Procarboxypeptidase in rat pancreas. Overall characterization and comparison of the activation processes

Complex Formation of Human Proelastases with Procarboxypeptidases A1 and A2

The pancreas secretes digestive proenzymes typically in their monomeric form. A notable exception is the ternary complex formed by proproteinase E, chymotrypsinogen C, and procarboxypeptidase A (proCPA) in cattle and other ruminants. In the human and pig pancreas binary complexes of proCPA with proelastases were found. To characterize complex formation among human pancreatic protease zymogens in a systematic manner, we performed binding experiments using recombinant proelastases CELA2A, CELA3A, and CELA3B; chymotrypsinogens CTRB1, CTRB2, CTRC, and CTRL1; and procarboxypeptidases CPA1, CPA2, and CPB1. We found that proCELA3B bound not only to proCPA1 (KD 43 nm) but even more tightly to proCPA2 (KD 18 nm), whereas proCELA2A bound weakly to proCPA1 only (KD 152 nm). Surprisingly, proCELA3A, which shares 92% identity with proCELA3B, did not form stable complexes due to the evolutionary replacement of Ala(241) with Gly. The polymorphic nature of position 241 in both CELA3A (∼4% Ala(241) alleles) and CELA3B (∼2% Gly(241) alleles) points to individual variations in complex formation. The functional effect of complex formation was delayed procarboxypeptidase activation due to increased affinity of the inhibitory activation peptide, whereas proelastase activation was unchanged. We conclude that complex formation among human pancreatic protease zymogens is limited to a subset of proelastases and procarboxypeptidases. Complex formation stabilizes the inhibitory activation peptide of procarboxypeptidases and thereby increases zymogen stability and controls activation.

Chymotrypsin C is a co-activator of human pancreatic procarboxypeptidases A1 and A2

Human digestive carboxypeptidases CPA1, CPA2, and CPB1 are secreted by the pancreas as inactive proenzymes containing a 94-96-amino acid-long propeptide. Activation of procarboxypeptidases is initiated by proteolytic cleavage at the C-terminal end of the propeptide by trypsin. Here, we demonstrate that subsequent cleavage of the propeptide by chymotrypsin C (CTRC) induces a nearly 10-fold increase in the activity of trypsin-activated CPA1 and CPA2, whereas CPB1 activity is unaffected. Other human pancreatic proteases such as chymotrypsin B1, chymotrypsin B2, chymotrypsin-like enzyme-1, elastase 2A, elastase 3A, or elastase 3B are inactive or markedly less effective at promoting procarboxypeptidase activation. On the basis of these observations, we propose that CTRC is a physiological co-activator of proCPA1 and proCPA2. Furthermore, the results confirm and extend the notion that CTRC is a key regulator of digestive zymogen activation.

Membrane-tethered proteins for basic research, imaging, and therapy

Secreted and intracellular proteins including antibodies, cytokines, major histocompatibility complex molecules, antigens, and enzymes can be redirected to and anchored on the surface of mammalian cells to reveal novel functions and properties such as reducing systemic toxicity, altering the in vivo distribution of drugs and extending the range of useful drugs, creating novel, specific signaling receptors and reshaping protein immunogenicity. The present review highlights progress in designing vectors to target and retain chimeric proteins on the surface of mammalian cells. Comparison of chimeric proteins indicates that selection of the proper cytoplasmic domain and introduction of oligiosaccharides near the cell surface can dramatically enhance surface expression, especially for single-chain antibodies. We also describe progress and limitations of employing surface-tethered proteins for preferential activation of prodrugs at cancer cells, imaging gene expression in living animals, performing high-throughput screening, selectively activating immune cells in tumors, producing new adhesion molecules, creating local immune privileged sites, limiting the distribution of soluble factors such as cytokines, and enhancing polypeptide immunogenicity. Surface-anchored chimeric proteins represent a rich source for developing new techniques and creating novel therapeutics.

Mast cell cathepsins C and S control levels of carboxypeptidase A and the chymase, mouse mast cell protease 5

Carboxypeptidase A (CPA) is a metalloprotease, residing in the mast cell secretory granules together with chymases and tryptases. Little information is available with respect to the mechanisms that maintain or regulate the levels of stored proteases in the mast cell secretory granules. In this study we examined whether cathepsins C and S may be involved in the control of the levels of mast cell proteases. Mast cells cultured from bone marrow of cathepsin C- or S-null mice expressed higher levels of CPA protein and activity than cells from wild-type mice. Similar increases in protein were observed for the mouse chymase, mast cell protease-5 (mMCP-5), but not for the tryptase, mMCP-6. Steady-state levels of CPA and mMCP-5 mRNA were similar in wild-type and cathepsin C-null mast cells, indicating that post-transcriptional mechanisms explain the observed cathepsin C-dependence of CPA and mMCP-5 expression. The present study thus indicates novel roles for cathepsins C and S in regulating the levels of stored proteases in the mast cell secretory granules.

