Purification and characterization of the prohormone convertase PC1(PC3)

The neuroendocrine chaperone 7B2 can enhance in vitro POMC cleavage by prohormone convertase PC2

We previously showed that the neuroendocrine polypeptide 7B2 transiently interacts with prohormone convertase PC2 in the secretory pathway of neuroendocrine cells. Here we demonstrate that the processed, but not the intact, form of 7B2 can enhance the in vitro cleavage of newly synthesized prohormone proopiomelanocortin (POMC) in lysates of Xenopus intermediate pituitary cells. PC2 is presumably the cleavage enzyme involved since intact 7B2 abolishes the enhancing effect of processed 7B2 and is known to act as a specific inhibitor of PC2. Furthermore, processed 7B2 stimulates in vitro POMC cleavage by immunopurified Xenopus PC2. Our results indicate that 7B2 can display chaperone activity towards PC2.

The post-translational processing and intracellular sorting of carboxypeptidase H in the islets of Langerhans

The post-translational processing and intracellular sorting of the proinsulin-converting enzyme carboxypeptidase H (CPH) was studied in isolated rat islets of Langerhans. Pulse-chase-radiolabelling experiments using sequence-specific antisera showed that CPH was synthesized initially as a 57-kDa glycoprotein which was processed to a 54-kDa mature form by proteolytic processing at the N-terminus. Processing of the CPH precursor occurred rapidly (t(1/2) = 30) after an initial delay of 15-30 min and the enzyme was secreted in parallel with the insulin-related peptides in response to glucose-stimulation within 1 h after radiolabelling. This indicated that the proteins were packaged into nascent secretory granules at approximately the same rate following synthesis. Conversion of proinsulin and the 57-kDa form was inhibited markedly by chase incubation of islets at 20 degrees C, indicating that maturation of both proteins occurs in a post-Golgi compartment. Affinity purification of the enzyme from insulinoma subcellular fractions showed that the 57-kDa form was associated with endoplasmic reticulum or Golgi elements, and the 54-kDa form was present in secretory granules. Structural analysis showed that the granule form of the enzyme had an N-terminal amino acid sequence beginning at residue 42 of rat CPH, thereby implicating cleavage of the precursor after the fourth Arg in a site containing five consecutive Arg residues. These findings indicate that post-translational processing of CPH is mediated by an endoprotease which cleaves at sites containing multiple basic amino acid residues upon segregation of the enzyme to the secretory granules.

Identification of the region within the neuroendocrine polypeptide 7B2 responsible for the inhibition of prohormone convertase PC2

The highly conserved polypeptide 7B2 and the subtilisin-related prohormone convertases PC1/PC3 and PC2 are broadly distributed in neurons and endocrine cells and are localized to secretory granules. We recently showed that recombinant 7B2 is in vitro a potent inhibitor of PC2 activity, but not of PC1/PC3, and that newly synthesized 7B2 is transiently associated with proPC2 in vivo. In the present study, in vitro mutagenesis was used to identify the region within the 7B2 sequence responsible for the inhibition of PC2. Mutant proteins were produced in a prokaryotic expression system and their effects on PC1/PC3 and PC2 activities were studied by two different in vitro enzyme assays. None of the 7B2 mutant proteins inhibited PC1/PC3 activity. Truncation studies revealed that a short segment within the COOH-terminal portion of 7B2 is critical for its inhibitory effect on PC2. This segment contains a pair of basic amino acid residues which may represent a recognition motif for PC2. Single amino acid substitutions within this Lys171-Lys172 site strongly diminished and a double mutation abolished the inhibitory potency of 7B2. Our results indicate that, although amino acid residues directly surrounding this dibasic pair also contribute to PC2 inhibition, the Lys171-Lys172 site is particularly important for the ability of 7B2 to inhibit PC2.

