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• W2019201285 abstract "The proteinase mPC1, a neuroendocrine member of the mammalian family of subtilisin-like enzymes, has previously been shown to be converted to a carboxyl-terminally truncated 66-kDa form during transport through the secretory pathway. The cleavage site and the function of this carboxyl-terminal truncation event are unknown. We have performed site-directed mutagenesis of two paired basic sites in the mPC1 carboxyl-terminal tail and expressed these constructs in PC12 cells, a rat pheochromocytoma known to lack endogenous PC1. We found that the most likely site for the truncation event was at Arg590-Arg591 since mutation of this site to Lys-His prevented processing of 87-kDa PC1. A PC1 mutant carboxyl-terminally truncated at this site and expressed in PC12 cells was efficiently routed to the secretory pathway and stored in secretory granules, indicating that the carboxyl-terminal extension is not required for sorting of this enzyme. The function of the various PC1 constructs was assessed by analyzing proneurotensin cleavage to various forms. The carboxyl-terminally truncated PC1 mutant was found to perform most of the cleavages of this precursor as well as wild-type PC1; however, the blockade mutant processed proneurotensin much less efficiently. Differences between the site preferences of the various enzymes were noted. Our results support the notion that carboxyl-terminal processing of PC1 serves to regulate PC1 activity. The proteinase mPC1, a neuroendocrine member of the mammalian family of subtilisin-like enzymes, has previously been shown to be converted to a carboxyl-terminally truncated 66-kDa form during transport through the secretory pathway. The cleavage site and the function of this carboxyl-terminal truncation event are unknown. We have performed site-directed mutagenesis of two paired basic sites in the mPC1 carboxyl-terminal tail and expressed these constructs in PC12 cells, a rat pheochromocytoma known to lack endogenous PC1. We found that the most likely site for the truncation event was at Arg590-Arg591 since mutation of this site to Lys-His prevented processing of 87-kDa PC1. A PC1 mutant carboxyl-terminally truncated at this site and expressed in PC12 cells was efficiently routed to the secretory pathway and stored in secretory granules, indicating that the carboxyl-terminal extension is not required for sorting of this enzyme. The function of the various PC1 constructs was assessed by analyzing proneurotensin cleavage to various forms. The carboxyl-terminally truncated PC1 mutant was found to perform most of the cleavages of this precursor as well as wild-type PC1; however, the blockade mutant processed proneurotensin much less efficiently. Differences between the site preferences of the various enzymes were noted. Our results support the notion that carboxyl-terminal processing of PC1 serves to regulate PC1 activity. We have shown previously that in AtT-20 cells, constitutively released PC1 is present mostly in the 87-kDa form, while PC1 (also known as PC3) released through stimulation predominantly consists of a carboxyl-terminally truncated, 66-kDa protein (Vindrola and Lindberg, 1992). This difference indicates that carboxyl-terminal processing of 87-kDa PC1 probably largely occurs within regulated secretory vesicles. Recent studies employing temperature block, brefeldin A, and determination of oligosaccharide maturation have supported the idea that PC1 is carboxyl-terminally cleaved within the post-trans-Golgi network compartments in the regulated secretory pathway (Benjannet et al., 1992; Lindberg, 1994; Milgram and Mains, 1994; Zhou and Mains, 1994a). Through purification and characterization of recombinant PC1, we have demonstrated that both the 87-and the 74/66-kDa forms of PC1 are enzymatically active (Zhou and Lindberg, 1994). The conversion from 87-kDa PC1 to the 74/66-kDa forms not only increases specific activity while decreasing overall stability, it also narrows the pH optimum, increases calcium-dependence, and alters susceptibility to certain proteinase inhibitors (Zhou and Lindberg, 1994). Thus, we speculate that proteolytic processing of PC1 may play an important role in the regulation of PC1 enzymatic activity in vivo. The carboxyl-terminal segment may also be involved in the targeting and sorting of PC1 to secretory vesicles. In the experiments described here, we have used site-directed mutagenesis to study the biosynthetic processing, sorting, and function of the various domains