Calcium- and pH-dependent aggregation and membrane association of the precursor of the prohormone convertase PC2

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

Pancreatic and duodenal homeobox protein 1 (Pdx-1) maintains endoplasmic reticulum calcium levels through transcriptional regulation of sarco-endoplasmic reticulum calcium ATPase 2b (SERCA2b) in the islet β cell

Although the pancreatic duodenal homeobox 1 (Pdx-1) transcription factor is known to play an indispensable role in β cell development and secretory function, recent data also implicate Pdx-1 in the maintenance of endoplasmic reticulum (ER) health. The sarco-endoplasmic reticulum Ca(2+) ATPase 2b (SERCA2b) pump maintains a steep Ca(2+) gradient between the cytosol and ER lumen. In models of diabetes, our data demonstrated loss of β cell Pdx-1 that occurs in parallel with altered SERCA2b expression, whereas in silico analysis of the SERCA2b promoter revealed multiple putative Pdx-1 binding sites. We hypothesized that Pdx-1 loss under inflammatory and diabetic conditions leads to decreased SERCA2b levels and activity with concomitant alterations in ER health. To test this, siRNA-mediated knockdown of Pdx-1 was performed in INS-1 cells. The results revealed reduced SERCA2b expression and decreased ER Ca(2+), which was measured using fluorescence lifetime imaging microscopy. Cotransfection of human Pdx-1 with a reporter fused to the human SERCA2 promoter increased luciferase activity 3- to 4-fold relative to an empty vector control, and direct binding of Pdx-1 to the proximal SERCA2 promoter was confirmed by chromatin immunoprecipitation. To determine whether restoration of SERCA2b could rescue ER stress induced by Pdx-1 loss, Pdx1(+/-) mice were fed a high-fat diet. Isolated islets demonstrated an increased spliced-to-total Xbp1 ratio, whereas SERCA2b overexpression reduced the Xbp1 ratio to that of wild-type controls. Together, these results identify SERCA2b as a novel transcriptional target of Pdx-1 and define a role for altered ER Ca(2+) regulation in Pdx-1-deficient states.

Manganese induces oligomerization to promote down-regulation of the intracellular trafficking receptor used by Shiga toxin

Manganese down-regulates the Shiga toxin receptor GPP130, which protects against lethal toxin doses. This study reveals a major aspect of the mechanism. Manganese binds GPP130, inducing GPP130 oligomerization, which is required and sufficient to redirect GPP130 out of the Golgi toward lysosomes.

Manganese (Mn) protects cells against lethal doses of purified Shiga toxin by causing the degradation of the cycling transmembrane protein GPP130, which the toxin uses as a trafficking receptor. Mn-induced GPP130 down-regulation, in addition to being a potential therapeutic approach against Shiga toxicosis, is a model for the study of metal-regulated protein sorting. Significantly, however, the mechanism by which Mn regulates GPP130 trafficking is unknown. Here we show that a transferable trafficking determinant within GPP130 bound Mn and that Mn binding induced GPP130 oligomerization in the Golgi. Alanine substitutions blocking Mn binding abrogated both oligomerization of GPP130 and GPP130 sorting from the Golgi to lysosomes. Further, oligomerization was sufficient because forced aggregation, using a drug-controlled polymerization domain, redirected GPP130 to lysosomes in the absence of Mn. These experiments reveal metal-induced oligomerization as a Golgi sorting mechanism for a medically relevant receptor for Shiga toxin.

Pituitary protein 7B2 plasma levels in patients with liver disease: Comparisons with other hormones and neuropeptides

7B2, a protein initially isolated from the porcine pituitary gland, has been identified in numerous animal and human tissues, with the highest concentrations in the pituitary and hypothalamus. The 7B2 molecule is highly evolutionarily conserved and is considered to be indispensable in the function and regulation of proprotein convertase 2 (PC2). In the present study, the plasma 7B2 immunoreactivity (7B2-IR) of 18 patients with liver disease was studied. Of these patients, seven (three male and four female), aged 37–67 [54.6±13.5 (SD)] years, suffered from liver cirrhosis of cryptogenic (n=2) or alcoholic (n=5) aetiology. The remaining 11 patients (four male and seven female), aged 22–76 [56.1±17.6 (SD)] years, suffered from miscellaneous liver abnormalities. The clinical diagnosis was confirmed in the majority of patients by the histological examination of a percutaneous liver biopsy or by appropriate radiological investigations. Plasma bilirubin, alkaline phosphatase, aspartate aminotransferase, albumin, prothrombin time, electrolytes, urea and creatinine were measured. The plasma 7B2-IR levels were estimated using a sensitive radioimmunoassay (RIA), and the elution position of 7B2-IR was verified by gel chromatography. The mean plasma 7B2-IR concentration in patients with liver disease was 99.44±15.9 pmol/l. In the patients with hepatocellular damage due to metastatic tumours [Ca bronchus, carcinoid (n=6)], the 7B2-IR concentrations were significantly higher [185±36.9 pmol/l, (P<0.05)] compared with the overall subjects with liver damage. The results of the present study demonstrate that 7B2-IR is increased in liver disease, with the highest levels detected in patients with tumourous liver conditions.