Processing of mouse proglucagon by recombinant prohormone convertase 1 and immunopurified prohormone convertase 2 in vitro

The mouse tumor cell line alpha TC1-6 was used as a model system to examine the post-translational processing of proglucagon. Determination of the mouse preproglucagon cDNA sequence and comparison with the published sequences of rat and human preproglucagons revealed nucleic acid homologies of 89.1 and 84%, respectively, and amino acid homologies of 94 and 89.4%, respectively. Immunohistochemical analyses with antibodies directed against PC2 and glucagon colocalized both the enzyme and substrate within the same secretory granules. PC1 was also immunolocalized in secretory granules. Cells were metabolically labeled with [3H]tryptophan, and extracts were analyzed by reverse-phase high pressure liquid chromatography. Radioactive peptides with retention times identical to those of synthetic peptide standards were recovered and subjected to peptide mapping to verify their identities. To determine the potential role of PC1 and PC2 in proglucagon processing, 3H-labeled proglucagon was incubated in vitro with recombinant PC1 and/or immunopurified PC2. Both enzymes cleaved proglucagon to yield the major proglucagon fragment, glicentin, and oxyntomodulin, whereas only PC1 released glucagon-like peptide-I from the major proglucagon fragment. Neither PC1 nor PC2 processed glucagon from proglucagon in vitro. These results suggest a potential role for PC1 and/or PC2 in cleaving several of the normal products, excluding glucagon, from the mouse proglucagon precursor.

Purification and characteristics of the candidate prohormone processing proteases PC2 and PC1/3 from bovine adrenal medulla chromaffin granules

The prohormone-processing proteases PC1/3 and PC2 belong to the family of mammalian subtilisin-related proprotein convertases (PC) possessing homology to the yeast Kex2 protease. The presence of PC1/3 and PC2 in secretory vesicles of bovine adrenal medulla (chromaffin granules) implicates their role in the processing the precursors of enkephalin, neuropeptide Y, somatostatin, and other neuropeptides that are present in chromaffin granules. In this study, PC1/3 and PC2 were purified to apparent homogeneity from the soluble fraction of chromaffin granules by chromatography on concanavalin A-Sepharose, Sephacryl S-200, pepstatin A-agarose, and anti-PC1/3 or anti-PC2 immunoaffinity resins. PC1/3 and PC2 were monitored during purification by measuring proteolytic activities with 35S-enkephalin precursor and Boc-Arg-Val-Arg-Arg-methylcoumarin amide (MCA) substrates and by following PC1/3 and PC2 immunoreactivity with specific anti-PC1/3 and anti-PC2 sera generated in this study. Purified PC1/3 and PC2 on SDS-polyacrylamide gels each show a molecular mass of 66 kDa. PC2 in the soluble fraction of chromaffin granules was present at 5- and 10-fold higher enzyme protein and activity, respectively, compared with that of PC1/3. PC1/3 and PC2 cleaved paired basic and monobasic sites within peptide-MCA substrates, with Boc-Arg-Val-Arg-Arg-MCA and pGlu-Arg-Thr-Lys-Arg-MCA as the most effectively cleaved peptides tested. PC1/3 and PC2 showed pH optima of 6.5 and 7.0, respectively. Kinetic studies indicated apparent Km values for hydrolysis of Boc-Arg-Val-Arg-Arg-MCA as 66 and 40 microM, with Vmax values of 255 and 353 nmol/h/mg for PC1/3 and PC2, respectively. Specificity of the PC enzymes for dibasic sites was confirmed by potent inhibition by the active site-directed peptide inhibitors (D-Tyr)-Glu-Phe-Lys-Arg-CH2Cl and Ac-Arg-Arg-CH2Cl. Inhibition by EGTA and activation by Ca2+ indicated PC1/3 and PC2 as Ca(2+)-dependent proteases. In addition, PC enzymes were activated by dithiothreitol and inhibited by thiol-blocking reagents, p-hydroxymercuribenzoate and mercuric chloride. These results illustrate the properties of endogenous PC1/3 and PC2 as prohormone-processing enzymes.

