Autocatalytic maturation of the prohormone convertase PC2

GGA function is required for maturation of neuroendocrine secretory granules

Secretory granule (SG) maturation has been proposed to involve formation of clathrin-coated vesicles (CCVs) from immature SGs (ISGs). We tested the effect of inhibiting CCV budding by using the clathrin adaptor GGA (Golgi-associated, gamma-ear-containing, ADP-ribosylation factor-binding protein) on SG maturation in neuroendocrine cells. Overexpression of a truncated, GFP-tagged GGA, VHS (Vps27, Hrs, Stam)-GAT (GGA and target of myb (TOM))-GFP led to retention of MPR, VAMP4, and syntaxin 6 in mature SGs (MSGs), suggesting that CCV budding from ISGs is inhibited by the SG-localizing VHS-GAT-GFP. Furthermore, VHS-GAT-GFP-overexpression disrupts prohormone convertase 2 (PC2) autocatalytic cleavage, processing of secretogranin II to its product p18, and the correlation between PC2 and p18 levels. All these effects were not observed if full-length GGA1-GFP was overexpressed. Neither GGA1-GFP nor VHS-GAT-GFP perturbed SG protein budding from the TGN, or homotypic fusion of ISGs. Reducing GGA3 levels by using short interfering (si)RNA also led to VAMP4 retention in SGs, and inhibition of PC2 activity. Our results suggest that inhibition of CCV budding from ISGs downregulates the sorting from the ISGs and perturbs the intragranular activity of PC2.

Xenopus egg extracts: a model system to study proprotein convertases

The Xenopus egg extract translation system has proved an ideal tool with which to study the biosynthesis of the prohormone convertases. It provides a robust coupled translation/translocation system capable of efficient translocation of any protein containing an N-terminal signal sequence into the lumen of its microsomal membranes, with cotranslational cleavage of the signal peptide. Its main advantage over rival in vitro translation systems is that it will also carry out posttranslational modification of proteins, such as N-glycosylation, and, in the case of the proprotein convertases, support autocatalytic proregion removal. The egg extract also contains an endogenous, acidic pH optimum enzyme activity, suggestive of a proprotein convertase, that can undertake limited proteolysis of precursors containing multibasic processing sites.

Functional characterization of Narc 1, a novel proteinase related to proteinase K

The NARC 1 gene encodes a novel proteinase K family proteinase. The domain structure of rat Narc 1 resembles that of the subtilisin-like proprotein convertases (SPCs), except that rNarc 1 lacks the canonical P-domain of SPCs, retaining only the RGD motif as part of what might be a cryptically functioning P-domain. Narc 1 undergoes autocatalytic intramolecular processing at the site LVFAQ/, resulting in the cleavage of its prosegment and the generation of an active proteinase with a broad alkaline pH optimum and no apparent calcium requirement for activity. Both primary and secondary structural determinants influence Narc 1 substrate recognition. Our functional characterization of Narc 1 reinforces the inference drawn from the analysis of its predicted structure that this enzyme is most closely related to representatives of the proteinase K family, but that it is also sufficiently different to warrant its possible classification in a separate sub-family.

pH-induced conformational transitions of a molten-globule-like state of the inhibitory prodomain of furin: implications for zymogen activation

The endoprotease furin, which belongs to the family of mammalian proprotein convertase (PC), is synthesized as a zymogen with an N-terminal, 81-residue inhibitory prodomain. It has been shown that the proenzyme form of furin undergoes a multistep 'autocatalytic' removal of the prodomain at the C-terminal side of the two consensus sites, R(78)-T-K-R(81) approximately and R(44)-G-V-T-K-R(49) approximately. The furin-mediated cleavage at R(44)-G-V-T-K-R(49) approximately, in particular, is significantly accelerated in an 'acidic' environment. Here, we show that under neutral pH conditions, the inhibitory prodomain of furin is partially folded and undergoes conformational exchanges as indicated by extensive broadening of the NMR spectra. Presence of many ring-current shifted methyl resonances suggests that the partially folded state of the prodomain may still possess a 'semirigid' protein core with specific packing interactions among amino acid side chains. Measurements of the hydrodynamic radii and compaction factors indicate that this partially folded state is significantly more compact than a random chain. The conformational stability of the prodomain appears to be pH sensitive, in that the prodomain undergoes an unfolding transition towards acidic conditions. Our NMR analyses establish that the acid-induced unfolding is mainly experienced by the residues from the C-terminal half of the prodomain (residues R(44)-R(81)) that contains the two furin cleavage sites. A 38-residue peptide fragment derived from the entire pH-sensitive C-terminal region (residues R(44)-R(81)) does not exhibit any exchange-induced line broadening and adopts flexible conformations. We propose that at neutral pH, the cleavage site R(44)-G-V-T-K-R(49) approximately is buried within the protein core that is formed in part by residues from the N-terminal region, and that the cleavage site becomes exposed under acidic conditions, leading to a facile cleavage by the furin enzyme.

Processing and sorting of the prohormone convertase 2 propeptide

The prohormone convertases (PCs) are synthesized as zymogens whose propeptides contain several multibasic sites. In this study, we investigated the processing of the PC2 propeptide and its function in the regulation of PC2 activity. By using purified pro-PC2 and directed mutagenesis, we found that the propeptide is first cleaved at the multibasic site separating it from the catalytic domain (primary cleavage site); the intact propeptide thus generated is then sequentially processed at two internal sites. Unlike the mechanism described for furin, our mutagenesis studies show that internal cleavage of the propeptide is not required for activation of pro-PC2. In addition, we identified a point mutation in the primary cleavage site that does not prevent the folding nor the processing of the zymogen but nevertheless results in the generation of an inactive PC2 species. These data suggest that the propeptide cleavage site is directly involved in the folding of the catalytic site. By using synthetic peptides, we found that a PC2 propeptide fragment inhibits PC2 activity, and we identified the inhibitory site as the peptide sequence containing basic residues at the extreme carboxyl terminus of the primary cleavage site. Finally, our study supplies information concerning the intracellular fate of a convertase propeptide by providing evidence that the PC2 propeptide is generated and is internally processed within the secretory granules. In agreement with this localization, an internally cleaved propeptide fragment could be released by stimulated secretion.