Sorting of neuropeptides and neuropeptide receptors into secretory pathways

There are two major secretory pathways in neurons, the regulated pathway and the constitutive pathway. Neuropeptides and other regulated secretory proteins are known to be sorted into large dense-core vesicles of the regulated pathway in the trans-Golgi network and are secreted upon stimulus-induced increases in intracellular Ca(2+). The newly synthesized cell surface receptors are usually sorted into microvesicles of the constitutive pathway and inserted into the plasma membrane by spontaneous exocytosis. Small-diameter sensory neurons in dorsal root ganglia and pheochromocytoma cells express neuropeptides (e.g., substance P) and several neuropeptide receptors including opioid receptors. The mu-opioid receptors are delivered to the cell surface through the constitutive pathway, whereas another type of opioid receptor, the delta-opioid receptor, is often found in the membrane of large dense-core vesicles and can be inserted into the plasma membrane when exocytosis occurs. Recent studies show that sequences with opposite electrical polarity within the prohormones of substance P are essential for their sorting into large dense-core vesicles. Moreover, the delta-opioid receptor is sorted into large dense-core vesicles by its interaction with protachykinin, a prohormone of substance P. These findings provide insight into the molecular mechanisms that determine the sorting and trafficking of neuropeptides and neuropeptide receptors in neurons.

Proteomics of regulated secretory organelles

Regulated secretory organelles are important subcellular structures of living cells that allow the release in the extracellular space of crucial compounds, such as hormones and neurotransmitters. Therefore, the regulation of biogenesis, trafficking, and exocytosis of regulated secretory organelles has been intensively studied during the last 30 years. However, due to the large number of different regulated secretory organelles, only a few of them have been specifically characterized. New insights into regulated secretory organelles open crucial perspectives for a better comprehension of the mechanisms that govern cell secretion. The combination of subcellular fractionation, protein separation, and mass spectrometry is also possible to study regulated secretory organelles at the proteome level. In this review, we present different strategies used to isolate regulated secretory organelles, separate their protein content, and identify the proteins by mass spectrometry. The biological significance of regulated secretory organelles-proteomic analysis is discussed as well.

Maturation of secretory granules

Exocrine, endocrine, and neuroendocrine cells store hormones and neuropeptides in secretory granules (SGs), which undergo regulated exocytosis in response to an appropriate stimulus. These cargo proteins are sorted at the trans-Golgi network into forming immature secretory granules (ISGs). ISGs undergo maturation while they are transported to and within the F-actin-rich cortex. This process includes homotypic fusion of ISGs, acidification of their lumen, processing, and aggregation of cargo proteins as well as removal of excess membrane and missorted cargo. The resulting mature secretory granules (MSGs) are stored in the F-actin-rich cell cortex, perhaps as segregated pools exhibiting specific responses to stimuli for regulated exocytosis. During the last decade our understanding of the maturation of ISGs advanced substantially. The use of biochemical approaches led to the identification of membrane molecules mechanistically involved in this process. Furthermore, live cell imaging in combination with fluorescently tagged marker proteins of SGs provided insights into the dynamics of maturing ISGs, and the functional implications of cytoskeletal elements and motor proteins.

Biogenesis of Dense-Core Secretory Granules

Dense core granules (DCGs) are vesicular organelles derived from outbound traffic through the eukaryotic secretory pathway. As DCGs are formed, the secretory pathway can also give rise to other types of vesicles, such as those bound for endosomes, lysosomes, and the cell surface. DCGs differ from these other vesicular carriers in both content and function, storing highly concentrated cores’ of condensed cargo in vesicles that are stably maintained within the cell until a specific extracellular stimulus causes their fusion with the plasma membrane. These unique features are imparted by the activities of membrane and lumenal proteins that are specifically delivered to the vesicles during synthesis. This chapter will describe the DCG biogenesis pathway, beginning with the sorting of DCG proteins from proteins that are destined for other types of vesicle carriers. In the trans-Golgi network (TGN), sorting occurs as DCG proteins aggregate, causing physical separation from non-DCG proteins. Recent work addresses the nature of interactions that produce these aggregates, as well as potentially important interactions with membranes and membrane proteins. DCG proteins are released from the TGN in vesicles called immature secretory granules (ISGs). The mechanism of ISG formation is largely unclear but is not believed to rely on the assembly of vesicle coats like those observed in other secretory pathways. The required cytosolic factors are now beginning to be identified using in vitro systems with purified cellular components. ISG transformation into a mature fusion-competent, stimulus-dependent DCG occurs as endoproteolytic processing of many DCG proteins causes continued condensation of the lumenal contents. At the same time, proteins that fail to be incorporated into the condensing core are removed by a coat-mediated budding mechanism, which also serves to remove excess membrane and membrane proteins from the maturing vesicle. This chapter will summarize the work leading to our current view of granule synthesis, and will discuss questions that need to be addressed in order to gain a more complete understanding of the pathway.

Short elements with charged amino acids form clusters to sort protachykinin into large dense-core vesicles

The sorting of neuropeptide tachykinins into large dense-core vesicles (LDCVs) is a key step in their regulated secretion from neurons. However, the sorting mechanism for protachykinin has not yet to be clearly resolved. In this study, we report that the clustered short elements with charged amino acids regulate the efficiency of protachykinin sorting into LDCVs. A truncation experiment showed that the propeptide and the mature peptide-containing sequence of protachykinin were sorted into LDCVs. These two regions exhibit a polarized distribution of charged amino acids. The LDCV localization of the propeptide was gradually decreased with an increasing number of neutral amino acids. Furthermore, the short element with four to five amino acids containing two charged residues was found to be a basic unit for LDCV sorting that enables regulated secretion. In the native propeptide sequence, these charged short elements were clustered to enhance the intermolecular aggregation by electrostatic interaction and produce a gradual and additive effect on LDCV sorting. The optimal conditions for intermolecular aggregation of protachykinin were at millimolar Ca(2+) concentrations and pH 5.5-6.0. These results demonstrate that the charged short elements are clustered such that they serve as aggregative signals and regulate the efficiency of protachykinin sorting into LDCVs. These findings reveal a novel mechanism for the sorting of neuropeptides into a regulated secretory pathway.