of PC1 in PC12 cells. A mouse PC1/CMV 1The abbreviations used are: CMVcytomegalovirusPCRpolymerase chain reactionbpbase pair(s)DMEMDulbecco's modified Eagle's mediumPBSphosphate-buffered salineHPLChigh performance liquid chromatographyPAGEpolyacrylamide gel electrophoresisNTneurotensini-immunoreactive. expression vector was generously provided by Dr. N. G. Seidah (Benjannet et al., 1992). Site-directed mutations were designed to convert mPC1 Arg590-Arg591 to Lys-His, Arg627-Arg628 to Lys-Ala, or Gly592 to a stop codon. cytomegalovirus polymerase chain reaction base pair(s) Dulbecco's modified Eagle's medium phosphate-buffered saline high performance liquid chromatography polyacrylamide gel electrophoresis neurotensin immunoreactive. The mutation at Arg590-Arg591 of mPC1 was carried out using the Muta-Gene™ in vitro mutagenesis kit (Bio-Rad). A mutagenesis primer, 5′-CTT TTC CAC TCC GTG CTT GTC ATT CTG GAC TG-3′, was synthesized by LSUMC Core Laboratories. Single-strand DNA was prepared as described by Ausubel et al.(1987). The mutation in mPC1591BL/CMV was confirmed by DNA sequencing using a DNA Sequenase™ version 2.0 kit (U. S. Biochemical Corp.). The mutations at Arg627-Arg628 and Gly592 were performed using a PCR mutagenesis method. Primers XBAI-3 (5′-CAC AAC AAC TCT AGA CCC AGG AAC-3′) and BSTXI-5 (5′-TAA ATG CCA AAG CTC TGG TGG-3′) were synthesized to match the sequences of mPC1 cDNA at bp 1405-1426 or 2392-2415 (with a mutation switching CG to TA at 2404 bp to introduce an XbaI cleavage site). Two pairs of mutagenesis primers were synthesized to introduce mutations at Arg627-Arg628 or at Gly592. These primers were 628BL-5 (5′-GCA-ATG-TGG-AGG-GTA-AAG CGG ATG AGC AGG TAC-3′), 628BL-3 (5′-GTA CCTGCT CAT CCG CTT TAC CCT CCA CAT TGC-3′), 591ST-5 (5′CCA GAA TGA CAG GAG ATA AGT GGA AAA GAT GGT G-3′), and 591ST-3 (5′-CAC CAT CTT TTC CAC TTA TCT CCT GTC ATT CTG G-3′). The mPC1/CMV and mPC1591BL/CMV vectors were transfected into JM101 Escherichia coli, then were amplified and purified using a Wizard™ Miniprep DNA purification system (Promega, Madison, WI). About 2 μg of mPC1/CMV DNA were digested with 1.5 μl of XbaI (30 units, New England Biolabs) in a 20-μl volume at 37°C for 1.5 h; after heating at 70°C for 10 min, 15 μl of this reaction mixture was further digested with 1.5 μl of BstXI (15 units, New England Biolabs) in a 30-μl volume at 55°C for 1.5 h. The reaction products were separated on 1% agarose; a 6800-bp cDNA fragment was recovered and extracted from the agarose gel using a Geneclean II kit (Bio 101, Inc.). This cDNA was used as the vector in the following ligation reaction. Using 0.5 μg of mPC1/CMV or mPC1591BL/CMV DNA as template, three separate PCR reactions were carried out. The BSTXI-5 and 591ST-3 (or 628 BL-3) were used as PCR primers in the first reaction, while 591ST-5 (or 628BL-5) and XBAI-3 were used in the second reaction. After 10 cycles of amplification, the products were separated from template and primers on 1.5% agarose gel; the major DNA product of each PCR reaction was recovered and extracted from an agarose gel using a Geneclean II kit. In the third PCR reaction, the major products of the first two reactions were mixed and annealed to generate a template, and BSTXI-5 and XBAI-3 were used as PCR primers. The amplification was performed for 15 cycles, and the product (1005 bp) was separated from template and primers on a 1% agarose gel and DNA recovered using a Geneclean II kit. The product of the third PCR was digested using BstXI and XbaI as described above; the largest product was separated from small DNA segments using the Geneclean II kit. In order to ligate the PCR products into the digested vector (6800 bp), 60 ng of vector DNA was mixed with half of the PCR product and 1 μl (400 units) T4 DNA ligase (New England Biolabs) in a 20-μl final volume for 45 min at room temperature. The ligation mixture was then transformed into Ultracomp™ INVαF‘ cells (Invitrogen). To confirm the mutations and accuracy of PCR amplification, the 1005-bp PCR product region was sequenced using a DNA Sequenase™ version 2.0 kit with three different sequencing primers. Plasmids of correctly mutated mPC1SB/CMV (Arg627-Arg628→ Lys-Ala), mPC1DB/CMV (Arg590-Arg591→ Lys-His and Arg627-Arg628→ Lys-Ala), and mPC1ST/CMV (truncation at Gly592) were then amplified in E. coli (XL1 Blue; Stratagene) and purified using a Qiagen plasmid kit. PC12 cells, obtained from Drs. L. Elferink and R. Scheller (Stanford University), were grown in DMEM-high glucose (4.5 g/L) medium containing 10% fetal bovine