Binding of BiP to the processing enzyme lymphoma proprotein convertase prevents aggregation, but slows down maturation

Lymphoma proprotein convertase (LPC) is a subtilisin-like serine protease of the mammalian proprotein convertase family. It is synthesized as an inactive precursor protein, and propeptide cleavage occurs via intramolecular cleavage in the endoplasmic reticulum. In contrast to other convertases like furin and proprotein convertase-1, propeptide cleavage occurs slowly. Also, both a glycosylated and an unglycosylated precursor are detected. Here we demonstrate that the unglycosylated precursor form of LPC is localized in the cytosol due to the absence of a signal peptide. Using a reducible cross-linker, we found that glycosylated pro-LPC is associated with the molecular chaperone BiP. In addition, we show that pro-LPC is prone to aggregation and forms large complexes linked via interchain disulfide bonds. BiP is associated mainly with non-aggregated pro-LPC and pro-LPC dimers and trimers, suggesting that BiP prevents aggregation. Overexpression of wild-type BiP or a dominant-negative BiP ATPase mutant resulted in reduced processing of pro-LPC. Taken together, these results suggest that binding of BiP to pro-LPC prevents aggregation, but results in slower maturation.

A temperature-sensitive Krp1 allows in vivo characterization of kexin activation

Members of the kexin family of processing enzymes are responsible for the cleavage of many proproteins during their transport through the secretory pathway. The enzymes are themselves made as inactive precursors and we have investigated the activation of Krp1, a kexin from the fission yeast Schizosaccharomyces pombe. As Krp1 is essential for cell growth, we have used a krp1ts strain to investigate the role of the prosequence in the activation process. Mutations that reduce either the efficiency with which the prosequence is released or the rate at which the released prosegment is subsequently cleaved at an internal site are less active when assayed in vivo. We also show that prosegments lacking an internal dibasic motif can act as autoinhibitors and prevent activation of the catalytic fragment. Krp1 constructs containing prosequences based on these inhibitors do not become active in vitro. Surprisingly, the same constructs do become active in the intact cell and appear to suggest that alternative activation processes can be used by these enzymes.

Mutations in the catalytic domain of prohormone convertase 2 result in decreased binding to 7B2 and loss of inhibition with 7B2 C-terminal peptide

Prohormone convertases 1 (PC1) and 2 (PC2) are members of a family of subtilisin-like proprotein convertases responsible for proteolytic maturation of a number of different prohormones and proneuropeptides. Although sharing more than 50% homology in their catalytic domains, PC1 and PC2 exhibit differences in substrate specificity and susceptibility to inhibitors. In addition to these differences, PC2, unlike PC1 and other members of the family, specifically binds the neuroendocrine protein 7B2. In order to identify determinants responsible for the specific properties of the PC2 catalytic domain, we compared its primary sequence with that of other PCs. This allowed us to distinguish a PC2-specific sequence at positions 242-248. We constructed two PC2 mutants in which residues 242 and 243 and residues 242-248 were replaced with the corresponding residues of PC1. Studies of in vivo cleavage of proenkephalin, in vivo production of alpha-MSH from proopiomelanocortin, and in vitro cleavage of a PC2-specific artificial substrate by mutant PC2s did not reveal profound alterations. On the other hand, both mutant pro-PC2s exhibited a considerably reduced ability to bind to 21-kDa 7B2. In addition, inhibition of mutant PC2-(242-248) by the potent natural inhibitor 7B2 CT peptide was almost completely abolished. Taken together, our results show that residues 242-248 do not play a significant role in defining the substrate specificity of PC2 but do contribute greatly to binding 7B2 and are critical for inhibition with the 7B2 CT peptide.

Maturation and specificity of Plasmodium falciparum subtilisin-like protease-1, a malaria merozoite subtilisin-like serine protease

Plasmodium falciparum subtilisin-like protease-1 (PfSUB-1) is a protein belonging to the subtilisin-like superfamily of serine proteases (subtilases). PfSUB-1 undergoes extensive posttranslational proteolytic processing. The primary translation product is converted in the parasite endoplasmic reticulum to p54. This is further processed to p47, which accumulates in secretory organelles within the merozoite. Here, we present a detailed study of this processing. In vitro translated PfSUB-1 showed no capacity to undergo autocatalytic processing. However, parasite extracts contain a protease that cleaves the in vitro translated proprotein between Asp(219) and Asn(220) to form two products of 31 (p31) and 54 kDa; the latter was indistinguishable from authentic p54 and remained complexed with p31 in a noncovalent interaction characteristic of that between a subtilase prodomain and its cognate catalytic domain. Cross-linking studies showed that this complex also exists in the parasite. Expression of PfSUB-1 in recombinant baculovirus also resulted in processing to p54. Mutation of the predicted active site serine abolished processing. Recombinant p54 was secreted in a complex with p31, and could be further converted to p47 in vitro. Conversion required calcium, was an intramolecular autocatalytic process, and involved a second cleavage between Asp(251) and Ala(252). A decapeptide based on sequence flanking Asp(219) was efficiently cleaved by recombinant PfSUB-1. We conclude that PfSUB-1 is a subtilase with an unusual substrate specificity and that it is activated by two autocatalytic processing steps.