7B2 prevents unfolding and aggregation of prohormone convertase 2

Prohormone convertase 2 (PC2) requires interaction with the neuroendocrine protein 7B2 for the production of an activatable zymogen; the mechanism for this effect is unknown. 7B2 could act proactively to generate an activation-competent form of pro-PC2 during synthesis, or block spontaneous generation of activation-incompetent forms. We here demonstrate that addition of exogenous recombinant 7B2 to CHO cells expressing pro-PC2 prevented the unfolding and aggregation of secreted PC2 forms in a dose-dependent manner, as assessed by aggregation assays, activity assays, cross-linking experiments, and sucrose density gradients. Intracellular pro-PC2 was also found to exist in part as higher-order oligomers that were reduced in the presence of coexpressed 7B2. 7B2 addition did not result in the acquisition of enzymatic competence unless added before or very rapidly after pro-PC2 secretion, indicating that an activation-competent structure cannot be maintained in the absence of 7B2. Velocity sedimentation experiments showed that addition of extracellular 7B2 solubilized three different PC2 species from a precipitable aggregate: two activatable pro-PC2 species, the intact zymogen and a zymogen with a partially cleaved propeptide, and an inactive 66-kDa form. Our results suggest that 7B2 possesses chaperone activity that blocks partially unfolded pro-PC2 forms from losing catalytic competence and then aggregating. The loss of the catalytically competent conformer appears to represent the earliest indicator of pro-PC2 unfolding and is followed on a slower time scale by the appearance of aggregates. Because 7B2 expression is not confined to areas expressing pro-PC2, 7B2 may represent a general intracellular and extracellular secretory chaperone.

Immunohistochemical expression and colocalization of somatostatin, carboxypeptidase-E and prohormone convertases 1 and 2 in rat brain

The processing of many peptides for their maturation in target tissue depends upon the presence of sorting receptor. Several previous studies have predicted that carboxypeptidase-E (CPE), prohormone convertase 1 (PC1) and prohormone convertase 2 (PC2) may function as sorting elements for somatostatin (SST) for its maturation and processing to appropriate targets. However, nothing is currently known about whether brain, neuronal culture or even endocrine cells express SST, CPE, PC1 and PC2 and exhibit colocalization. Accordingly, in the present study using peroxidase immunohistochemistry, double-labeled indirect immunofluorescence immunohistochemistry and Western blot analysis, we mapped the distributional pattern of SST, CPE, PC1 and PC2 in different rat brain regions. Additionally, we also determined the colocalization of SST with CPE, PC1 and PC2 as well as colocalization of CPE with PC1 and PC2. The localization of SST, CPE, PC1 and PC2 reveals a distinct and region specific distribution pattern in the rat brain. Using an indirect double-label immunofluorescence method we observed selective neuron specific colocalization in a region specific manner in cortex, striatum and hippocampus. These studies provide the first evidence for colocalization between SST, CPE, PC1 and PC2 as well as CPE with PC1 and PC2. SST in cerebral cortex colocalized in pyramidal and non-pyramidal neurons with CPE, PC1 and PC2. Most importantly, in striatum and hippocampus colocalization was mostly observed selectively and preferentially in interneurons. CPE is also colocalized with PC1 and PC2 in a region specific manner. The data presented here provide a new insight into the distribution and colocalization of SST, CPE, PC1 and PC2 in rat brain. Taken together, our data anticipate the possibility that CPE, PC1 and PC2 might be potential target for the maturation of SST.

Identification of a segment in the precursor of pulmonary surfactant protein SP-B, potentially involved in pH-dependent membrane assembly of the protein

In the present work, the hydrophobic properties of proSP-B, the precursor of pulmonary surfactant protein SP-B, have been analyzed under different pH conditions, and the sequence segment at position 111-135 of the N-terminal domain of the precursor has been detected as potentially possessing pH-dependent hydrophobic properties. We have studied the structure and lipid-protein interactions of the synthetic peptides BpH, with sequence corresponding to the segment 111-135 of proSP-B, and BpH-W, bearing the conservative substitution F127W to use the tryptophan as an intrinsic fluorescent probe. Peptide BpH-W interacts with both zwitterionic and anionic phospholipid vesicles at neutral pH, as monitored by the blue-shifted maximum emission of its tryptophan reporter. Insertion of tryptophan into the membranes is further improved at pH 5.0, especially in negatively-charged membranes. Peptides BpH and BpH-W also showed pH-dependent properties to insert into phospholipid monolayers. We have also found that the single sequence variation F120K decreases substantially the interaction of this segment with phospholipid surfaces as well as its pH-dependent insertion into deeper regions of the membranes. We hypothesize that this region could be involved in pH-triggered conformational changes occurring in proSP-B along the exocytic pathway of surfactant in type II cells, leading to the exposure of the appropriate segments for processing and assembly of SP-B within surfactant lipids.

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.