serum and 5% heat-inactivated horse serum on collagen-coated plates. PC12 cells expressing wild-type mPC1 have been described previously (Lindberg et al., 1994). Mutated PC1 forms were transfected into PC12 cells using Lipofectin (Life Technologies, Inc.). The day prior to transfection, 1-2 million PC12 cells were subcultured on 10-cm plates. The next day, after the cells were well attached, the plates were rinsed twice with prewarmed serum-free DMEM-high glucose medium. A 30-μl aliquot of Lipofectin was placed into 3 ml of serum-free medium in a sterile 15-ml tube with 30 μg of DNA from mPC1SB/CMV, mPC1DB/CMV, or mPC1ST/CMV, and kept for 10 min at room temperature. Experimental plates were incubated with 3 ml of medium containing Lipofectin/DNA, while the control plate was incubated with Lipofectin only. The medium was replaced with regular medium containing 300 μg/ml G418 after 5 h of incubation at 37°C. Transfected cells produced colonies after approximately 3 weeks. The colonies were then subcloned using the agarose method described by Lindberg and Zhou(1995), and the cell lines producing the highest quantities of PC1 proteins were selected by immunoblotting 12-24 cell extracts with PC1 amino-terminal antiserum (Vindrola and Lindberg, 1992). PC12 cells stably transfected with mPC1s were cultured in medium containing 300 μg/ml G418 in collagen-coated 35-mm plates. After the cells reached about 70-80% confluence, the synthesis of proneurotensin was induced using medium containing 1 μM forskolin, 1 μM dexamethasone, 100 ng/ml nerve growth factor, and 10 mM LiCl (Rovere et al., 1993). The plates were maintained in inducing medium in an atmosphere of 5% CO2 at 37°C for 48 h. In stimulation experiments, two 35-mm wells of induced PC12 cells were each washed twice with 5 ml of prewarmed PBS. To one plate was added 1 ml of Earle's balanced salt solution (prepared as described by Lindberg et al.(1994)), while to the other plate was added 1 ml of Earle's balanced salt solution containing 50 mM KCl (and lacking a corresponding concentration of NaCl). The plates were incubated for 40 min at 37°C and the conditioned media were collected on ice, centrifuged at low speed to remove any floating cells, and then concentrated to 80 μl using a Microcon 10 device (Amicon) prior to the addition of 10 μl of 10 × Laemmli sample buffer (Laemmli, 1970). For cell extraction, the cells in each 35-mm well were washed twice in 5 ml of prewarmed PBS and homogenized in 300 μl of ice-cold Laemmli sample buffer. All samples were boiled for 5 min. Aliquots (70 μl) of each sample were subjected to Western blotting for PC1 as described previously. PC12 cells expressing wild-type and mutated forms of PC1 were grown in collagen-coated 35-mm wells in six-well plates to 80% confluence. The cells were rinsed and incubated in 1 ml of Met/Cys-free DMEM at for 30 min at 37°C, then labeled for 20 min at 37°C in 1 ml of Met/Cys-free DMEM containing 0.5 mCi of [35S]Pro-Mix (Amersham Corp.). They were then chased in 1 ml of regular DMEM with 2% fetal bovine serum for various periods of time as indicated. The cells and the conditioned media were transferred into ice-cold Eppendorf tubes and centrifuged at 2,000 rpm for 5 min at 4°C. The supernatants were saved, and the pellets were resuspended in 100 μl of boiling buffer containing 50 mM sodium phosphate, pH 7.4, 1% SDS, and 50 mM β-mercaptoethanol (Milgram and Mains, 1994). Both conditioned media and cell extracts were boiled for 5 min. One ml of AG buffer (Vindrola and Lindberg, 1992) was added to cell extracts (in 100 μl of boiling buffer), and the solution was centrifuged to remove any pellet. Aliquots (1 ml) of each sample were subjected to immunoprecipitation, SDS-PAGE analysis, and autoradiography as described previously (Vindrola and Lindberg, 1992). Pulse-chase experiments were repeated three times. The ability of transfected mPC1s to process endogenous prohormone was assessed by measuring the various forms of mature and unprocessed neurotensin-derived peptides in PC12 cell extracts. PC12 cells were cultured and induced in 10-cm plates as described above. After 48 h of induction, the cells were washed three times with 10 ml of Dulbecco's PBS, placed on ice, scraped into 1 ml of ice-cold 0.1 N HCl, and stored frozen at −70°C. Upon thawing, the samples were vortexed to produce a homogeneous suspension. A 100-μl aliquot of cell extract was removed for protein determination and Western blotting of mPC1 (Fig. 2). The remainder was centrifuged on a microcentrifuge for 5 min, and the