The subtilisin/kexin family of precursor convertases. Emphasis on PC1, PC2/7B2, POMC and the novel enzyme SKI-1

Proopiomelanocortin (POMC) is a precursor to various, bioactive peptides including ACTH, beta LPH, alpha MSH, and beta endorphin (beta END). Processing of POMC at dibasic residues is tissue-specific and is performed by either PC1 alone (resulting in ACTH and beta LPH, anterior pituitary corticotrophes) or by a combination of PC1 and PC2 (yielding alpha MSH and beta END, pituitary neurointermediate lobe and hypothalamus). The PC2-specific binding protein 7B2 is intimately involved in the zymogen activation of proPC2 into PC2. Structure-function studies of these enzymes demonstrated the presence of N- and C-terminal domains, as well as specific amino acids within the catalytic segment that influence the degree of activity of each enzyme and the interaction of PC2 with 7B2. The tissue distribution, plasticity of expression, and the multiple precursors that are differentially cleaved by PC1 and/or PC2, predict a wide array of combinatorial activities of these convertases within the endocrine and neuroendocrine system. The phenotypic consequences of the absence of genetic expression of either PC1 or PC2 are now explored using knockout mice and in human patients suffering from obesity and diabetes.

The cell biology of the prohormone convertases PC1 and PC2

Mature peptide hormones and neuropeptides are typically synthesized from much larger precursors and require several posttranslational processing steps--including proteolytic cleavage--for the formation of the bioactive species. The subtilisin-related proteolytic enzymes that accomplish neuroendocrine-specific cleavages are known as prohormone convertases 1 and 2 (PC1 and PC2). The cell biology of these proteases within the regulated secretory pathway of neuroendocrine cells is complex, and they are themselves initially synthesized as inactive precursor molecules. ProPC1 propeptide cleavage occurs rapidly in the endoplasmic reticulum, yet its major site of action on prohormones takes place later in the secretory pathway. PC1 undergoes an interesting carboxyl terminal processing event whose function appears to be to activate the enzyme. ProPC2, on the other hand, exhibits comparatively long initial folding times and exits the endoplasmic reticulum without propeptide cleavage, in association with the neuroendocrine-specific protein 7B2. Once the proPC2/7B2 complex arrives at the trans-Golgi network, 7B2 is internally cleaved into two domains, the 21-kDa fragment and a carboxy-terminal 31 residue peptide. PC2 propeptide removal occurs in the maturing secretory granule, most likely through autocatalysis, and 7B2 association does not appear to be directly required for this cleavage event. However, if proPC2 has not encountered 7B2 intracellularly, it cannot generate a catalytically active mature species. The molecular mechanism behind the intriguing intracellular association of 7B2 and proPC2 is still unknown, but may involve conformational rearrangement or stabilization of a proPC2 conformer mediated by a 36-residue internal segment of 21-kDa 7B2.

Expression of human prohormone convertase PC2 in a baculovirus-insect cell system

PC2 is a member of the eukaryotic family of subtilisin-related proprotein convertases which are thought to be involved in the intracellular proteolytic processing of prohormones and proneuropeptides. The presence of only small amounts of PC2 in the secretory granules of certain mammalian neuroendocrine cell types has made the characterization and further study of this enzyme difficult. We report here the expression of proteolytically active human PC2 protein in the insect cell-baculovirus system. Human PC2 expressed in insect cells is a calcium-dependent intracellular protein active at neutral pH. In insect cells, human PC2 was found intracellularly as 75-kDa and 71-kDa proteins. Both 73-kDa and 68-kDa forms were found in the conditioned medium, but no PC2 proteolytic activity was detected. We demonstrated the presence of a soluble inhibitor in infected-cell medium which may block PC2 activity.

The proteolytic maturation of prohormone convertase 2 (PC2) is a pH-driven process

Recombinant proPC2 purified from the medium of CHO cells overexpressing both the prohormone convertase (PC) precursor proPC2 and the 21-kDa amino terminal portion of the neuroendocrine protein 7B2 can spontaneously convert to an active species. In the present report, we have characterized the proPC2 zymogen conversion process. Sequencing of the mature 66 kDa enzyme revealed a single site of cleavage at the paired basic site amino terminal to the GYRDI sequence. In contrast to mature PC2 activity, proPC2 conversion was inhibited neither by the eukaryotic subtilisin inhibitor pCMS nor by the specific PC2 inhibitor, 7B2 CT peptide, suggesting significant differences between the proPC2 conversion reaction and the hydrolysis of synthetic substrates by mature PC2. In support of this idea, proPC2 conversion was not calcium dependent and was unaffected by 5 mM EDTA. The rate of conversion of proPC2 remained similar with a 10-fold difference in zymogen concentration, implicating an intramolecular rather than intermolecular mechanism of activation. Interestingly, the rate of proPC2 conversion was extremely pH dependent, occurring most extensively between pHs 4.0 and 4.9. Taken together, our results suggest that cellular proPC2 maturation occurs via an autocatalytic, intramolecular process controlled not by 7B2 inhibition nor by calcium levels, but by the decreasing pH gradient along the secretory pathway.

Biosynthesis of secretogranin II in Xenopus intermediate pituitary

Secretogranin II (SgII) is a sulphated secretory protein found in a broad variety of neuroendocrine cells. We have raised an antiserum against SgII to monitor its fate in Xenopus intermediate pituitary. Pulse-chase incubations in combination with immunoprecipitation analysis showed that SgII was synthesised as an 84-kDa precursor protein which was processed to fragments of 69, 54, 34, 21 and 15 kDa. Secretion of these cleavage products was sensitive to the dopamine D2 receptor agonist apomorphine, and thus occurred via the regulated secretory pathway. When cells were treated with the fungal metabolite brefeldin A or with the specific vacuolar H+-ATPase inhibitor bafilomycin A1, the processing of SgII and the release of its cleavage products were strongly inhibited, indicating that its processing commenced in the later compartments of the secretory pathway. Pulse-chase and immunoblot analysis showed that the 21-kDa fragment was the major SgII-derived cleavage and release product, and carried secretoneurin, a highly conserved peptide flanked by potential dibasic processing sites. Hence, SgII is cleaved to a variety of products that are released via the regulated secretory pathway, while secretoneurin does not seem to represent a major end-product of SgII processing in Xenopus intermediate pituitary.