Molecular cloning and cDNA sequence analysis of carboxypeptidases A1, A2 and B from the Japanese flounder Paralichthys olivaceus

Although pancreatic serine proteases have been cloned in teleosts, no sequence data are currently available on members of the carboxypeptidase (CP) family. Here, we cloned cDNAs coding for two preproCPAs, corresponding to mammalian preproCPA1 and preproCPA2, and one preproCPB from a pancreatic cDNA library of the Japanese flounder, Paralichthys olivaceus. The activation peptides of flounder proCPs completely retained the sequences for inhibition of enzymatic activity of proCPs just like mammalian proCPs. Of 306-309 amino acids in total, 95 amino acids are completely conserved between bovine CPA1 and CPB and flounder CPs. Notably, amino acid residues for Zn(2+) ligands, catalysis and substrate anchoring are completely conserved between flounder and bovine CPs. Three species of flounder preproCPs are all expressed in the pancreas of first feeding larvae.

Cloning, expression, and substrate specificity of MeCPA, a zinc carboxypeptidase that is secreted into infected tissues by the fungal entomopathogen Metarhizium anisopliae

To date zinc carboxypeptidases have only been found in animals and actinomycete bacteria. A cDNA clone (MeCPA) for a novel fungal (Metarhizium anisopliae) carboxypeptidase (MeCPA) was obtained by using reverse transcription differential display polymerase chain reaction to identify pathogenicity genes. MeCPA resembles pancreatic carboxypeptidases in being synthesized as a precursor species (418 amino acids) containing a large amino-terminal fragment (99 amino acids). The mature (secreted) form of MeCPA shows closest amino acid identity to human carboxypeptidases A1 (35%) and A2 (37%). MeCPA was expressed in an insect cell line yielding an enzyme with dual A1 + A2 specificity for branched aliphatic and aromatic COOH-terminal amino acids. However, in contrast to the very broad spectrum A + B-type bacterial enzymes, MeCPA lacks B-type activity against charged amino acids. This is predictable as key catalytic residues determining the specificity of MeCPA are conserved with those of mammalian A-type carboxypeptidases. Thus, in evolutionary terms the fungal enzyme is an intermediate between the divergence of A and B forms and the differentiation of the A form into A1 and A2 isoforms. Ultrastructural immunocytochemistry of infected host (Manduca sexta) cuticle demonstrated that MeCPA participates with the concurrently produced endoproteases in procuring nutrients; an equivalent function to digestive pancreatic enzymes.

Toward an enzyme/prodrug strategy for cancer gene therapy: endogenous activation of carboxypeptidase A mutants by the PACE/Furin family of propeptidases

In an effort to develop a gene-dependent enzyme/prodrug therapy (GDEPT) for tumor-specific delivery of methotrexate (MTX) we have chosen to construct mutant forms of carboxypeptidase A1 (CPA) that circumvent the requirement for trypsin-dependent activation. The basis of this strategy is that methotrexate-alpha-peptides are inefficient substrates for the reduced folate carrier (RFC) and hence cannot be internalized by cells. However, the blocking amino acid can be cleaved by CPA to liberate MTX, which is then internalized by the RFC, resulting in inhibition of dihydrofolate reductase and cytotoxicity. A battery of mutant CPAs was generated, in which the putative trypsin cleavage sites in the propeptide were mutated to the consensus recognition sequence for mammalian subtilisin-like propeptidases. These mutant forms of CPA were evaluated for expression, activation, and catalytic activity by transiently transfecting them into COS-1 cells both in the absence and in the presence of cotransfected propeptidases. CPA95 was identified as the most efficiently cleaved mutant, and further studies of this mutant indicated that the endogenously activated enzyme had kinetic parameters identical to those of the trypsin-activated wild-type protein. In addition, endogenously activated CPA95 could effectively sensitize cells to MTX-Phe in culture, decreasing the IC50 of MTX-Phe from 25- to 250-fold in squamous cell carcinoma cells expressing active CPA as compared with the parental lines.