supernatant was removed and lyophilized. For the experiment shown in Fig. 5, the cell pellet obtained after removal of the HCl was solubilized in various amounts (between 0.3 and 0.6 ml) of Laemmli sample buffer containing 5 M urea to obtain a constant protein concentration of 3 mg/ml. Fifty μl of each sample were subjected to Western blotting using the amino-terminal PC1 antiserum and the blot subjected to video densitometry for quantitation of the amount of immunoreactive PC1 in each dish. Proteins remaining in the gel following blotting were stained with Coomassie Blue; this procedure verified that the lanes had indeed been equally loaded with protein.Figure 5Analysis of proneurotensin processing in PC12 cells expressing various forms of PC1. Products of proneurotensin processing were detected by radioimmunoassay using specific radioimmunoassays directed against the epitopes indicated. A schematic diagram of proneurotensin is shown in panel a. Within each cell line, the percentage of each particular processing product is shown in panel b as a percentage of total proneurotensin. Numbers in parentheses refer to dibasic sites detected by each assay. Quantitation of total proneurotensin in each cell extract was determined by assaying iK6L following tryptic digestion (CTiK6L); the amounts of CTiK6L in PC12 Ctr (control), mPC1DB/PC12, mPC1/PC12, and mPC1ST/PC12 cells were 37.4 ± 4.9, 10.1 ± 0.9, 11.6 ± 2.5, and 34.5 ± 1.9 pmol/mg protein, respectively (mean ± S.E., n = 4).View Large Image Figure ViewerDownload Hi-res image Download (PPT) The rat proneurotensin precursor (Kislaukis et al., 1988) is depicted in Fig. 5a. It consists of a 169-residue polypeptide, which begins with a NH2-terminal signal peptide(1Ausubel F.M. Brent R. Kingston R.E. Moore D.D. Smith J.A. Seidman J.G. Struhl K. Current Protocols in Molecular Biology. John Wiley & Sons, New York1987Google Scholar, 2Benjannet S. Reudelhuber T. Mercure C. Rondeau N. Chretien M. Seidah N.G. J. Biol. Chem. 1992; 267: 11417-11423Abstract Full Text PDF PubMed Google Scholar, 3Bidard J. de Nadai F. Rovere C. Moinier D. Laur J. Martinez J. Cuber J. Kitabgi P. Biochem. J. 1993; 291: 225-233Crossref PubMed Scopus (34) Google Scholar, 4Bloomquist B.T. Eipper B.A. Mains R.E. Mol. Endocrinol. 1991; 5: 2014-2024Crossref PubMed Scopus (206) Google Scholar, 5Carraway R.E. Bullock B.P. Dobner P.R. Peptides. 1993; 14: 991-999Crossref PubMed Scopus (10) Google Scholar, 6Chun J.Y. Kerner J. Kreiner T. Scheller R.H. Axel R. Neuron. 1994; 12: 831-844Abstract Full Text PDF PubMed Scopus (47) Google Scholar, 7Goodman L.J. Gorman C.M. Biochem. Biophys. Res. Commun. 1994; 201: 795-804Crossref PubMed Scopus (45) Google Scholar, 8Jean F. Basak A. Rondeau N. Benjannet S. Hendy G.N. Seidah N.G. Chretien M. Lazure C. Biochem. J. 1993; 292: 891-900Crossref PubMed Scopus (87) Google Scholar, 9Kislaukis E. Bullock B. McNeil S. Dobner P.R. J. Biol. Chem. 1988; 263: 4963-4968Abstract Full Text PDF PubMed Google Scholar, 10Laemmli U.K. Nature. 1970; 227: 680-685Crossref PubMed Scopus (205531) Google Scholar, 11Lindberg I. Mol. Cell. Neurosci. 1994; 5: 263-268Crossref PubMed Scopus (51) Google Scholar, 12Lindberg I. Zhou Y. Methods Neurosci. 1995; 23: 94-108Crossref Scopus (21) Google Scholar, 13Lindberg I. Ahn S. Breslin M.B. Mol. Cell. Neurosci. 1994; 5: 614-622Crossref PubMed Scopus (27) Google Scholar, 14Loh Y.P. Loh Y.P. Mechanisms of Intracellular Trafficking and Processing of Proproteins. CRC Press, Boca Raton, FL1992: 181-183Google Scholar, 15Milgram S. Mains R.E. J. Cell Sci. 1994; 107: 737-745Crossref PubMed Google Scholar, 16Palmer D.J. Christie D.L. J. Biol. Chem. 1992; 267: 19806-19812Abstract Full Text PDF PubMed Google Scholar, 17Roth W.W. Mackin R.B. Noe B.D. Endocrine J. 1993; 1: 131-140Google Scholar, 18Rovere C. Nadai F.D. Bidard J. Cuber J. Kitabgi P. Peptides. 1993; 14: 982-989Crossref Scopus (17) Google Scholar, 19Rufaut N.W. Brennan S.O. Hakes D.J. Dixon J.E. Birch N.P. J. Biol. Chem. 1993; 268: 20291-20298Abstract Full Text PDF PubMed Google Scholar, 20Seidah N.G. Marcinkiewicz M. Benjannet S. Gaspar L. Beaubien G. Mattei M.G. Lazure C. Mbikay M. Chretien M. Mol. Endocrinol. 1991; 5: 111-122Crossref PubMed Scopus (405) Google Scholar, 21Seidah N.G. Hamelin J. Gaspar A.M. Day R. Chretien M. DNA Cell Biol. 1992; 11: 283-289Crossref PubMed Scopus (42) Google Scholar, 22Smeekens S.P. Avruch A.S. LaMendola J. Chan S.J. Steiner D.F. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 340-344Crossref PubMed Scopus (391) Google Scholar). Neurotensin is located near the COOH terminus of the precursor and is flanked by two Lys-Arg sequences at positions 148-149 and 163-164. Neurotensin is preceded by a neuromedin N sequence located between Lys140-Arg141 and Lys148-Arg149. A fourth Lys-Arg sequence occurs near the middle of the precursor at position 85-86. This doublet and the one at the NH2 terminus of neuromedin N delimit a 53-residue peptide, which starts with a neuromedin N-like sequence (Lys-Leu-Pro-Leu-Val-Leu, designated K6L) and ends with an acidic sequence (Glu-Lys-Glu-Glu-Val-Ile, designated E6I). The specificities of the neurotensin, neuromedin N, E6I, and K6L antisera used here have been described previously in detail (Bidard et al., 1993; Rovere et al., 1993). Briefly, the neurotensin and E6I antisera react with the free COOH termini, while the neuromedin N and K6L antisera recognize the free NH2 termini, of their respective antigens. These antisera cross-react poorly (<1%) with antigenic sequences that are internal to proneurotensin or proneurotensin fragments. Thus, the neurotensin antiserum will detect all precursor products with a COOH-terminal neurotensin sequence (including authentic neurotensin). Similarly, the E6I antiserum will measure all the precursor forms ending with the E6I sequence, while the neuromedin N and K6L antiserum will assay the precursor products bearing NH2-terminal neuromedin N and K6L sequences, respectively. The radioimmunoassay and reverse-phase HPLC procedures employed here to quantitate the various proneurotensin-derived peptides have been fully described elsewhere (Bidard et al., 1993; Rovere et al., 1993). All cell extracts were directly assayed for their content in immunoreactive neurotensin (iNT), E6I (iE6I), and K6L (iK6L). Because of the above-described antisera specificity, the iNT, iE6I, and iK6L assays measure the amounts of precursor products that are processed at the Lys163-Arg164, Lys140-Arg141, and Lys84-Arg85 sequences, respectively. Portions of the cell extracts were submitted to Arg-directed tryptic digestion (Bidard et al., 1993; Rovere et al., 1993) and then assayed for immunoreactive K6L. The value of CTiK6L thus obtained provides an index of the total amount of proneurotensin (either processed or unprocessed) that was synthesized and stored in the cells during the induction period. The remainder of the trypsin-treated samples was applied to reverse-phase HPLC, and the fractions were assayed for their immunoreactive neuromedin N (iNN) content. Previous studies have shown that trypsin-generated iNN can be resolved by HPLC into two peaks, one comigrating with synthetic neuromedin N and the other with neuromedin N bearing a COOH-terminal Lys-Arg extension (Bidard et al., 1993; Rovere et al., 1993). The latter peptide is produced by Arg-directed tryptic digestion of precursor forms in which the neuromedin N sequence is internal, whereas the former is generated by cleavage of peptides that end with a COOH-terminal neuromedin N sequence. Thus, the post-HPLC assay of trypsin-generated neuromedin N provides a measurement of all the precursor products that are processed at the Lys148-Arg149 sequence (including authentic neuromedin N). The results were normalized for the amount of protein in each extract. The percentages of cleavage at the Lys163-Arg164, Lys148-Arg149, Lys140-Arg141, and Lys84-Arg85 sequences were calculated by dividing, respectively, iNT, trypsin-generated neuromedin N, iE6I, and iK6L by CTiK6L and by multiplying these ratio values by 100. Duplicate independent samples were analyzed on two separate occasions. It has been documented that four paired basic residues exist within the mPC1 carboxyl-terminal region, however, only Arg590-Arg591 and Arg627-Arg628 are conserved across human, mouse, rat, and anglerfish PC1 (Seidah et al., 1991, 1992; Smeekens et al., 1991; Bloomquist et al., 1991; Roth et al., 1993). If 87-kDa PC1 is indeed cleaved at these two sites, the larger products should approximate 62 and 70 kDa, respectively; these molecular masses are close to the sizes of the PC1 forms obtained by spontaneous cleavage of 87-kDa PC1 (66 and 74 kDa; Zhou and Lindberg(1994)). Therefore, we theorized that these two sites could represent the cleavage sites for PC1 carboxyl-terminal processing. To test this hypothesis, site-directed mutagenesis reactions were carried out to convert Arg627-Arg628 to Lys-Ala or Arg590-Arg591 to Lys-His and Arg627-Arg628 to Lys-Ala, generating mPC1SB (single blockade at Arg627-Arg628) and mPC1DB (double blockade at both sites). A site-directed mutation