Secretion and pH-dependent self-processing of the pro-form of the Yarrowia lipolytica acid extracellular protease

The secretion and maturation of the acid extracellular protease (AXP) of the yeast Yarrowia lipolytica have been characterized using antiserum raised against this enzyme. A 42 kDa pro-enzyme form of AXP was identified from lysates of radiolabelled Y. lipolytica cells and found to contain no N-linked carbohydrate moieties. Using pulse-chase immune precipitation it was demonstrated that the AXP precursor was secreted into the extracellular medium where, under conditions of low pH, it underwent autocatalytic activation forming the mature enzyme. Conversion of the AXP pro-form in the presence of the protease inhibitor pepstatin indicated that an intramolecularly-catalysed reaction mechanism was involved in AXP maturation. Further evidence supporting the role of autocatalytic processing came from the side-chain specificity of mature AXP towards the oxidized B-chain of insulin.

Specificity of prohormone convertase 2 on proenkephalin and proenkephalin-related substrates

In the central and peripheral nervous systems, the neuropeptide precursor proenkephalin must be endoproteolytically cleaved by enzymes known as prohormone convertases 1 and 2 (PC1 and PC2) to generate opioid-active enkephalins. In this study, we have investigated the specificity of recombinant mouse PC2 for proenkephalin-related internally quenched (IQ) peptides, for methylcoumarin amide-based fluorogenic peptides, and for recombinant rat proenkephalin. IQ peptides exhibited specificity constants (kcat/Km) between 9.4 x 10(4) M-1 s-1 (Abz-Val-Pro-Arg-Met-Glu-Lys-Arg-Tyr-Gly-Gly-Phe-Met-Gln-EDDnp+ ++; where Abz is ortho-aminobenzoic acid and EDDnp is N-(2, 4-dinitrophenyl)ethylenediamine)) and 0.24 x 10(4) M-1 s-1 (Abz-Tyr-Gly-Gly-Phe-Met-Arg-Arg-Val-Gly-Arg-Pro-Glu-EDDnp), with the peptide B to Met-enk-Arg-Phe cleavage preferred (Met-enk is met-enkephalin). Fluorogenic substrates with P1, P2, and P4 basic amino acids were hydrolyzed with specificity constants ranging between 2.0 x 10(3) M-1 s-1 (Ac-Orn-Ser-Lys-Arg-MCA; where MCA is methylcoumarin amide) and 1.8 x 10(4) M-1 s-1 (<Glu-Arg-Thr-Lys-Arg-MCA; where <Glu is pyroglutamic acid). Substrates containing only a single basic residue were not appreciably hydrolyzed, and substrates lacking a P4 Arg exhibited kcat of less than 0.05 s-1. Substitution of ornithine for Lys at the P4 position did not significantly affect the kcat but increased the Km 2-fold. Data from both sets of fluorogenic substrates supported the contribution of a P4 Arg to PC2 preference. Analysis of proenkephalin reaction products using immunoblotting and gel permeation chromatography demonstrated that PC2 can directly cleave proenkephalin and that the generation of small opioid peptides from intermediates is mediated almost entirely by PC2 rather than by PC1. These results are in accord with the analysis of PC2 knock-out brains, in which the amounts of three mature enkephalins were depleted by more than three-quarters.

Biosynthetic processing and quaternary interactions of proprotein convertase SPC4 (PACE4)

SPC4 (PACE4), a member of the eukaryotic family of subtilisin-like proprotein convertases, is synthesized as a proenzyme (proSPC4) which undergoes proteolytic removal of N-terminal propeptide during transit through the secretory pathway. As this propeptide processing seems to be a key event in the functional expression of SPC4, we have investigated its mechanism and the intracellular site where it occurs. In transfected fibroblast cells, the 110-kDa proSPC4 undergoes slow cleavage to generate a 103-kDa mature enzyme in the endoplasmic reticulum (ER). Site-directed mutagenesis studies demonstrate that the proteolytic activation of SPC4 occurs mainly through a unimolecular autocatalytic process and propeptide cleavage is a prerequisite for its export from the ER. Sedimentation velocity and chemical cross-linking analysis demonstrate that the precursor protein in the cells exists as both a monomer and a dimer-sized complex whereas mature SPC4 exists only as a monomer. These results suggest that the cleavage of the N-terminal propeptide of SPC4 plays a regulatory role in its activation and secretion through the change in its oligomeric state.

Studies of endothelin-converting enzyme in bovine endothelial cells and vascular smooth-muscle cells: further characterization of the biosynthetic process

Endothelin-1 (ET-1) is synthesized by a number of cell types, including endothelial, epithelial, and smooth muscle cells. Initial biosynthesis occurs as a protein precursor, preproendothelin-1 (preproET-1). This is processed intracellularly to the inactive intermediate big ET-1, which is hydrolyzed by endothelin-converting enzyme (ECE) to generate ET-1, but the precise identity of the physiologically relevant ECE has yet to be confirmed. Although ET-1 is synthesized in the constitutive secretory pathway, many features of the selective processing of proET-1 are comparable to those of peptide hormones. We describe here experimental investigations aimed at defining the regulation of ET-1 synthesis and its relationship to the biosynthesis of ECE.