The proprotein convertase PC2 is involved in the maturation of prosomatostatin to somatostatin-14 but not in the somatostatin deficit in Alzheimer's disease

A somatostatin deficit occurs in the cerebral cortex of Alzheimer's disease patients without a major loss in somatostatin-containing neurons. This deficit could be related to a reduction in the rate of proteolytic processing of peptide precursors. Since the two proprotein convertases (PC)1 and PC2 are responsible for the processing of neuropeptide precursors directed to the regulated secretory pathway, we examined whether they are involved first in the proteolytic processing of prosomatostatin in mouse and human brain and secondly in somatostatin defect associated with Alzheimer's disease. By size exclusion chromatography, the cleavage of prosomatostatin to somatostatin-14 is almost totally abolished in the cortex of PC2 null mice, while the proportions of prosomatostatin and somatostatin-28 are increased. By immunohistochemistry, PC1 and PC2 were localized in many neuronal elements in human frontal and temporal cortex. The convertases levels were quantified by Western blot, as well as the protein 7B2 which is required for the production of active PC2. No significant change in PC1 levels was observed in Alzheimer's disease. In contrast, a marked decrease in the ratio of the PC2 precursor to the total enzymatic pool was observed in the frontal cortex of Alzheimer patients. This decrease coincides with an increase in the binding protein 7B2. However, the content and enzymatic activity of the PC2 mature form were similar in Alzheimer patients and controls. Therefore, the cortical somatostatin defect is not due to convertase alteration occuring during Alzheimer's disease. Further studies will be needed to assess the mechanisms involved in somatostatin deficiency in Alzheimer's disease.

Differential aggregation properties of secretory proteins that are stored in exocrine secretory granules of the pancreas and parotid glands

Low-pH- and calcium-induced aggregation of regulated secretory proteins has been proposed to play a role in their retention and storage in secretory granules. However, this has not been tested for secretory proteins that are stored in the exocrine parotid secretory granules. Parotid granule matrix proteins were analyzed for aggregation in the presence or absence of calcium and in the pH range of 5.5 to 7.5. Amylase did not aggregate under these conditions, although <10% of parotid secretory protein (PSP) aggregated below pH 6.0. To test aggregation directly in isolated granules, rat parotid secretory granules were permeabilized with 0.1% saponin in the presence or absence of calcium and in the pH range of 5.0 to 8.4. In contrast to the low-pH-dependent retention of amylase in exocrine pancreatic granules, amylase was quantitatively released and most PSP was released from parotid granules under all conditions. Both proteins were completely released upon granule membrane solubilization. Thus neither amylase nor PSP show low-pH- or calcium-induced aggregation under physiological conditions in the exocrine parotid secretory granules.

The lumenal domain of the integral membrane protein phogrin mediates targeting to secretory granules

Phogrin, a transmembrane glycoprotein of neuroendocrine cells, is localized to dense-core secretory granules. We have investigated the subcellular targeting of phogrin by analyzing the sorting of a series of deletion mutants to the regulated pathway of secretion in AtT20 cells. The lumenal domain as a soluble protein was efficiently routed to granules, based on a combination of morphological analysis and secretion studies. Sorting was not dependent on a candidate targeting signal consisting of an N-terminal conserved cysteine-rich motif. Both the pro-region and the lumenal domain of mature, post-translationally processed phogrin independently reached the granule, although the pro-region was sorted more efficiently. Once within the regulated secretory pathway, all phogrin lumenal domain proteins were stored in functional granules for extended periods of time. Thus, phogrin possesses several domains contributing to its targeting to the secretory granule. Our findings support a model of granule biogenesis where proteins are sorted on the basis of their biochemical properties rather than via signal-dependent binding to a targeting receptor. Sorting of integral membrane proteins mediated by the lumenal domain may ensure that functionally important transmembrane molecules are included in the forming granule.

Dibasic cleavage site is required for sorting to the regulated secretory pathway for both pro- and neuropeptide Y

To investigate the signals governing routing of biologically active peptides to the regulated secretory pathway, we have expressed mutated and non-mutated proneuropeptide Y (ProNPY) in pituitary-derived AtT20 cells. The mutations were carried out on dibasic cleavage site and or ProNPY C-terminal sequence. Targeting to the regulated secretory pathway was studied using protein kinase A (8-BrcAMP), protein kinase C (phorbol myristate acetate) specific activators and protein synthesis inhibitor cycloheximide, and by pulse chase. The analysis of expressed peptides in cells and culture media indicated that: neuropeptide Y (NPY) and ProNPY were differently secreted, whilst NPY was exclusively secreted via regulatory pathway; ProNPY was secreted via regulated and constitutive-like secretory pathways. ProNPY secretion behaviour was not Proteolytic cleavage efficiency-dependent. The dibasic cleavage was essential for ProNPY and NPY cAMP-dependent regulated secretion and may have function as a retention signal.

Access of a membrane protein to secretory granules is facilitated by phosphorylation

Peptidylglycine alpha-amidating monooxygenase (PAM), an integral membrane protein essential for the biosynthesis of amidated peptides, was used to assess the role of cytosolic acidic clusters in trafficking to regulated secretory granules. Casein kinase II phosphorylates Ser(949) and Thr(946) of PAM, generating a short, cytosolic acidic cluster. P-CIP2, a protein kinase identified by its ability to interact with several juxtamembrane determinants in the PAM cytosolic domain, also phosphorylates Ser(949). Antibody specific for phospho-Ser(949)-PAM-CD demonstrates that a small fraction of the PAM-1 localized to the perinuclear region bears this modification. Pituitary cell lines expressing PAM-1 mutants that mimic (TS/DD) or prevent (TS/AA) phosphorylation at these sites were studied. PAM-1 TS/AA yields a lumenal monooxygenase domain that enters secretory granules inefficiently and is rapidly degraded. In contrast, PAM-1 TS/DD is routed to regulated secretory granules more efficiently than wild-type PAM-1 and monooxygenase release is more responsive to secretagogue. Furthermore, this acidic cluster affects exit of internalized PAM-antibody complexes from late endosomes; internalized PAM-1 TS/DD accumulates in a late endocytic compartment instead of the trans-Golgi network. The increased ability of solubilized PAM-1 TS/DD to aggregate at neutral pH may play an important role in its altered trafficking.

