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

The immunomodulatory functions of chromogranin A-derived peptide pancreastatin

Chromogranin A (CgA) is a 439 amino acid protein secreted by neuroendocrine cells. Proteolytic processing of CgA results in the production of different bioactive peptides. These peptides have been associated with inflammatory bowel disease, diabetes, and cancer. One of the chromogranin A-derived peptides is ∼52 amino acid long Pancreastatin (PST: human (h)CgA250-301, murine (m)CgA263-314). PST is a glycogenolytic peptide that inhibits glucose-induced insulin secretion from pancreatic islet β-cells. In addition to this metabolic role, evidence is emerging that PST also has inflammatory properties. This review will discuss the immunomodulatory properties of PST and its possible mechanisms of action and regulation. Moreover, this review will discuss the potential translation to humans and how PST may be an interesting therapeutic target for treating inflammatory diseases.

Abnormal Expression of an Insulin Synthesizing Enzyme in Islets of Adult Autoantibody Positive Donors

The observation that the two active forms of proprotein convertase 1/3 (PC1/3) were differentially expressed in beta cells of normal islets raised the possibility that this heterogeneity is lost during type 1 diabetes (T1D) progression. To test this hypothesis, the expression of the convertase was evaluated by confocal microscopy in sections of human pancreas of autoantibody positive (AA+) and T1D donors and compared with that of control. Islets of T1D pancreas were comprised of beta cells expressing either low or high PC1/3 levels and all islets of a pancreatic section contained only one beta cell type. Pancreata of AA+ donors contained either of these two classes of islets intermixed with normal islets comprised of beta cells with heterogeneous PC1/3 expression. This alteration affected the expression of proinsulin and insulin, which in most AA+ and T1D donors were lower than in controls. The present results indicate that the heterogeneity of PC1/3 expression is lost in all beta cells in a subset islets of AA+ donors and in all islets of T1D donors. These findings suggest that the heterogeneity of PC1/3 expression is a biomarker of human beta cell health and that its loss coincides with the initial stages of T1D.

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.

Similar sequences but dissimilar biological functions of GDF11 and myostatin

Growth differentiation factor 11 (GDF11) and myostatin (MSTN) are closely related TGFβ family members that are often believed to serve similar functions due to their high homology. However, genetic studies in animals provide clear evidence that they perform distinct roles. While the loss of Mstn leads to hypermuscularity, the deletion of Gdf11 results in abnormal skeletal patterning and organ development. The perinatal lethality of Gdf11-null mice, which contrasts with the long-term viability of Mstn-null mice, has led most research to focus on utilizing recombinant GDF11 proteins to investigate the postnatal functions of GDF11. However, the reported outcomes of the exogenous application of recombinant GDF11 proteins are controversial partly because of the different sources and qualities of recombinant GDF11 used and because recombinant GDF11 and MSTN proteins are nearly indistinguishable due to their similar structural and biochemical properties. Here, we analyze the similarities and differences between GDF11 and MSTN from an evolutionary point of view and summarize the current understanding of the biological processing, signaling, and physiological functions of GDF11 and MSTN. Finally, we discuss the potential use of recombinant GDF11 as a therapeutic option for a wide range of medical conditions and the possible adverse effects of GDF11 inhibition mediated by MSTN inhibitors.

Molecular characterization and spatiotemporal expression of prohormone convertase 2 in the Pacific abalone, Haliotis discus hannai

Prohormone convertases (PCs) are subtilisin-like proteases responsible for the intracellular processing of prohormones and proneuropeptides in vertebrates and invertebrates. The full-length PC2 cDNA sequence was cloned from pleuropedal ganglion of Haliotis discus hannai, consisted of 2254-bp with an open reading frame of 1989-bp and encoded a protein of 662 amino acid residues. The architecture of Hdh PC2 displayed key features of PCs, including a signal peptide, a pro-segment domain with sites for autocatalytic activation, a catalytic domain, and a pro-protein domain (P-domain). It shares the highest homology of its amino acid sequence with the PC2 from H. asinina and to lesser extent with that of Homo sapiens and Rana catesbeiana PC2. Sequence alignment analysis indicated that Hdh PC2 was highly conserved in the catalytic domain, including a catalytic triad of serine proteinases of the subtilisin family at positions Asp-195, His-236, and Ser-412. The cloned sequence contained a canonical integrin binding sequence, and four cysteine residues involved in the formation of an intramolecular disulfide link. Phylogenetic analysis revealed that the Hdh PC2 is robustly clustered with the Has PC2. Quantitative PCR assay demonstrated that the Hdh PC2 was predominantly expressed in the pleuropedal ganglion rather than in other examined tissues. Although PC2 mRNA was expressed throughout the gametogenetic cycle of male and female abalone, the expression level was significantly higher in the ripening stage of female abalone. Also, a significantly higher expression was observed in the pleuropedal ganglion and gonadal tissues at a higher effective accumulative temperature (1000°C). In situ hybridization revealed that the PC2 mRNA expressing neurosecretory cells were distributed in the cortex region of the pleuropedal ganglion. According to the results, it can be concluded that pleuropedal ganglion is the highest site of PC2 activity, and this enzyme might be involved in the abalone reproduction process.