Residues unique to the pro-hormone convertase PC2 modulate its autoactivation, binding to 7B2 and enzymatic activity

The prohormone convertase PC2 is one of the major subtilisin/kexin-like enzymes responsible for the formation of small bioactive peptides in neural and endocrine cells. This convertase is unique among the members of the subtilisin/kexin-like mammalian serine proteinase family in that it undergoes zymogen processing of its inactive precursor proPC2 late along the secretory pathway and requires the help of a PC2-specific binding protein known as 7B2. We hypothesized that some of these unique properties of PC2 are dictated by the presence of PC2-specific amino acids, which in the six other known mammalian convertases are otherwise conserved but distinct. Accordingly, six sites were identified within the catalytic segment of PC2. Herein we report on the site-directed mutagenesis of Tyr194 and of the oxyanion hole Asp309 and the consequences of such mutations on the cellular expression and enzyme activity of PC2. The data show that the Y194D mutation markedly increases the ex vivo ability of PC2 to process proopiomelanocortin (POMC) into beta-endorphin in cells devoid of 7B2, e.g. BSC40 cells. In these cells, expression of native PC2 does not result in the secretion of measurable in vitro activity against a pentapeptide fluorogenic substrate. In contrast, secreted Y194D-PC2 exhibited significant enzymatic activity, even in the absence of 7B2. Based on co-immunoprecipitations and Western blots, binding assays indicate that Tyr194 participates in the interaction of PC2 with 7B2, and that the oxyanion hole Asp309 is critical for the binding of proPC2 with pro7B2.

Structural elements of PC2 required for interaction with its helper protein 7B2

The structures of the eukaryotic subtilisin protease family members can be divided into four distinct domains as follows: the proregion, the catalytic domain, the P domain, and the carboxyl-terminal region. Although these enzymes are evolutionarily related, only prohormone convertase 2 (PC2) requires 7B2 for activation. To examine the potential contribution of each domain of PC2 to PC2-7B2 interactions, we performed sequential deletions, site-directed mutagenesis, and domain swapping to replace individual domains or particular amino acids of pro-PC2 with the corresponding segments/amino acids of pro-PC1. These chimeras and mutant enzyme molecules were then expressed in AtT-20 cells and analyzed for 7B2 binding, maturation ability, and enzymatic activity. The results revealed that 1) the PC2 proregion is required but is not sufficient to confer 7B2 binding; 2) the P domain is required for the stabilization of PC2 structure and is not exchangeable with the P domain of PC1; and 3) the carboxyl-terminal domain is not involved in 7B2 binding. Site-directed mutagenesis of pro-PC2 further showed that a single residue replacement in the catalytic domain, Tyr-194 --> Asp, prevented pro-PC2 from binding 7B2 and blocked activation. This residue is present within a loop rich in aromatic amino acids which appears to be on the surface of the molecule as extrapolated from the crystal structure of subtilisin. This loop may represent the primary recognition site for 7B2 within the catalytic domain.

Activation of the kexin from Schizosaccharomyces pombe requires internal cleavage of its initially cleaved prosequence

Members of the kexin family of processing enzymes are responsible for the cleavage of many proproteins during their transport through the secretory pathway. The enzymes themselves are made as inactive precursors, and we investigated the activation process by studying the maturation of Krp1, a kexin from the fission yeast Schizosaccharomyces pombe. Using a cell-free translation-translocation system prepared from Xenopus eggs, we found that Krp1 is made as a preproprotein that loses the presequence during translocation into the endoplasmic reticulum. The prosequence is also rapidly cleaved in a reaction that is autocatalytic and probably intramolecular and is inhibited by disruption of the P domain. Prosequence cleavage normally occurs at Arg-Tyr-Lys-Arg102/ (primary cleavage site) but can occur at Lys-Arg82 (internal cleavage site) and/or Trp-Arg99 when the basic residues are removed from the primary site. Cleavage of the prosequence is necessary but not sufficient for activation, and Krp1 is initially unable to process substrates presented in trans. Full activation is achieved after further incubation in the extract and is coincident with the addition of O-linked sugars. O glycosylation is not, however, essential for activity, and the crucial event appears to be cleavage of the initially cleaved prosequence at the internal site. Our results are consistent with a model in which the cleaved prosequence remains noncovalently associated with the catalytic domain and acts as an autoinhibitor of the enzyme. Inhibition is then relieved by a second (internal) cleavage of the inhibitory prosequence. Further support for this model is provided by our finding that overexpression of a Krp1 prosequence lacking a cleavable internal site dramatically reduced the growth rate of otherwise wild-type S. pombe cells, an effect that was not seen after overexpression of the normal, internally cleavable, prosequence or prosequences that lack the Lys-Arg102 residues.

Mechanism of the facilitation of PC2 maturation by 7B2: involvement in ProPC2 transport and activation but not folding

Among the members of the prohormone convertase (PC) family, PC2 has a unique maturation pattern: it is retained in the ER for a comparatively long time and its propeptide is cleaved in the TGN/ secretory granules rather than in the ER. It is also unique by its association with the neuroendocrine protein 7B2. This interaction results in the facilitation of proPC2 maturation and in the production of activatable proPC2 from CHO cells. In the present study, we have investigated the mechanism of this interaction. ProPC2 binds 7B2 in the ER, but exits this compartment much more slowly than 7B2. We found that proPC2 was also slow to acquire the capacity to bind 7B2, whereas 7B2 could bind proPC2 rapidly after synthesis. This indicated that proPC2 folding was the limiting step in the formation of the complex. Indeed, sensitivity of native proPC2 to N-glycanase F digestion and inhibition of proPC2 folding supported the notion that 7B2 is not involved in the early steps of proPC2 folding, and that proPC2 must fold before binding 7B2. Under experimental conditions that prevent propeptide cleavage, 7B2 expression increased proPC2 transport to the Golgi. This increase exhibited the same kinetics as the facilitation of the removal of the propeptide. Finally, proPC2 activation could be reconstituted in Golgi- enriched subcellular fractions. In vitro, 7B2 was required for proPC2 activation at an acidic pH. Taken together, our results demonstrate that rather than promoting proPC2 folding, 7B2 acts as a helper protein involved in proPC2 transport and is required in the proPC2 activation process. We propose, therefore, that 7B2 stabilizes proPC2 in a conformation already competent for these two events.