Response of an integral granule membrane protein to changes in pH

A key feature of the regulated secretory pathway in neuroendocrine cells is lumenal pH, which decreases between trans-Golgi network and mature secretory granules. Because peptidylglycine alpha-amidating monooxygenase (PAM) is one of the few membrane-spanning proteins concentrated in secretory granules and is a known effector of regulated secretion, we examined its sensitivity to pH. Based on antibody binding experiments, the noncatalytic linker regions between the two enzymatic domains of PAM show pH-dependent conformational changes; these changes occur in the presence or absence of a transmembrane domain. Integral membrane PAM-1 solubilized from rat anterior pituitary or from transfected AtT-20 cells aggregates reversibly at pH 5.5 while retaining enzyme activity. Over 35% of the PAM-1 in anterior pituitary extracts aggregates at pH 5.5, whereas only about 5% aggregates at pH 7.5. PAM-1 recovered from secretory granules and endosomes is highly responsive to low pH-induced aggregation, whereas PAM-1 recovered from a light, intracellular recycling compartment is not. Mutagenesis studies indicate that a transmembrane domain is necessary but not sufficient for low pH-induced aggregation and reveal a short lumenal, juxtamembrane segment that also contributes to pH-dependent aggregation. Taken together, these results demonstrate that several properties of membrane PAM serve as indicators of granule pH in neuroendocrine cells.

A conserved alpha-helix at the amino terminus of prosomatostatin serves as a sorting signal for the regulated secretory pathway

Mammalian prosomatostatin (PSST) contains the bioactive peptides SST-14 and SST-28 at the COOH-terminal end of the molecule and a putative sorting signal in the propeptide segment for targeting the precursor to the regulated secretory pathway. The NH(2)-terminal segment of PSST consists of an amphipathic alpha-helix, which has been totally conserved throughout vertebrate evolution. We have analyzed the PSST-(3--15) region for sorting function by alanine scanning and deletional mutagenesis. Mutants created were stably expressed in AtT-20 cells. Regulated secretion was studied by analyzing basal and stimulated release of SST-14 LI and by immunocytochemistry for staining of SST-14 LI in punctate granules. Deletion of the PSST-(3--15) segment blocked regulated secretion and rerouted PSST for constitutive secretion as unprocessed precursor. Alanine scanning mutagenesis identified the region Pro(5)--Gln(12) as being important in precursor targeting, with Leu(7) and Leu(11) being critical. Molecular modeling demonstrated that these two residues are located in close proximity on a hydrophobic surface of the alpha-helix. Disruption of the alpha-helix did not impair the ability of PSST to be processed at the COOH terminus to SST-14 and SST-28. Processing, however, was shifted to the early compartments of the secretory pathway rather than storage granules and was relatively inefficient.

Oligomerization of pro-opiomelanocortin is independent of pH, calcium and the sorting signal for the regulated secretory pathway

Studies indicate that pro-opiomelanocortin (POMC) is sorted to the regulated secretory pathway by binding to a sorting receptor identified as membrane-bound carboxypeptidase E (CPE) [Cool et al. (1997) Cell 88, 73-83]. The efficiency of this sorting mechanism could be enhanced if POMC molecules were to self-associate to form oligomers, prior or subsequent to binding to CPE. Using cross-linking and gel filtration techniques, we demonstrated that POMC forms oligomers at both neutral and acidic pHs and calcium was not necessary. delta N-POMC, which lacks the N-terminal sorting signal for the regulated secretory pathway, also formed similar oligomers, indicating that the sorting and oligomerization domains are different.

Trafficking/sorting and granule biogenesis in the beta-cell

Proinsulin is packaged into nascent (immature, clathrin-coated) secretory granules in the trans-Golgi network (TGN) of the beta -cell along with other granular constituents including the proinsulin conversion enzymes. It is assumed that such packaging is dependent on an active sorting process, separating granular proteins from other secretory or membrane proteins, but the mechanism remains elusive. As granules mature, the clathrin coat is lost, the intragranular milieu is progressively acidified, and proinsulin is converted to insulin and C-peptide. Loss of clathrin is believed to arise by budding of clathrin-coated vesicles from maturing granules, carrying with them any inappropriate or unnecessary products and providing an additional means for refinement of granular content.

Prohormone transport through the secretory pathway of neuroendocrine cells

En route through the secretory pathway of neuroendocrine cells, prohormones pass a series of membrane-bounded compartments. During this transport, the prohormones are sorted to secretory granules and proteolytically cleaved to bioactive peptides. Recently, progress has been made in a number of aspects concerning secretory protein transport and sorting, particularly with respect to transport events in the early regions of the secretory pathway. In this review we will deal with some of these aspects, including: i) selective exit from the endoplasmic reticulum via COPII-coated vesicles and the potential role of p24 putative cargo receptors in this process, ii) cisternal maturation as an alternative model for protein transport through the Golgi complex, and iii) the mechanisms that may be involved in the sorting of regulated secretory proteins to secretory granules. Although much remains to be learned, interesting new insights into the functioning of the secretory pathway have been obtained.Key words: regulated secretory pathway, p24 family, vesicular transport, POMC, protein sorting, secretory granule, Xenopus laevis.

Signal-mediated sorting of neuropeptides and prohormones: secretory granule biogenesis revisited

Neuropeptides and hormones, in contrast to constitutive secretory proteins, are sorted to and stored in secretory granules and released upon a stimulus. During the last two decades, signals and mechanisms involved in their sorting to the regulated pathway of protein secretion have been addressed in numerous studies. Taken together these studies revealed three important features of regulated secretory proteins: aggregation, sorting signal motifs and membrane binding. Here we try to dissect the sorting process with regard to these features and discuss their relevance in the context of current sorting models. We especially address the question where in the secretory pathway sorting takes place and discuss a possible role of sorting receptors.