Role of amino acid sequences flanking dibasic cleavage sites in precursor proteolytic processing. The importance of the first residue C-terminal of the cleavage site

The amino acid sequences flanking 352 dibasic moieties contained in 83 prohormones and pro-proteins listed in a database were examined. Frequency calculations on the occurrence of given residues at positions P6 to P'4 allowed us to delineate a number of features which might be in part responsible for the in vivo discrimination between cleaved and uncleaved dibasic sites. These include the following: amino acids at these positions were characterized by a large variability in composition and properties; no major contribution of a given precursor subsite to endoprotease specificity was observed; some amino acid residues appeared to occupy preferentially certain precursor subsites (for instance, Met in P6 and P3, Asp and Ala in P'1, Pro in P6, Gly in P3 and P'2 etc.) whereas some others appeared to be excluded. Most amino acid residues occupying the P'1 position in these precursor cleavage sites were tolerated. But the beta-carbon branched side chain residues (Thr, Val, Leu, Ile) and Pro, Cys, Met and Trp were either totally excluded or poorly represented, suggesting that they might be unfavourable to cleavage. The biological relevance of these observations to the efficacy of dibasic cleavage by model propeptide convertases was in vitro tested using both pro-ocytocin convertase and Kex2 protease action on a series of pro-ocytocin related synthetic substrates reproducing the Pro7-->Leu15 sequence of the precursor in which the Ala13 residue (P'1 in the LysArg-Ala motif) was replaced by various amino acid residues. A good correlation was obtained on this model system indicating that P'1 residue of precursor dibasic processing sites is an important feature and may play the role of anchoring motif to S'1 convertase subsite. We tentatively propose that the present database, and the corresponding model, may be used for further investigation of dibasic endoproteolytic processing of propeptides and pro-proteins.

Differences in pH optima and calcium requirements for maturation of the prohormone convertases PC2 and PC3 indicates different intracellular locations for these events

PC2 and PC3, which is also known as PC1, are subtilisin-like proteases that are involved in the intracellular processing of prohormones and proneuropeptides. Both enzymes are synthesized as propolypeptides that undergo proteolytic maturation within the secretory pathway. An in vitro translation/translocation system from Xenopus egg extracts was used to investigate mechanisms in the maturation of pro-PC3 and pro-PC2. Pro-PC3 underwent rapid (t1/2 < 10 min) processing of the 88-kDa propolypeptide at the sequence RSKR83 to generate the 80-kDa active form of the enzyme. This processing was blocked when the active site aspartate was changed to asparagine, suggesting that an autocatalytic mechanism was involved. In this system, processing of pro-PC3 was optimal between pH 7.0 and 8.0 and was not dependent on additional calcium. These results are consistent with pro-PC3 maturation occurring at an early stage in the secretory pathway, possibly within the endoplasmic reticulum, where the pH would be close to neutral and the calcium concentration less than that observed in later compartments. Processing of pro-PC2 in the Xenopus egg extract was much slower than that of pro-PC3 (t1/2 = 8 h). It exhibited a pH optimum of 5.5-6.0 and was dependent on calcium (K0.5 = 2-4 mM). The enzymatic properties of pro-PC2 processing were similar to that of the mature enzyme. Further studies using mutant pro-PC2 constructs suggested that cleavage of pro-PC2 was catalyzed by the mature 68-kDa PC2 molecule. The results were consistent with pro-PC2 maturation occurring within a late compartment of the secretory pathway that contains a high calcium concentration and low pH.

Distribution and regulation of the candidate prohormone processing enzymes SPC2 and SPC3 in adult rat brain