Convertase PC2 and the neuroendocrine polypeptide 7B2 are co-induced and processed during neuronal differentiation of P19 embryonal carcinoma cells

Convertases of the subtilisin/kexin family are responsible for the biological activation of a variety of pro-proteins, pro-hormones, and pro-trophic factors, and thus can modulate various aspects of embryonic development. We investigated the expression of each convertase by Northern hybridization during cell differentiation in vitro, using the mouse embryonal carcinoma cell line P19 as a model. The neuroendocrine convertase PC2 and 7B2, its specific binding protein, are co-induced during neuronal differentiation of P19 cells with retinoic acid, whereas the other convertases are not or follow different patterns of temporal expression. The mature forms of PC2 and 7B2 proteins are detected together by immunoblotting following induction of mRNA expression, indicating that these proteins are processed early during brain development. These results demonstrate that PC2 and 7B2 gene expression and protein processing are in a close temporal association during neuronal differentiation and point to the value of the P19 cell model to study the significance and the regulation of this relationship in mammalian brain development.

Activation of the zymogen form of prostate-specific antigen by human glandular kallikrein 2

Prostate-specific antigen (PSA) and human glandular kallikrein 2 (hK2) are glandular kallikreins secreted by the prostate gland. Both enzymes are synthesized with a propeptide that is supposedly cleaved off in the prostate to yield the mature forms found in semen. We have purified and characterised recombinant PSA and hK2 produced in eucaryotic cells. Recombinant PSA was recovered as a zymogen and recombinant hK2 was recovered in mature form. The zymogen form of PSA had no or very low enzymatic activity. After incubation with hK2, proPSA was activated, as shown by the cleavage of the seminal gel proteins and a peptide substrate; the hK2-proPSA ratio used was similar to the enzyme-substrate ratio that prevails under phyciological conditions. Our results indicate that hK2 is responsible for the activation of proPSA, a finding that may be very important for understanding of the role of these two kallikreins in the reproductive system and in prostate cancer biology.

Processing of prothyrotropin-releasing hormone by the family of prohormone convertases

The post-translational processing of prothyrotropin-releasing hormone (pro-TRH25-255) has been extensively studied in our laboratory, and the processing pathway to mature TRH has been elucidated. We have also demonstrated that recombinant PC1 and PC2 process partially purified pro-TRH to cryptic peptides in vitro and that pro-TRH and PC1 mRNAs are coexpressed in primary cultures of hypothalamic neurons. To further define the role of each convertase, and particularly PC1 and PC2, in pro-TRH processing, recombinant vaccinia viruses were used to coexpress the prohormone convertases PC1, PC2, PACE4, PC5-B, furin, or control dynorphin together with rat prepro-TRH in constitutively secreting LoVo cells or in the regulated endocrine GH4C1 cell line. Radioimmunoassays from LoVo-derived secreted products indicated that furin cleaves the precursor to generate both N- and C-terminal intermediates. PC1, PC2, and PACE4 only produced N-terminal intermediates, but less efficiently than furin. In GH4C1 cells, PC1, PC2, furin, PC5-B, and PACE4 produced both N-terminal and C-terminal forms. Significantly, TRH-Gly and TRH were mostly produced by PC1, PC2, and furin. Utilizing gel electrophoresis to further analyze the cleavage specificities of PC1 and PC2, we found that PC1 seems primarily responsible for cleavage to both intermediates and mature TRH, since it generated all products at significantly higher levels than PC2. The addition of 7B2 to the coinfection did not augment the ability of PC2 to cleave pro-TRH to either N- or C-terminal forms.

Dissociation of the complex between the neuroendocrine chaperone 7B2 and prohormone convertase PC2 is not associated with proPC2 maturation

7B2 is a highly conserved neuroendocrine protein that is associated with the proform of the prohormone convertase PC2 in the early stages of the secretory pathway in intermediate pituitary cells of Xenopus laevis. Subsequent processing of 7B2 and dissociation of the 7B2/proPC2 complex is thought to be associated with the conversion of proPC2 to the mature enzyme. Here, we report that, in both Xenopus and mouse intermediate cells, proPC2 maturation does not take place when the proenzyme is associated with the 7B2 precursor and that, in contrast to the previous notion, dissociation of the complex between proPC2 and the N-terminal 7B2 fragment precedes, and is thus not directly linked to, proPC2 maturation. In vitro, conversion of newly synthesized proPC2 was efficiently blocked by recombinant 7B2 and studies with truncation mutants indicated that a short segment in the C-terminal region of 7B2 is necessary and sufficient for this inhibitory effect. Our results indicate that, after 7B2 precursor processing and dissociation of the N-terminal fragment, the C-terminal fragment of 7B2 may remain associated with proPC2, thereby preventing autocatalytic conversion of the proenzyme until the appropriate site for activation in the secretory pathway is reached.

Processing of pro-islet amyloid polypeptide (proIAPP) by the prohormone convertase PC2

Islet amyloid polypeptide (IAPP), 'amylin', is the component peptide of islet amyloid formed in Type 2 diabetes. IAPP is expressed in islet beta-cells and is derived from a larger precursor, proIAPP, by proteolysis. An in vitro translation/translocation system was used to separately examine processing of human proIAPP by the beta-cell endopeptidases PC2, PC3 or furin. ProIAPP was converted to mature IAPP by PC2 but there was little conversion by furin or PC3. These data are consistent with processing of proIAPP in beta-cell secretory granules. Abnormal cellular proteolysis associated with type 2 diabetes could contribute to IAPP amyloidosis.