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.

Neuroregulation of ProTRH biosynthesis and processing

This review presents an overview of the current knowledge on proTRH biosynthesis, its processing, its tissue distribution, and the role of known processing enzymes in proTRH maturation. The neuroendocrine regulation of TRH biosynthesis, the biological actions of its products, and the signal transduction and catabolic pathways used by those products are also reviewed. The widespread expression of proTRH, PC1, and PC2 rnRNAs in hypophysiotropic and extrahypophysiotropic areas of the brain, with their overlapping distribution in many areas, indicates the striking versatility provided by tissue-specific processing in generating quantitative and qualitative differences in nonTRH peptide products as well as TRH. Evidence is presented suggesting that differential processing for proTRH at the intracellular level is physiologically relevant. It is clear that control over the diverse range of proTRH-derived peptides within a specific cell is accomplished most from the regulation at the posttranslational level rather than the translational or transcriptional levels. Several examples supporting this hypothesis are presented in this review. A better understanding of proTRH-derived peptides role represents an exciting new frontier in proTRH research. These connecting sequences in between TRH molecules to form the precursor protein may function as structural or targeting elements that guide the folding and sorting of proTRH and its larger intermediates so that subsequent processing and secretion are properly regulated. The particular anatomical distribution of the proTRH end products, as well as regulation of their levels by neuroendocrine or pharmacological manipulations, supports a unique potential biologic role for these peptides.

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.

Carboxypeptidase E, a peripheral membrane protein implicated in the targeting of hormones to secretory granules, co-aggregates with granule content proteins at acidic pH

Carboxypeptidase E (CPE) is a prohormone-processing enzyme and peripheral membrane protein of endocrine/neuroendocrine secretory granules. CPE has been shown to bind to an amino-terminal peptide of pro-opiomelanocortin (N-POMC) at pH 5.5 and hypothesized to be critically involved in the targeting of hormones such as POMC to the regulated secretory pathway [Cool, D. R., Normant, E., Shen, F., Chen, H. C., Pannell, L., Zhang, Y., and Loh, Y. P. (1997) Cell 88, 73-83]. To further explore the possibility that CPE serves to mediate the association of content proteins with the membrane during granule biogenesis, the binding of CPE to granule content proteins was investigated using an in vitro aggregation assay in which the selective precipitation of granule content proteins is induced by titration of the pH to <6.0. CPE was observed to co-aggregate efficiently with pituitary and chromaffin granule content proteins at concentrations well below those that promote its self-aggregation. In addition, CPE co-precipitated at pH 5.8 with purified prolactin and with insulin, which homophillically self-aggregate yet are structurally distinct from N-POMC. N-POMC when added to the assays did not inhibit the aggregation of CPE with prolactin or insulin, indicating that these interactions do not involve a binding site for N-POMC. The data show that CPE interacts at acidic pH with a variety of different content proteins, resembling in this regard other granule membrane proteins. The results support the idea that co-aggregation of abundant membrane proteins with content proteins is an important general mechanism for the sorting and retention of secretory granule proteins during granule maturation.

Biogenesis of secretory granules in the trans-Golgi network of neuroendocrine and endocrine cells

Secretory granule formation requires selection of soluble and membrane proteins into nascent secretory granules, and exclusion of proteins not required for the function of secretory granules. Both selection and exclusion presumably can occur in the compartment where assembly of the secretory granule begins, the trans most cisternae of the Golgi complex. Current research focused on the initial stages of secretory granule formation includes a search for the 'signals' which may mediate active sorting of components into secretory granules, and the role of aggregation of regulated secretory proteins in sorting. In addition, the temporal sequence of the sorting events in the Golgi, and post-Golgi compartments has gained much attention, as summarized by the alternative but not mutually exclusive 'sorting for entry' vs. 'sorting by retention' models. 'Sorting for entry' which encompasses the most popular models requires selection of cargo and membrane and exclusion of non-secretory granule proteins in the TGN prior to secretory granule formation. 'Sorting by retention' stipulates that protein selection or exclusion may occur after secretory granule formation: secretory granule specific components are retained during maturation of the granule while non-secretory granule molecules are removed in vesicles which bud from maturing secretory granules. Finally, some progress has been made in the identification of cytosolic components involved in the budding of nascent secretory granules from the TGN. This review will focus on the recent data concerning the events in secretory granule formation which occur, in the trans-Golgi network.

Regulatory roles of the P domain of the subtilisin-like prohormone convertases

A unique feature of the eukaryotic subtilisin-like proprotein convertases (SPCs) is the presence of an additional highly conserved sequence of approximately 150 residues (P domain) located immediately downstream of the catalytic domain. To study the function of this region, which is required for the production of enzymatically active convertases, we have expressed and characterized various P domain-related mutants and chimeras in HEK293 cells and alpha-TC1-6 cells. In a series of C-terminal truncations of PC3 (also known as PC1 or SPC3), PC3-Thr594 was identified as the shortest active form, thereby defining the functional C-terminal boundary of the P domain. Substitutions at Thr594 and nearby sites indicated that residues 592-594 are crucial for activity. Chimeric SPC proteins with interchanged P domains demonstrated dramatic changes in several properties. Compared with truncated wild-type PC3 (PC3-Asp616), both PC3/PC2Pd and PC3/FurPd had elevated activity on several synthetic substrates as well as reduced calcium ion dependence, whereas Fur/PC2Pd was only slightly decreased in activity as compared with truncated furin (Fur-Glu583). Of the three active SPC chimeras tested, all had more alkaline pH optima. When PC3/PC2Pd was expressed in alpha-TC1-6 cells, it accelerated the processing of proglucagon into glicentin and major proglucagon fragment and cleaved major proglucagon fragment to release GLP-1 and tGLP-1, similar to wild-type PC3. Thus, P domain exchanges generated fully active chimeric proteases in several instances but not in all (e.g. PC2/PC3Pd was inactive). The observed property changes indicate a role for the P domain in regulating the stability, calcium dependence, and pH dependence of the convertases.

