the pancreas might lower protective SPINK1 levels and contribute to the development of human pancreatitis. In this regard, it is noteworthy that the well known pathological trypsinogen activator cathepsin B exhibited a preference for the activation of mesotrypsinogen of all three human trypsinogen isoforms, suggesting a biochemical mechanism for mesotrypsinogen activation in pancreatic acinar cells.     

Nucleotide sequence of the gene encoding the Streptomyces albus G beta-lactamase precursor

A 1400-base DNA fragment, which contains the gene encoding the extracellular active-site serine beta-lactamase of Streptomyces albus G previously cloned into Streptomyces lividans [Dehottay et al. (1986) Gene 42, 31-36], was sequenced. The gene codes for a 314-amino-acid precursor, the N-terminal region of which has the characteristics of a signal peptide. The beta-lactamase as excreted by the host strain S. lividans PD6 has a ragged N-terminus, indicating either the presence of a leader peptidase of poor specificity or the action of an aminopeptidase. The primary structure (as deduced from the nucleotide sequence) was confirmed by amino acid sequencing of a 16-residue stretch at the amino terminus of the protein, a 12-residue stretch containing the active-site serine [De Meester et al. (1987) Biochem. J. 244, 427-432] and a 23-residue stretch obtained by trypsin digestion of the protein. The beta-lactamase belongs to class A, has three half-cystine residues (one of which occurs on the amino side of the active-site serine) and is inactivated by thiol reagents. Putative ribosome binding site and terminator region were identified.     

Conjugative pili of IncP plasmids, and the Ti plasmid T pilus are composed of cyclic subunits.

TrbC propilin is the precursor of the pilin subunit TrbC of IncP conjugative pili in Escherichia coli. Likewise, its homologue, VirB2 propilin, is processed into T pilin of the Ti plasmid T pilus in Agrobacterium tumefaciens. TrbC and VirB2 propilin are truncated post-translationally at the N terminus by the removal of a 36/47-residue leader peptide, respectively. TrbC propilin undergoes a second processing step by the removal of 27 residues at the C terminus by host-encoded functions followed by the excision of four additional C-terminal residues by a plasmid-borne serine protease. The final product TrbC of 78 residues is cyclized via an intramolecular covalent head-to-tail peptide bond. The T pilin does not undergo additional truncation but is likewise cyclized. The circular structures of these pilins, as verified by mass spectrometry, represent novel primary configurations that conform and assemble into the conjugative apparatus.     

Expression of mutated cationic trypsinogen reduces cellular viability in AR4-2J cells

Mutations in the human cationic trypsinogen are associated with hereditary pancreatitis. The cDNA coding for human cationic trypsinogen was subcloned into the expression vector pcDNA3. The mutations R122H, N29I, A16V, D22G, and K23R were introduced by site directed mutagenesis. We constructed an expression vector coding for active trypsin by subcloning the cDNA of trypsin lacking the coding region for the trypsin activating peptide behind an appropriate signal peptide. Expression of protein was verified by Western blot and measurement of enzymatic activity. AR4-2J cells were transiently transfected with the different expression vectors and cell viability and intracellular caspase-3 activity were quantified. In contrast to wild-type trypsinogen, expression of active trypsin and mutated trypsinogens reduced cell viability of AR4-2J cells. Expression of trypsin and R122H trypsinogen induced caspase-3 activity. Acinar cells might react to intracellular trypsin activity by triggering apoptosis.     

Mengovirus leader is involved in the inhibition of host cell protein synthesis.

The presence of a leader peptide in picornaviruses is restricted to the Cardiovirus and Aphthovirus genera. However, the leader peptides of these two genera are structurally and functionally unrelated. The aphthovirus leader is a protease involved in viral polyprotein processing and host cell translation shutoff. The function of the cardiovirus leader peptide is still unknown. To gain an insight into the function of the cardiovirus leader peptide, a mengovirus leader peptide deletion mutant was constructed. The deletion mutant was able to grow at a reduced rate in baby hamster kidney cells (BHK-21). Mutant virus production in mouse fibroblasts (L929 cells), however, could be demonstrated only after inoculation of BHK-21 cells with the transfected L929 cells. Analysis of cellular and viral protein synthesis in mutant virus-infected cells showed a delayed inhibition of host cell protein synthesis and a reduced production of viral proteins. In a single-cycle infection, mutant virus produced only 1% of wild-type virus yield at 8 h postinfection. Host cell translation shutoff in L929 cells infected with mutant virus was restored by the addition of the kinase inhibitor 2-aminopurine. Mutant virus production in 2-aminopurine-treated L929 cells was increased to 60% of wild-type virus yield at 8 h postinfection. Our results suggest that the cardiovirus leader peptide is involved in the inhibition of host cell protein synthesis.     

Nucleotide sequence of a γ gliadin gene: Comparisons with other γ gliadin sequences show the structure of γ gliadin genes and the general primary structure of γ gliadins

A low stringency screening of a wheat (Triticum aestivum L.) genomic library produced three types of γ gliadin clones. The sequence of one clone, λ10–20, encoded a γ gliadin of 34.3 kDa. Comparisons of this protein with the proteins encoded by other γ gliadin DNA sequences revealed a general γ gliadin structure: a 19-residue signal peptide; 12-residue mature amino terminus; 12–16 copies of a proline- and glutamine-rich heptapeptide repeat; a 76-residue region high in glutamine containing most of the cyysteines and charged residues: a 6–16-residue polyglutamine region; and the 41-residue carboxyl terminus. Comparisons of the 5′ and 3′ flanking regions of several γ gliadins reveals the high homology and general structure of γ gliadin genes. A further comparison of the 5′ flanking regions with the 5′ flanking regions of other prolamin genes showed that γ gliadin genes contain three copies of a conserved sequence seen within approx. 600 b.p. upstream of the translation start sites of prolamin genes.     

Glycosylation and membrane insertion of newly synthesized rat dopamine beta-hydroxylase in a cell-free system without signal cleavage.

Dopamine beta-hydroxylase (DBH, EC 1.14.17.1) is present in both membrane-bound and soluble forms in neurosecretory vesicles. This study was designed to investigate the differences between membrane-bound and soluble DBH and how they may arise from translation of a single mRNA. Antisera to a peptide corresponding to the carboxyl terminus of rat DBH was found to specifically immunoprecipitate the 77- and 73-kDa subunits of newly synthesized DBH in rat brain. Thus, both soluble and membrane-bound forms contain the same carboxyl terminus. To investigate differences at the amino terminus, full-length rat DBH mRNA, translated in a cell-free system, produced a 66-kDa peptide. An additional higher molecular mass product was synthesized upon co-translational addition of microsomal membranes. This product was glycosylated since it bound to concanavalin A-Sepharose and reverted to the 66-kDa polypeptide after treatment with endoglycosidase H. This glycosylated product was resistant to protease digestion and fractionated with microsomal membranes on sucrose gradients, indicating that it is incorporated into the microsomal membranes. Amino-terminal sequencing of the glycosylated translation product indicated that the amino-terminal "signal" sequence was not cleaved. The results indicate that in the cell-free system newly synthesized DBH undergoes glycosylation and incorporation into microsomal membranes without cleavage of the NH2-terminal signal sequence.