Structural elements that direct specific processing of different mammalian subtilisin-like prohormone convertases

PC1 and PC2 are two important subtilisin-like prohormone convertases (PC) that undergo differential endoproteolytic processing steps and sequentially mediate proopiomelanocortin (POMC) processing. To investigate the structural elements directing the processing of different PCs, we constructed a series of mutant and chimeric PC proteins and expressed them in cell lines with different patterns of expression of endogenous PCs: AtT-20, hEK293, and hLoVo cells. The COOH-terminally truncated PC1 underwent efficient proregion cleavage and rapid secretion in all three cell lines, while proregion cleavage and secretion were completely blocked in an active-site mutant of PC1. The truncated PC1 produced dramatic changes in POMC processing in AtT-20 cells. PC2 with the potential oxyanion hole Asp residue changed to Asn was processed and altered several aspects of POMC processing in a manner similar to that of wild-type PC2. PC1 protein with its proregion substituted with that of furin was cleaved after its proregion, producing active PC1 enzyme. A similar furin/PC2 fusion protein underwent proregion cleavage at low efficiency. By contrast, when the proregions of PC1 and PC2 were substituted with one another, both fusion proteins failed to cleave the foreign prosequences, were unable to undergo oligosaccharide maturation, and remained in the ER. Although inactive PC mutants could theoretically function as dominant negatives, none interfered with the processing of endogenous active PCs or with POMC processing. We conclude that the COOH-terminal of PC1 plays an important role in the routing or storage of PC1, the proregions of these PC proteins are replaceable in a molecule-specific manner, removal of proregion is essential for routing and for endoproteolytic activity, and the role of the potential oxyanion hole in PC2 is still unclear.

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.

Addition of an endoplasmic reticulum retention/retrieval signal does not block maturation of enzymatically active peptidylglycine alpha-amidating monooxygenase

Peptidylglycine alpha-amidating monooxygenase (PAM) catalyzes the COOH-terminal alpha-amidation of neural and endocrine peptides via a two-step reaction carried out in sequence by the monooxygenase and lyase domains contained in this bifunctional protein. Peptide alpha-amidation is thought to take place primarily in the secretory granules in which mature bioactive peptides are stored, and it is not known where in the secretory compartment newly synthesized PAM protein becomes enzymatically active. To address this question, PAM-3, a soluble bifunctional protein, was modified by addition of the KDEL endoplasmic reticulum (ER) retention/retrieval signal to its COOH terminus. PAM-3-KDEL protein stably expressed in hEK-293 cells or in AtT-20 cells was efficiently retained in the ER based on immunocytochemistry, pulse-chase experiments, and maintained endoglycosidase H sensitivity. The effect of the KDEL sequence was specific since PAM-3 with an inactive ER retention/retrieval signal (PAM-3-KDEV) moved through the secretory pathway like wild type PAM-3. In AtT-20 cells, PAM-3-KDEL was not subjected to the COOH-terminal endoproteolytic cleavage that generates a 75-kDa PAM protein from PAM-3 and PAM-3-KDEV. PAM-3-KDEL protein exhibited both monooxygenase and lyase activities with specific activities similar to those of the wild type PAM-3 and PAM-3-KDEV proteins. Thus, although PAM catalyzes a reaction that occurs primarily in the secretory granules, newly synthesized PAM protein becomes enzymatically competent in the ER.

Processing of the papain precursor. The ionization state of a conserved amino acid motif within the Pro region participates in the regulation of intramolecular processing

The cysteine protease papain is synthesized as a 40-kDa inactive precursor with a 107-amino-acid N-terminal pro region. Although sequence conservation in the pro region is lower than in the mature proteases, a conserved motif (Gly-Xaa-Asn-Xaa-Phe-Xaa-Asp-36, papain precursor numbering) was found within the pro region of cysteine proteases of the papain superfamily. To determinate the function to this conserved motif, we have mutagenized at random each of the 4 residues individually within the pro region of the papain precursor. Precursor mutants were expressed in yeast, screened according to their ability to be processed through either a cis or trans reaction, into mature active papain. Three classes of mutants were found. Non-functional propapain mutants of the first class are completely degraded by subtilisin indicating that they are not folded into a native state. Mutants of the second class were neutral with respect to cis and trans processing. The third class included mutants that mostly accumulated as mature papain in the yeast vacuole. They had mutations that had lost the negatively charged Asp-36 residues and a mutation that probably introduces a positive charge, Phe-38His. The precursor of the Phe-38His mutant could be recovered by expression in a vph1 mutant yeast strain which has a vacuolar pH of about 7. The Phe-38His propapain mutant has an optimum pH of autoactivation about one pH unit higher than the wild type molecule. These results indicate that the electrostatic status of the conserved motif participates in the control of intramolecular processing of the papain precursor.

Structure-function studies on the biosynthesis and bioactivity of the precursor convertase PC2 and the formation of the PC2/7B2 complex

Site directed mutagenesis of the prohormone convertase PC2 was used to define the effect of certain residues on the zymogen activation of proPC2 and on its binding to the neuroendocrine protein 7B2. These included the oxyanion hole Asp309 (D309N), the N-terminal Glu25 (E25Q and E25K) of proPC2 and the Asp519 (D519E) of the RGD motif within the P-domain of PC2. Heterologous vaccinia virus expression of the wild type and mutant PC2's in endocrine pituitary cells such as AtT20 and GH3 cells demonstrated that the most dramatic effect was observed with the D309N mutant which no longer bound pro7B2 and which exhibited a significant reduction in its capacity to produce beta-endorphin from pro-opiomelanocortin (POMC).