Overexpression of human procarboxypeptidase A2 in Pichia pastoris and detailed characterization of its activation pathway

The cDNA of human procarboxypeptidase A2 has been overexpressed in the methylotrophic yeast Pichia pastoris and secreted into the culture medium by means of the alpha-mating factor signal sequence, yielding a major protein of identical size and N-terminal sequence as the wild-type form. Two other forms containing the proenzyme have also been overexpressed: one of them resulted from an incomplete processing of the signal peptide, whereas the other was a glycosylated derivative. Recombinant procarboxypeptidase A2 was purified to homogeneity, and it was shown that its mature active form displays functional properties similar to those of the enzyme directly isolated from human pancreas. The overall yield was approximately 250 mg of proenzyme or 180 mg of mature enzyme/liter of cell culture. The proteolysis-promoted activation process of the recombinant proenzyme has been studied in detail. During maturation by trypsin, the increase in activity of the enzyme is a rapid and monotonic event, which reflects the rate of the proteolytic release of the inhibitory pro-segment and the weaker nature of its interactions with the enzyme moiety compared with procarboxypeptidases of the A1 type. Three main forms of the pro-segment (96, 94, and 92 amino acids), with no inhibitory capability in the severed state, and a single mature carboxypeptidase A2 are produced during this process. No further proteolysis of these pro-segments by the generated carboxypeptidase A2 occurs, in contrast with observations made in other procarboxypeptidases (A1 and B). This differential behavior is a result of the extreme specificity of carboxypeptidase A2.

Comparative analysis of the sequences and three-dimensional models of human procarboxypeptidases A1, A2 and B

A full-length cDNA clone for preprocarboxypeptidase B from human pancreas has been isolated and sequenced. The open reading frame is 1254 bp in length, encoding a protein of 417 amino acids that includes a leader signal peptide of 15 amino acids and a 95-amino acid-long pro-segment. It contains two differences when compared to the sequence reported for pancreas-specific protein, a human serum marker for acute pancreatitis identified as procarboxypeptidase B. The main difference is a previously unreported Cys at position 138, which is needed for the formation of one of the three disulphide bridges. Sequence alignments between human procarboxypeptidases A1, A2 and B and other known forms show that the most conserved region is the enzyme moiety followed by the globular domain of the pro-segment. The maximum variability is found in the connecting region between moieties. The known three-dimensional structures of procarboxypeptidases from bovine and porcine species have been used to model all three human procarboxypeptidases and also to estimatethe interaction energies between the different parts of the molecules, in an attempt to gain insight into the structural features responsible for the differences observed in the functionality of the proenzymes, particularly in their proteolytic activation pathways. Taken together, the results obtained confirm that the main determinant for the rate and mode of activation of procarboxypeptidases is the strength of the interaction between the enzyme and the globular domain of the pro-segment, the connecting segment playing a complementary role.

Characterisation and preliminary X-ray diffraction analysis of human pancreatic procarboxypeptidase A2

Human procarboxypeptidase A2 has been expressed in a Pichia pastoris heterologous system and purified by hydrophobic interaction and anion exchange chromatographies. The hydrolytic action of carboxypeptidase A2 on peptide substrates with different lengths and residues at the C-terminus was analysed, and a preference towards long substrates with aromatic amino acids in their C-terminal end, particularly tryptophan, was found; with such substrates its activity is similar or higher than that of bovine carboxypeptidase A1. Procarboxypeptidase A2 has been crystallised using a vapour diffusion approach; the crystals obtained belong to the monoclinic system, spacegroup P2(1), and present one procarboxypeptidase A2 molecule per asymmetric unit. The crystals diffract beyond 1.8 A resolution and are suitable for detailed X-ray analysis.

The three-dimensional structure of human procarboxypeptidase A2. Deciphering the basis of the inhibition, activation and intrinsic activity of the zymogen

The three-dimensional structure of human procarboxypeptidase A2 has been determined using X-ray crystallography at 1.8 A resolution. This is the first detailed structural report of a human pancreatic carboxypeptidase and of its zymogen. Human procarboxypeptidase A2 is formed by a pro-segment of 96 residues, which inhibits the enzyme, and a carboxypeptidase moiety of 305 residues. The pro-enzyme maintains the general fold when compared with other non-human counterparts. The globular part of the pro-segment docks into the enzyme moiety and shields the S2-S4 substrate binding sites, promoting inhibition. Interestingly, important differences are found in the pro-segment which allow the identification of the structural determinants of the diverse activation behaviours of procarboxypeptidases A1, B and A2, particularly of the latter. The benzylsuccinic inhibitor is able to diffuse into the active site of procarboxypeptidase A2 in the crystals. The structure of the zymogen-inhibitor complex has been solved at 2.2 A resolution. The inhibitor enters the active site through a channel formed at the interface between the pro-segment and the enzyme regions and interacts with important elements of the active site. The derived structural features explain the intrinsic activity of A1/A2 pro-enzymes for small substrates.