Aggregation as a determinant of protein fate in post-Golgi compartments: role of the luminal domain of furin in lysosomal targeting

The mammalian endopeptidase furin is a type 1 integral membrane protein that is predominantly localized to the TGN and is degraded in lysosomes with a t1/2 = 2-4 h. Whereas the localization of furin to the TGN is largely mediated by sorting signals in the cytosolic tail of the protein, we show here that targeting of furin to lysosomes is a function of the luminal domain of the protein. Inhibition of lysosomal degradation results in the accumulation of high molecular weight aggregates of furin; aggregation is also dependent on the luminal domain of furin. Temperature and pharmacologic manipulations suggest that furin aggregation occurs in the TGN and thus precedes delivery to lysosomes. These findings are consistent with a model in which furin becomes progressively aggregated in the TGN, an event that leads to its transport to lysosomes. Our observations indicate that changes in the aggregation state of luminal domains can be potent determinants of biosynthetic targeting to lysosomes and suggest the possible existence of quality control mechanisms for disposal of aggregated proteins in compartments of the secretory pathway other than the endoplasmic reticulum.

Supramolecular assemblies from lysosomal matrix proteins and complex lipids

Most lysosomal hydrolases are soluble enzymes. Lamp-II (lysosome-associated membrane protein-II) is a major constituent of the lysosomal membrane. We studied the aggregation of a series of lysosomal molecules. The aggregation-sensitive lysosomal marker enzymes were optimally aggregated at intralysosomal pH. A similar pH dependence was recorded for aggregation of Lamp-II. The pH-dependent loss of solubility of isolated Lamp-II required components of the lysosome extract. Conditions of mild acid pH promoting aggregation triggered the formation of complexes with lipids of lysosomal origin. We fractionated a membrane-free lysosome extract by gel-filtration chromatography and could reconstitute assemblies in vitro from separated fractions. We found some selectivity in the lysosomal proteins binding to complex lipids, phosphatidylcholine, sphingomyelin, and phosphatidylethanolamine being most effective. We propose that the formation at pH 5.0 of such supramolecular assemblies between lysosomal proteins and lipids occurs within the intralysosomal environment. Some possible consequences of such an intralysosomal matrix formation on organelle function are discussed.

In vitro characterization of the novel proprotein convertase PC7

Biochemical and enzymatic characterization of the novel proprotein convertase rat PC7 (rPC7) was carried out using vaccinia virus recombinants overexpressed in mammalian BSC40 cells. Pro-PC7 is synthesized as a glycosylated zymogen (101 kDa) and processed into mature rPC7 (89 kDa) in the endoplasmic reticulum. No endogenously produced soluble forms of this membrane-anchored protein were detected. A deletion mutant (65 kDa), truncated well beyond the expected C-terminal boundary of the P-domain, produced soluble rPC7 in the culture medium. Enzymatic activity assays of rPC7 using fluorogenic peptidyl substrates indicated that the pH optimum, Ca2+ dependence, and cleavage specificity of this enzyme are largely similar to those of furin. However, with some substrates, cleavage specificity more closely resembled that of yeast kexin, suggesting differential processing of proprotein substrates by this novel convertase. We examined the rPC7- and human furin-mediated cleavage of synthetic peptides containing the processing sites of three proteins known to colocalize in situ with rPC7. Whereas both enzymes correctly processed the pro-parathyroid hormone tridecapeptide and the pro-PC4 heptadecapeptide, neither enzyme cleaved a pro-epidermal growth factor hexadecapeptide. Thus, this study establishes that rPC7 is an enzymatically functional subtilisin/kexin-like serine proteinase with a cleavage specificity resembling that of hfurin. In addition, we have demonstrated that rPC7 can correctly process peptide precursors that contain the processing sites of at least two potential physiological substrates.

Protein secretion: puzzling receptors

All known sorting receptors for soluble cargo in the secretory pathway are transmembrane proteins. For sorting to the regulated pathway, however, a subpopulation of secretory proteins, associated with the membrane but not membrane-spanning, appears to link cargo and membrane in storage granule biogenesis.

Overall lack of regulated secretion in a PC12 variant cell clone

A stable clone of PC12 neuroendocrine cells, named 27, known from previous studies to exhibit a defect of regulated secretion (lack of regulated secretory proteins, of synaptophysin, of dense granules and of catecholamine uptake and release; Clementi, E., Racchetti, G., Zacchetti, D., Panzeri, M. C., and Meldolesi, J. (1992) Eur. J. Neurosci. 4, 944-953) was characterized in detail to clarify the nature of its phenotype and the mechanisms of its establishment. The neuroendocrine nature of the PC12-27 phenotype was documented by specific markers: synapsins, neurofilament subunit H, neuronal kinesin, and alpha-latrotoxin receptor. Moreover, various intracellular membrane systems of PC12-27, including the endoplasmic reticulum and the Golgi complex, appeared similar to control PC12 in both morphology and marker expression. In contrast, all the investigated markers located either in dense granules (dopamine-beta-hydroxylase), in synaptic-like microvesicles (the acetylcholine transporter) or in both these regulated secretory organelles (VAMP2/synaptobrevin-2, synaptotagmin) were missing in PC12-27 cells, and the same was true also for the cytosolic and plasmalemma proteins involved in regulated exocytosis (Rab3, SNAP25, syntaxin). Pulse labeling and in vitro translation experiments revealed the defect to consist in a protein synthesis blockade that mRNA studies (reverse transcription-polymerase chain reaction, Northern blotting, and actinomycin D experiments) revealed to take place primarily at the transcriptional level. The secretion defect of PC12-27 cells was modified neither by various types of long term stimulation nor by nerve growth factor treatment. Moreover, when one of the missing regulated secretory proteins, chromogranin B, was expressed by cDNA transfection, it was secreted, however via the constitutive pathway. Our results demonstrate that PC12-27 cells are fully incompetent for both branches of regulated secretion, those of dense granules and synaptic-like microvesicles, possibly because of the impairment of a general expression control system that appears to operate independently of neuroendocrine cell differentiation.