Regulated secretion. Helper proteins for neuroendocrine secretion

The granins, known to be general constituents of neuroendocrine secretory granules, are regulators of the proteolytic processing of peptide precursors and promote their aggregation-mediated sorting into secretory granules.

The Dfur2 gene of Drosophila melanogaster: genetic organization, expression during embryogenesis, and pro-protein processing activity of its translational product Dfurin2

The gene structure and expression of the Dfur2 gene of Drosophila melanogaster, which encodes the subtilisin-like serine endoprotease Dfurin2, was studied. The Dfur2 gene is very compact in contrast to the related Dfur1 gene, which has an estimated size of over 100 kbp. The 6-kb Dfur2 mRNA is encoded by 16 exons dispersed over a genomic region of about 9 kbp. The exon/intron organization shows conservation of intron positions not only in comparison with Dfur1, but also with the related mammalian genes FUR, PC1/PC3, PC2, and PC4. This conservation supports the hypothesis that all genes belonging to the family of subtilisin-like pro-protein processing enzymes are evolutionary related by descent from a common ancestral gene. In primer extension experiments, Dfur2 transcription initiation sites were identified in the presumed Dfur2 promoter region. This region was found to contain general RNA polymerase II promoter elements like a potential TATA box, a potential CAP signal, and several potential CCAAT boxes. Also, several sequence motifs putatively corresponding to binding sites for Drosophila transcription factors like zeste, bicoid, and engrailed were found to be present. RNA in situ hybridization experiments on Drosophila embryos revealed presumably maternal Dfur2 expression until the syncytial blastoderm (stage 5 of embryogenesis), no expression during gastrulation (stage 9), transient expression in a subset of neurons in the central nervous system of stage 12-13 embryos, and, from stage 13 onwards, expression in the developing tracheal tree. In a vaccinia expression system, the endoprotease Dfurin2 not only cleaved wild-type precursor of von Willebrand factor (pro-vWF) with pro-region cleavage site R-S-K-R decreases, but also, although to a lesser extent, pro-vWF mutants in which the P2 (vWFK-2A) or P4 (vWFR-4A) basic residue with respect to the pro-region cleavage site had been mutated. This cleavage specificity resembles that of human furin. The cleavage of pro-vWF by Dfurin2 shows that the previously reported lack of cleavage of the precursor of the beta A-chain of activin-A by Dfurin2 in this vaccinia expression system is substrate determined.

Structural basis of substrate specificity in the serine proteases

Structure-based mutational analysis of serine protease specificity has produced a large database of information useful in addressing biological function and in establishing a basis for targeted design efforts. Critical issues examined include the function of water molecules in providing strength and specificity of binding, the extent to which binding subsites are interdependent, and the roles of polypeptide chain flexibility and distal structural elements in contributing to specificity profiles. The studies also provide a foundation for exploring why specificity modification can be either straightforward or complex, depending on the particular system.

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.

7B2 is a neuroendocrine chaperone that transiently interacts with prohormone convertase PC2 in the secretory pathway

The neuroendocrine polypeptide 7B2 is a highly conserved secretory protein selectively present in prohormone-producing cells equipped with a regulated secretory pathway. We find that the amino-terminal half of 7B2 is distantly related to chaperonins, a subclass of molecular chaperones. When incubated in vitro with newly synthesized pituitary proteins, recombinant 7B2 specifically associates with prohormone convertase PC2. Metabolic cell labeling combined with coimmunoprecipitation studies showed that, in vivo, the precursor form of 7B2 interacts with the proform of PC2. Pulse-chase analysis revealed that this association is transient in that it commences early in the secretory pathway, while dissociation in the later stages appears to coincide with the cleavages of 7B2, proPC2, and prohormone. Our results suggest that 7B2 is a novel type of molecular chaperone preventing premature activation of proPC2 in the regulated secretory pathway.

The family of subtilisin/kexin like pro-protein and pro-hormone convertases: divergent or shared functions

Six mammalian processing enzymes were recently discovered which exhibit significant similarities to both yeast kexin and bacterial subtilisins. These subtilisin/kexin-like convertases were called furin/PACE, PC1/PC3, PC2, PACE4, PC4 and PC5/PC6. The analysis of the mRNA expression of these convertases in rat tissues and cell lines by Northern blot analysis demonstrated a unique pattern for each enzyme. Thus, although furin and PACE4 mRNA (4.4 kb each) exhibit a widespread tissue distribution only furin is ubiquitously expressed. PACE4 exhibits a major 4.4 kb mRNA form, and in some tissues a 3.9 kb form is detected. PC5 mRNA (3.8 kb major) is more restricted in its distribution than PACE4 and furin, and it exhibits the presence of multiple mRNA forms, resulting in variable lengths of the C-terminal Cys-rich domain. In addition, like furin and PACE4, PC5 is expressed in both regulated and constitutively secreting cells. In contrast, PC1 (3 and 5 kb) and PC2 (2.8 and 5 kb) are primarily expressed in tissues and cells containing secretory granules. Multiple mRNA forms are also detected, but as far as is known none affect their open reading frame and only result in a variable length of the 3' non-coding sequence. Finally, PC4 mRNA (2.8 kb major and 1.9 kb minor) is only expressed in testicular germ cells. Biosynthetic analysis of the zymogen activation of PC1 and PC2 and their cleavage specificity following their cellular co-expression with a number of precursors, demonstrated that although pro-PC1 is rapidly activated to PC1 in the endoplasmic reticulum, pro-PC2 conversion into PC2 is rather slow. The cleavage of pro-PC2 into PC2 starts in the trans Golgi network and is regulated by an endogenous endocrine and neural precursor called 7B2. Although the genetic organization of the convertase genes is very similar, they exhibit unique promoter sequences and only furin and PACE4 genes are localized on the same chromosome.