A number of candidate mammalian prohormone processing enzymes related to the yeast Kex2 endoprotease have been cloned and demonstrated to cleave several prohormone precursors at single, pairs and tetra basic amino acid processing sites. We have mapped the distribution of the mRNAs encoding two of these endoproteases in adult rat brain. SPC3 message levels showed a more restricted distribution and generally lower levels than SPC2 transcripts. The highest levels of SPC2 mRNA were found in the pyramidal cells of the hippocampus, several thalamic nuclei, the habenula and selected nuclei in the hypothalamus. SPC3 mRNA was most abundant in dentate gyrus granule cells, the habenula and selected hypothalamic nuclei. In the hypothalamus overlapping and unique distributions of the two transcripts were seen in the paraventricular nucleus with SPC3 mRNA predominantly expressed in lateral magnocellular cells. Both SPC2 and SPC3 mRNA were upregulated in the paraventricular and supraoptic hypothalamic nuclei following chronic salt loading. Combined immunocytochemistry/in situ hybridization histochemistry demonstrated that SPC2 and SPC3 transcripts were both expressed in the vasopressinergic subpopulation of magnocellular neurons in the supraoptic nucleus. SPC3 mRNA, but not SPC2 transcripts, also colocalized with immunoreactive vasopressin-associated neurophysin in the suprachiasmatic nucleus. These results remain consistent with roles for SPC2 and SPC3 in the biosynthesis of neuropeptides and for a specific role for SPC3 in the processing of provasopressin. Increased levels of SPC2 and SPC3 transcripts following a chronic osmotic stimulus suggests these proteases are coregulated with prohormone substrates and may be useful as an indicator of peptidergic activity.

Differential effects of temperature blockade on the proteolytic processing of three secretory granule-associated proteins

Vesicular transport within the secretory pathway can be arrested by incubating cells at 15 degrees C or 20 degrees C to block exit from the endoplasmic reticulum or trans-Golgi network, respectively. Using this powerful tool we have compared the intracellular sites of endoproteolytic processing of proopiomelanocortin and two prohormone processing enzymes in AtT-20 mouse pituitary corticotrope tumor cells. For comparison, proopiomelanocortin processing was also evaluated in primary neurointermediate pituitary cultures. AtT-20 cells synthesize and store endogenous proopiomelanocortin and prohormone convertase 1; AtT-20 cells expressing high levels of integral membrane or soluble peptidylglycine alpha-amidating monooxygenase were generated by stable transfection. Cells were incubated with [35S]methionine and chased at 4 degrees C, 15 degrees C, 20 degrees C or 37 degrees C. The endoproteolytic processing of peptidylglycine alpha-amidating mono-oxygenase, prohormone convertase 1, and proopiomelanocortin was compared following immunoprecipitation. Endoproteolytic processing of integral membrane and soluble peptidylglycine alpha-amidating monooxygenase proteins was completely blocked by incubation of cells at 20 degrees C. In contrast, prohormone convertase 1 processing from the 87 kDa precursor to the 81 kDa intermediate proceeded to completion at both 15 degrees C and 20 degrees C, while cleavage to generate the 63 kDa prohormone convertase 1 protein was completely blocked at 20 degrees C. In AtT-20 cells and neurointermediate pituitary cultures, generation of beta-lipotropin from proopiomelanocortin continued at a slow but significant rate at 20 degrees C, while processing of beta-lipotropin to beta-endorphin was blocked. Thus prohormone convertase 1 processing begins in the endoplasmic reticulum and is not completed until after the trans-Golgi network, while peptidylglycine alpha-amidating monooxygenase processing begins after the trans-Golgi network. Selected proopiomelanocortin cleavages begin before entry into immature granules.

Furin has the proalbumin substrate specificity and serpin inhibitory properties of an in situ hepatic convertase

Furin, a KEX2 protease homolog with high RNA expression in the liver is an excellent candidate as a hepatic proprotein convertase. Here we show that purified recombinant furin has the same proalbumin specificity and serpin inhibitory properties as the in situ hepatic convertase. There was rapid cleavage at the -RRD- site of normal human proalbumin but there no significant cleavage of natural unprocessed variants with cleavage site sequences of -RRV-, -HRD-, -RQD-, or -CRD-. Cleavage of the latter was not increased by S-aminoethylation. Furin was specifically inhibited by alpha 1-antitrypsin Pittsburgh (358 Met-->Arg), (K1/2 = 3 microM) but not by 50 microM normal antitrypsin M or by antithrombin, however, antithrombin/heparin was a good inhibitor (K1/2 = 9 microM). The pH optimum for proalbumin cleavage was between pH 5.5 and 6.0, indicating that furin is potentially fully active within secretory vesicles, the site of proalbumin cleavage.