Crystal structure of an oligomer of proteolytic zymogens: detailed conformational analysis of the bovine ternary complex and implications for their activation

The pancreas of ruminants secretes a 100 kDa non-covalent ternary complex of the zymogen of a metalloexopeptidase, carboxypeptidase A, and the proforms of two serine endopeptidases, chymotrypsin C and proteinase E. The crystal structure of the bovine complex has been solved and refined to an R-factor of 0.192 using synchrotron radiation X-ray data to 2.35 A resolution. In this heterotrimeric complex, the 403 residue procarboxypeptidase A takes a central position, with chymotrypsinogen C and proproteinase E attached to different surface sites of it. The procarboxypeptidase A subunit is composed of the active enzyme part and the 94 residue prodomain, similar to the monomeric porcine homologous form. The 251 residue subunit chymotrypsinogen structure, the first solved of an anionic (acidic pI) chymotrypsinogen, exhibits characteristics of both chymotrypsinogen A and elastases, with a potential specificity pocket of intermediate size (to accommodate apolar medium-sized residues) although not properly folded, as in bovine chymotrypsinogen A; this pocket displays a "zymogen triad" characteristic for zymogens of the chymotrypsinogen family, consisting of three non-catalytic residues (one serine, one histidine, and one aspartate) arranged in a fashion similar to the catalytic residues in the active enzymes. Following the traits of this family, the N terminus is clamped to the main molecular body by a disulphide bond, but the close six residue activation segment is completely disordered. The third zymogen, the 253 residue proproteinase E, bears close conformational resemblance to active porcine pancreatic elastase; its specificity pocket is buried, displaying the second "zymogen triad". Its five N-terminal residues are disordered, although the close activation site is fixed to the molecular surface. The structure of this native zymogen displays large conformational differences when compared with the recently solved crystal structure of bovine subunit III, an N-terminally truncated, non-activatable, proproteinase E variant lacking the first 13 residues of the native proenzyme. Most of the prosegment of procarboxypeptidase A and its activation sites are buried in the centre of the oligomer, whilst the activation sites of chymotrypsinogen C and proproteinase E are surface-located and not involved in intra or inter-trimer contacts. This organization confers a functional role to the oligomeric structure, establishing a sequential proteolytic activation for the different zymogens of the complex. The large surface and number of residues involved in the contacts among subunits, as well as the variety of non-bonded interactions, account for the high stability of the native ternary complex.

Carboxypeptidase A isoforms produced by distinct genes or alternative splicing in brain and other extrapancreatic tissues

The presence of carboxypeptidase A (EC 3.4.17.1; CPA) gene transcripts and corresponding catalytic activity was investigated in brain and other extradigestive rat tissues in which presence of the pancreatic enzyme had not been reported so far. Transcripts of two known CPA genes, CPA1 and CPA2, were identified in extremely low abundance in brain and several other extrapancreatic tissues using Northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Whereas the CPA1 gene transcripts in brain, heart, stomach, or colon had a size similar to that in pancreas (1.35 kilobases), the CPA2 gene transcripts in brain, testis, or lung were of a smaller size (1.1 kilobases). Northern blot analysis using various probes, RT-PCR, and 5'-rapid amplification of cDNA 5'-end (5' RACE analysis) all indicated that this smaller size of the brain transcript was attributable to production by alternative splicing of the pro-mRNA. This process corresponds to deletion of the first four exons, leading to a mRNA encoding a protein in which the signal peptide and activation peptide of prepro-CPA2 are absent but the active site remains. The prediction that the shorter CPA2 isoform, designated CPA2(S), should correspond to a cytoplasmic metallopeptidase that does not require tryptic activation was verified by characterization of the recombinant protein and comparing it with the native CPA-like activity in brain. Both recombinant CPA2(S) generated in Escherichia coli and a soluble protein from brain displayed similar sizes on Western blots (32 kDa to be compared to 34 kDa for pancreatic CPA2). Recombinant CPA2(S) and a soluble CPA-like activity from brain displayed similar sensitivity to a series of inhibitors, contrasting with that of the pancreatic enzyme. It is concluded that alternative splicing produces a truncated CPA2 with distinct subcellular localization and modified catalytic activity. In spite of the presence of the CPA1 mRNA, no corresponding CPA activity could be detected in brain extracts, even after tryptic activation. This apparent discrepancy seems attributable to the presence of an endogenous peptide inhibitor which remains to be identified.