to replace Gly592 with a stop codon was also performed to generate mPC1ST, the carboxyl-terminally truncated form of PC1. These mutations are diagrammed in Fig. 1. In order to determine the effect of the mutations at the mPC1 carboxyl-terminal region, the carboxyl-terminal processing of wild-type mPC1 and the mutated forms (mPC1SB, mPC1DB, and mPC1ST) were studied in PC12 cells stably transfected with the various constructs. Western blotting using PC1 amino-terminal antiserum revealed that wild-type mPC1 and mPC1SB were largely converted to a 66-kDa form, while the majority of mPC1DB still remained as 87-kDa PC1 (Fig. 2), suggesting that mutation at Arg590-Arg591 (but not mutation at Arg627-Arg628) could effectively block PC1 carboxyl-terminal conversion. Western blotting also showed that mPC1ST (truncated at residues 592) exhibited a molecular mass identical to that of the 66-kDa PC1, and this form did not undergo any further carboxyl-terminal cleavage. Following stimulation of PC12 cells with 50 mM KCl, wild-type mPC1 as well as mPC1DB and mPC1ST could be released from regulated secretory pathway (Fig. 3). These data indicate that neither deletion of the carboxyl-terminal region (residues 592 to 726) nor mutations at Arg590-Arg591 and Arg627-Arg628 could block PC1 targeting into the regulated secretory pathway. It was noted that a small portion of mPC1DB was still cleaved, but that the cleavage product possessed a molecular mass slightly larger than that of cleaved wild-type mPC1, indicative of the involvement of an alternative cleavage site. In pulse-chase labeling experiments, we found that amino-terminal conversions of pro-mPC1DB and pro-mPC1ST were completed within the first 20 min of synthesis, similar to wild-type pro-mPC1 (data not shown). This result indicates that substitution of Arg590-Arg591 and Arg627-Arg628 or deletion of the PC1 carboxyl-terminal region (residues 592-726) apparently had little effect on proPC1 conversion. During the later stages of biosynthesis, intracellular 87-kDa mPC1DB remained intact after a 4-h chase period, while wild-type PC1 was converted to the 66-kDa form (Fig. 4). Constitutive secretion of the 87-kDa form of both mPC1DB and wild-type PC1 occurred within 1 h after the pulse period (Fig. 4). Similar studies of the mPC1ST mutant demonstrated efficient synthesis of a 66-kDa form of PC1 and constitutive secretion into the medium over the same time period (results not shown). PC12 cells are known to greatly increase their synthesis of proneurotensin under inducing conditions (Rovere et al., 1993). We have shown previously that transfection of mPC1 into PC12 cells can promote proneurotensin processing (Lindberg et al., 1994). To determine the physiological function of the mutated mPC1DB and mPC1ST, we compared the maturation of proneurotensin synthesized in PC12 cells transfected with the mutated mPC1DB and mPC1ST with that in PC12 cells expressing wild-type mPC1 and untransfected PC12 cells. It should be noted that the various cell lines expressed varying amounts of mPC1, with the double blockade mutant (mPC1DB) exhibiting the highest expression, and mPC1ST exhibiting the lowest. The relative amounts of PC1 in the cells used for this experiment were estimated by Western blotting a constant amount of protein from each dish used for neurotensin analysis and performing video densitometry of the blot. The ratios of the amount of mutant mPC1 (all forms) to wild-type mPC1 were approximately 0.3 (mPC1ST) and 9 (mPC1DB). As expected, untransfected PC12 cells exhibited no PC1 immunoreactivity. The amount of proneurotensin (processed and unprocessed) stored in the various cell lines during the induction period ranged between approximately 10 and 30 pmol/mg protein (CTiK6L values are given in the legend to Fig. 5). Similarly to wild-type mPC1, mPC1DB and mPC1ST both possessed the ability to process proneurotensin (Fig. 5). All forms of PC1 markedly increased proneurotensin cleavage at the Lys140-Arg141 and Lys148-Arg149 dibasic sites as compared to the control (Fig. 5b). They also cleaved, though less efficiently, the Lys84-Arg85 site. This cleavage event was not observed in control PC12 cells. Only mPC1 and mPC1ST were able to increase processing at the Lys163-Arg164 site above the level seen in the control, whereas mPC1DB appeared inactive in that respect. In general, and especially given the fact that it had the highest level of expression, mPC1DB was much less efficient in processing proneurotensin than mPC1 and mPC1ST. Interestingly, mPC1ST, the enzyme expressed