Identification of a transferable sorting domain for the regulated pathway in the prohormone convertase PC2

The mammalian subtilisin-like endoproteases furin and PC2 catalyze similar reactions but in different parts of the cell: furin in the trans-Golgi network and PC2 in dense-core granules. To map targeting domains within PC2, chimeras were constructed of the pro-, catalytic, and middle domains of furin with the carboxyl-terminal domain of PC2 (F-S-P) or of the pro- and catalytic domains of furin with the middle and carboxyl-terminal domains of PC2 (F-N-P). Their behavior in stable transfected AtT-20 cells was compared to a furin mutant truncated after the middle domain (F-S), wild-type furin, and with wild-type PC2. F-S-P, F-N-P, and F-S were catalytically active and underwent post-translational proteolysis and N-glycosylation with similar kinetics to wild-type furin. The truncated furin mutant was not stored intracellularly, whereas both chimeras, like PC2, showed intracellular retention and regulated release. Immunofluorescence and immuno-electron microscopy showed the presence of the chimeras and PC2 in dense-cored secretory granules together with proopiomelanocortin immunoreactivity. PC2 was sorted more efficiently than F-S-P, and the inclusion of the middle domain (F-N-P) further enhanced intracellular retention. It is concluded that sorting of PC2 into the regulated pathway depends on its carboxyl terminus. The middle domain may provide additional sorting determinants or a conformational framework for expression of the sorting signal.

The C-terminal region of carboxypeptidase E involved in membrane binding is distinct from the region involved with intracellular routing

Carboxypeptidase E (CPE) is involved in the biosynthesis of numerous peptide hormones and neurotransmitters. Previously, the C-terminal region of CPE has been shown to participate in the binding of the protein to membranes and to also contribute to the sorting of CPE into the regulated pathway. In this study, the role of the C-terminal region of CPE was further examined using several approaches. A series of CPE mutants with C-terminal deletions was expressed in the baculovirus system; constructs with a deletion of 14 or 23 residues were expressed at levels comparable to wild-type CPE. In contrast, deletion of 33 or more residues eliminated CPE activity, and the resulting protein was not secreted from the cells. Even though CPE mutants with a deletion of 14 or 23 residues were expressed normally, the resulting protein was mainly soluble, whereas approximately 55% of wild-type CPE was membrane associated. When expressed in AtT-20 cells, CPE with a deletion of 43 C-terminal amino acids was not secreted, whereas CPE with a deletion of 23 residues was secreted via the regulated pathway. Pulse-chase analysis revealed the protein with a deletion of 43 residues to be degraded in a non-acidic intracellular compartment. To investigate whether the C-terminal region of CPE can confer membrane binding and regulated pathway sorting to another protein, portions of the CPE C-terminal region were attached to the C terminus of albumin and the fusion proteins expressed in AtT-20 cells. Of the constructs examined, only the protein containing 51 amino acids of CPE was sorted to the regulated pathway, although with reduced efficiency compared to endogenous CPE. Although the C-terminal 14 amino acids of CPE are sufficient to target albumin to membranes, this fusion protein is not sorted into the regulated pathway. Taken together, these results indicate that the C-terminal 14 amino acids of CPE are important for membrane binding and that membrane binding and sorting require distinct signals.

Calcium- and pH-dependent aggregation of carboxypeptidase E

Carboxypeptidase E (CPE) is involved with the biosynthesis of numerous peptide hormones and neurotransmitters. Several forms of CPE have been previously detected in neuroendocrine cells, including a form which is soluble at pH 5.5 (S-CPE), a form which can be extracted from membranes with 1 M NaCl at pH 5.5 (M1-CPE), and a form which requires both 1% Triton X-100 and 1 M NaCl for extraction from membranes at pH 5.5 (M2-CPE). Like other peptide processing enzymes, CPE is known to be sorted into peptide-containing secretory vesicles of the regulated pathway. One mechanism that has been proposed to be important for sorting of regulated pathway proteins is Ca2+ and pH-induced aggregation. CPE purified from bovine pituitary membranes aggregates at pH 5.5 when the concentration of CPE is 0.3 micrograms/microliters or higher, but not when the concentration is 0.01 microgram/microliters. Aggregation of CPE is pH-dependent, with very little aggregation occurring at pH 6 or above. At pH 5.0-5.5, the M2 form of CPE shows a greater tendency to aggregate than the other two forms. At pH 6, Ca2+ concentrations from 1-30 mM increase the aggregation of M1- and M2-CPE, but not S-CPE. The aggregation of M2-CPE does not explain the apparent membrane binding of this protein since the aggregate is solubilized by 1% Triton X-100 at pH 5.5 or by pH 6.0, whereas M2-CPE is not extracted from membranes under these conditions. Taken together, these results are consistent with a model in which the decreasing pH and increasing Ca2+ levels in the trans Golgi network induce the aggregation of CPE, which contributes to the sorting of this protein into regulated pathway secretory vesicles.

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.