Regional distribution of neuropeptide processing endopeptidases in adult rat brain

Many peptide hormone and neuropeptide precursors undergo post-translational processing at mono- and/or dibasic residues. An enzymatic activity capable of processing prodynorphin at a monobasic processing site designated 'dynorphin converting enzyme' has been previously reported in rat rain and bovine pituitary. In this study the distribution of dynorphin converting enzyme activity in ten regions of rat brain has been compared with the distribution of subtilisin-like processing enzymes and with the immuno-reactive dynorphin peptides. The distribution of dynorphin converting enzyme activity generally matches the distribution of immuno-reactive dynorphin B-13 in most but not all brain regions. The regions that are known to have a relatively large number of immuno-reactive dynorphin-neurons also contain high levels of dynorphin converting enzyme activity. The distribution of dynorphin converting enzyme activity does not match the distribution of subtilisin-like processing enzyme or carboxypeptidase E activities. Taken together the data support the possibility that the dynorphin converting enzyme is involved in the maturation of dynorphin, as well as other neuropeptides, and peptide hormones.

Prohormone processing in permeabilized cells: endoproteolytic cleavage of prosomatostatin in the trans-Golgi network

Many peptide hormones are synthesized as larger precursors which undergo endoproteolytic cleavage at paired basic residues to generate a bioactive molecule. Morphological evidence has implicated either the trans-Golgi network (TGN) or immature secretory granules as the site of prohormone cleavage. To identify the site where prohormone cleavage is initiated, we have used retrovirally infected rat anterior pituitary GH3 cells which express high levels of prosomatostatin, proSRIF, (Stoller TJ, Shields D (1988) J Cell Biol 107, 2087-2095). By incubating these cells at 20 degrees C, a temperature that prevents exit from the Golgi apparatus, proSRIF accumulated quantitatively in the TGN and no proteolytic processing was evident. Following the 20 degrees C block, the cells were permeabilized and proSRIF processing determined. Cleavage of proSRIF to the mature hormone was approximately 35-50% efficient, required incubation at 37 degrees C and ATP hydrolysis, but was independent of GTP or cytosol. The in vitro ATP-dependent proSRIF processing was inhibited by inclusion of chloroquine, a weak base, CCCP, a protonophore, or by pre-incubating the permeabilized cells with low concentrations of N-ethylmaleimide or bafilomycin, both inhibitors of vacuolar-type ATP-dependent proton pumps. These data suggest that ATP is required for generation of an acidic pH in the lumen of the TGN which is necessary for the activity of prohormone processing enzymes. By exploiting a permeabilized cell system, we have demonstrated that proSRIF cleavage is initiated in the TGN, in a reaction which is facilitated by a Golgi-associated vacuolar type ATPase.

Regulation of proteolytic activity in tissues

Degradation of tissue proteins is controlled by multiple means. These include regulation of the synthesis of proteinases, activation of the zymogen forms, the activity of the mature proteinase, and the degradation of these enzymes and the substrates. Mature proteinases can be controlled by pH, calcium ions, ATP, lipids and the formation of complexes with other proteinases, proteoglycans, and inhibitors.

Purified yeast aspartic protease 3 cleaves anglerfish pro-somatostatin I and II at di- and monobasic sites to generate somatostatin-14 and -28

Anglerfish somatostatin-14 (SS-14) and somatostatin-28 (aSS-28) are derived from pro-somatostatin I (aPSS-I) and pro-somatostatin II (PSS-II), respectively. Purified yeast aspartic protease 3 (YAP3), was shown to cleave aPSS-I at the Arg81-Lys82 to yield SS-14 and Lys-1SS-14. In contrast, YAP3 cleaved aPSS-II only at the monobasic residue, Arg73 to yield aSS-28. Since the paired basic and monobasic sites are present in both precursors, the results indicate that the structure and conformation of these substrates dictate where cleavage occurs. Furthermore, the data show that YAP3 has specificity for both monobasic and paired basic residues.