at the lowest level, was the most active in processing the Lys163-Arg164 site, in contrast to mPC1DB which apparently did not cleave this dibasic despite its high level of expression. Thus, there appear to be certain differences in proneurotensin processing efficiency and site usage between the 66- and 87-kDa PC1 forms. PC1 is known to be cleaved within its carboxyl-terminal region at a late stage of its biosynthesis (Vindrola and Lindberg, 1992). Previous work has suggested that an autocatalytic mechanism may be involved in this process (Zhou and Lindberg, 1994); however, the site of this carboxyl-terminal cleavage event has not yet been identified. In this work, we have assumed that this cleavage occurs at a paired basic site, since these are known to represent consensus sequences for PC1 cleavage. Four paired basic residues are located in the mPC1 carboxyl-terminal region; cleavage at these sites can generate products with estimated molecular masses between 62 and 73 kDa. However, among these sites, only Arg590-Arg591 and Arg627-Arg628 are conserved among the PC1 sequences of human, rat, mouse, and anglerfish (Seidah et al., 1991, 1992; Smeekens et al., 1991; Bloomquist et al., 1991; Roth et al., 1993); thus, these two sites were thought to represent likely candidate sites for PC1 carboxyl-terminal cleavage. By performing site-directed mutagenesis at these two sites, we found that mutation of Arg627-Arg628 alone had little effect on the generation of the 66-kDa form, while mutations of both Arg590-Arg591 and Arg627-Arg628 were able to block the conversion of 87-kDa PC1 to the 66-kDa form. Furthermore, mPC1ST (truncation at Gly592) exhibited a molecular mass on SDS-PAGE identical to that of endogenous 66-kDa PC1 converted from the 87-kDa wild-type mPC1. Subsequent to the generation of our mutants, the sequence of Aplysia PC1 was published (Chun et al., 1994); PC1a from this species contains the first of these dibasics, but not the second. Taken together, these data strongly suggest that Arg590-Arg591 is the major cleavage site for the generation of 66-kDa PC1 in vivo. Since wild-type PC12 cells possess a regulated secretory pathway, but lack the ability to process prohormones at paired basic residues, PC1 cleavage at Arg590-Arg591 within PC12 cells is likely to be attributable to an autocatalytic mechanism. This interpretation is supported by our in vitro work (Zhou and Lindberg, 1994) and in vivo results obtained in AtT-20 cells, which indicate that overexpression of PC1 results in increased COOH-terminal proteolytic processing (Zhou and Mains, 1994a). The presence of an intermediate form of PC1 of approximately 74 kDa has been observed in in vitro studies (Zhou and Lindberg, 1994); however, little 74-kDa mPC1 is found in PC12 cells (this study) or in AtT-20 cells (Vindrola and Lindberg, 1992; Milgram and Mains, 1994). Taken together with the finding of lesser effects of the mutation of Arg627-Arg628 in PC12 cells, these results suggest that 74-kDa PC1 may represent a minor product during the carboxyl-terminal processing of PC1 in vivo. A possible explanation for these differences is that the cleavage site generating the 74-kDa form is blocked in vivo, possibly due to an association of PC1 carboxyl-terminal region with membrane or with other proteins. The association of PC1 with membranes and association with other granule proteins have both been reported (Vindrola and Lindberg, 1992; Palmer and Christie, 1992). Although mutation at Arg590-Arg591 and Arg627-Arg628 substantially blocked PC1 carboxyl-terminal conversion, a small portion of 87-kDa mPC1DB was still cleaved. The product was slightly larger than wild-type 66-kDa PC1 on SDS-PAGE, suggesting that an alternative cleavage site is involved. Through limited digestion using chymotrypsin, trypsin, and subtilisin, which possess different substrate specificities, we found that all three proteinases were able to convert 87-kDa recombinant PC1 to 66- and 74-kDa-like products in its carboxyl-terminal region (Zhou and Lindberg, 1994). These results suggest that the cleavage site in the PC1 carboxyl terminus is located in an exposed region which can readily be attacked. Therefore, the alternative cleavage site usage in PC12 cells may be due to the action of other proteinases located in the regulated secretory pathway. Alternatively, PC1 itself may also act at alternative cleavage sites. This idea is supported by our in vitro studies that demonstrate spontaneous carboxyl-terminal cleavage of 87-kDa mPC1DB purified from Chinese hamster ovary