The activation pathway of procarboxypeptidase B from porcine pancreas: participation of the active enzyme in the proteolytic processing

The activation process of porcine pancreatic procarboxypeptidase B (pro-CPB) has been studied in detail by a number of complementary methodologies, and a description of the molecular events that lead to the generation of active carboxypeptidase B (CPB) has been deduced. The generated CPB participates in the degradation of its own activation segment by excising C-terminal residues from fragments produced by tryptic proteolysis. The trimming action of CPB is, however, not essential for the release of a fully functional enzyme, in contrast to what was previously reported for porcine procarboxypeptidase A (pro-CPA). In the model presented here, the activation process is solely dependent on the first tryptic cleavage, at the limit between the activation segment and the enzyme region, and the former piece loses all of its inhibitory capacity once severed from the proenzyme. The use of heterologous inhibitors of CPB activity during the study of the tryptic activation process of pro-CPB has been required for the capture of short-lived, otherwise nondetectable, intermediates. This has allowed a complete description of the process and shown that the first proteolytic action of trypsin can also take place on a second target bond. Structural considerations that take into account the three-dimensional structures of the A and B forms of the proenzymes lead us to propose that the differences in conformation at the region that connects the globular activation domain to the enzyme are the main responsible elements for the differences observed in the activation processes of both proenzymes.

Crystallization and preliminary X-ray analysis of the ternary complex of procarboxypeptidase A from bovine pancreas

The ternary complex of procarboxypeptidase A, chymotrypsinogen C and proproteinase E from bovine pancreas has been crystallized using the sitting drop vapour diffusion method. The success in obtaining crystals has been found to be critically dependent on the prevention of autolysis of the complex. In preliminary stages, crystals twinned by merohedry were obtained from a solution containing MgCl2 and polyethylenglycol 400 as precipitating agent. Later on, untwinned ones could be grown employing CaCl2 instead of MgCl2. These latter crystals belong to the rhombohedral system and to the spacegroup R3 with cell dimensions a = b = 188.5 A and c = 82.5 A. Consideration of the possible values of Vm accounts for the presence of one ternary complex molecule-oligomere per asymmetric unit. The crystals diffract beyond 2.6 A resolution and are suitable for X-ray analysis.

The sequence and conformation of human pancreatic procarboxypeptidase A2. cDNA cloning, sequence analysis, and three-dimensional model

A full-length cDNA clone coding for human pancreatic preprocarboxypeptidase A2 has been isolated from a lambda gt 11 human pancreatic library. Expression clones were identified by specific interaction with antisera raised against the native protein. The open reading frame of the polynucleotide sequence is 1254 base pairs in length and encodes a protein of 417 amino acids. This cDNA includes a short leader signal peptide of 16 amino acids and a 94-amino acid-long activation segment. The amino acid sequence shows 89% identity to that of rat procarboxypeptidase A2, the only A2 form sequenced so far, and 64% identity to that of human procarboxypeptidase A1. The newly determined sequence was modeled to the three-dimensional crystal structures of both bovine carboxypeptidase A and porcine procarboxypeptidase A1 by a novel distance geometry approach. Biases in the modeling were avoided by relying exclusively on automatic procedures and by using random structures as starting points. Information taken from the known homologous structures refers only to the backbone since no explicit data describing the conformation of side chains were transferred. Ten structures of human carboxypeptidase A2 were determined on the basis of each of the two known crystal structures. The root-mean-square distance for the backbone atoms between the 10 structures and their mean for 237 selected residues is 0.7 A when starting from the bovine protein and 0.8 A for 251 selected residues when starting from the porcine protein. The 94 residue-long activation segment was also determined in the modeling based on the porcine zymogen; its structure is well defined but not its orientation with respect to the enzyme moiety. The model obtained for human procarboxypeptidase A2 is discussed with respect to the specificity and activation of the enzyme.