cells amplified for the production of this protein (results not shown). A possible alternative cleavage site for transfected mPC1 may be Lys602-Arg603, although this site is present only in mouse PC1. Cleavage at the Lys602-Arg603 site can generate a product 12 amino acids longer than the product cleaved at Arg590-Arg591 (66-kDa PC1); this molecular mass is also consistent with our observed molecular masses on SDS-PAGE. Construction of a PC1 vector encoding a further mutation at Lys602-Arg603 will be required to investigate this possibility. Carboxyl-terminal conversion of PC1 occurs mainly in regulated secretory granules, as evidenced by previous studies in AtT-20 and PC12 cells (Vindrola and Lindberg, 1992; Benjannet et al., 1992; Lindberg et al., 1994; Lindberg, 1994; Milgram and Mains, 1994; Zhou and Mains, 1994a). However, the functional significance of this conversion event is not clear. The activation of proPC1 occurs within the endoplasmic reticulum (Lindberg, 1994; Milgram and Mains, 1994; Goodman and Gorman, 1994), while peptide hormone precursors are thought to be cleaved within the later stages of the secretory pathway (reviewed by Loh et al.(1992)). It may thus be necessary for the cell to regulate PC1 function during intracellular transport. The decreasing pH gradient from the endoplasmic reticulum to the secretory granules may represent one important aspect of this regulation. The pH within the trans-Golgi network and regulated secretory granules corresponds well with the optimal pH of PC1 activity, between 5.0 and 6.5 (Zhou and Lindberg, 1993; Jean et al., 1993; Rufaut et al., 1993). On the other hand, since the timing and location of PC1 carboxyl-terminal processing coincide with the timing and location of prohormone processing, truncation of PC1 may also play a role in the regulation of enzyme activity. In vitro studies have shown that that carboxyl-terminal cleavage of PC1 dramatically increases PC1 activity against peptide and prohormone substrates (Zhou and Lindberg, 1994), suggesting that carboxyl-terminal cleavage of PC1 could potentially possess physiological significance. In order to determine the function of the PC1 carboxyl-terminal region in vivo we compared the processing of proneurotensin in PC12 cells stably transfected with wild-type mPC1, mutated mPC1DB, or mPC1ST. In contrast to AtT-20 cells, PC12 cells do not express prohormone convertases; thus, neurotensin is stored mainly in precursor form (Carraway et al., 1993; Rovere et al., 1993). When the varying expression levels are taken into account, the 66-kDa form of PC1 was found to be the most active against proneurotensin, especially relative to the 87-kDa blockade mutant, which was expressed at much higher levels. The comparatively low activity of mPC1DB against proneurotensin confirms our previous in vitro results, which indicate that the 87-kDa PC1 represents only a partially active PC1 form (Zhou and Lindberg, 1994); removal of the carboxyl-terminal region appears to be required to fully activate PC1. The finding that the various forms of PC1 are differentially active against proneurotensin supports our in vitro results showing that the 87- and 74/66-kDa recombinant PC1s exhibit differing specific activities (Zhou and Lindberg, 1994). A recent study has also demonstrated that expression of carboxyl-terminally truncated PC1 (StopD616) in AtT-20 cells increases the rate of conversion of proopiomelanocortin (Zhou and Mains, 1994b). Based on these in vivo and in vitro observations, we speculate that carboxyl-terminal processing of PC1 during transport through the secretory pathway may control the amount of PC1 activity available for the processing of prohormones. In conclusion, we have demonstrated that PC1 carboxyl-terminal conversion largely occurs at Arg590-Arg591 site through a possible autocatalytic mechanism; the PC1 carboxyl-terminal domain (Gly592 to Asn726) is required neither for activation nor for intracellular transport of PC1 to secretory granules. However, removal of this domain appears to increase total PC1 activity and alters cleavage site preference. In line with our previous in vitro data, these results support the idea that carboxyl-terminal processing is important for the regulation of PC1 function. We are grateful to L. Elferink and R. Scheller for supplying PC12 cells and N. G. Seidah for Rc/CMV encoding mPC1. We thank J. F. Finley for expert assistance with cell culture and C. Bui for capable assistance with Western blots." @default.
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