The dynamic plasticity of insulin production in β-cells

BACKGROUND

Although the insulin-producing pancreatic β-cells are quite capable of adapting to both acute and chronic changes in metabolic demand, persistently high demand for insulin will ultimately lead to their progressive dysfunction and eventual loss. Recent and historical studies highlight the importance of 'resting' the β-cell as a means of preserving functional β-cell mass.

SCOPE OF REVIEW

We provide experimental evidence to highlight the remarkable plasticity for insulin production and secretion by the pancreatic β-cell alongside some clinical evidence that supports leveraging this unique ability to preserve β-cell function.

MAJOR CONCLUSIONS

Treatment strategies for type 2 diabetes mellitus (T2DM) targeted towards reducing the systemic metabolic burden, rather than demanding greater insulin production from an already beleaguered β-cell, should be emphasized to maintain endogenous insulin secretory function and delay the progression of T2DM.

Prohormone convertase 2 activity is increased in the hippocampus of Wfs1 knockout mice

Background: Mutations in WFS1 gene cause Wolfram syndrome, which is a rare autosomal recessive disorder, characterized by diabetes insipidus, diabetes mellitus, optic nerve atrophy, and deafness. The WFS1 gene product wolframin is located in the endoplasmic reticulum. Mice lacking this gene exhibit disturbances in the processing and secretion of peptides, such as vasopressin and insulin. In the brain, high levels of the wolframin protein have been observed in the hippocampus, amygdala, and limbic structures. The aim of this study was to investigate the effect of Wfs1 knockout (KO) on peptide processing in mouse hippocampus. A peptidomic approach was used to characterize individual peptides in the hippocampus of wild-type and Wfs1 KO mice.

Results: We identified 126 peptides in hippocampal extracts and the levels of 10 peptides differed between Wfs1 KO and wild-type mice at P < 0.05. The peptide with the largest alteration was little-LEN, which level was 25 times higher in the hippocampus of Wfs1 KO mice compared to wild-type mice. Processing (cleavage) of little-LEN from the Pcsk1n gene product proSAAS involves prohormone convertase 2 (PC2). Thus, PC2 activity was measured in extracts prepared from the hippocampus of Wfs1 KO mice. The activity of PC2 in Wfs1 mutant mice was significantly higher (149.9 ± 2.3%, p < 0.0001, n = 8) than in wild-type mice (100.0 ± 7.0%, n = 8). However, Western blot analysis showed that protein levels of 7B2, proPC2 and PC2 were same in both groups, and so were gene expression levels.

Conclusion: Processing of proSAAS is altered in the hippocampus of Wfs1-KO mice, which is caused by increased activity of PC2. Increased activity of PC2 in Wfs1 KO mice is not caused by alteration in the levels of PC2 protein. Our results suggest a functional link between Wfs1 and PC2. Thus, the detailed molecular mechanism of the role of Wfs1 in the regulation of PC2 activity needs further investigation.

Molecular determinants for subcellular trafficking of the malarial sheddase PfSUB2

The malaria merozoite invades erythrocytes in the vertebrate host. Iterative rounds of asexual intraerythrocytic replication result in disease. Proteases play pivotal roles in erythrocyte invasion, but little is understood about their mode of action. The Plasmodium falciparum malaria merozoite surface sheddase, PfSUB2, is one such poorly characterized example. We have examined the molecular determinants that underlie the mechanisms by which PfSUB2 is trafficked initially to invasion-associated apical organelles (micronemes) and then across the surface of the free merozoite. We show that authentic promoter activity is important for correct localization of PfSUB2, likely requiring canonical features within the intergenic region 5' of the pfsub2 locus. We further demonstrate that trafficking of PfSUB2 beyond an early compartment in the secretory pathway requires autocatalytic protease activity. Finally, we show that the PfSUB2 transmembrane domain is required for microneme targeting, while the cytoplasmic domain is essential for surface translocation of the protease to the parasite posterior following discharge from micronemes. The interplay of pre- and post-translational regulatory elements that coordinate subcellular trafficking of PfSUB2 provides the parasite with exquisite control over enzyme-substrate interactions.

Curcumin affects proprotein convertase activity: elucidation of the molecular and subcellular mechanism

Proprotein convertases (PCs) form a group of serine endoproteases that are essential for the activation of proproteins into their active form. Some PCs have been proposed to be potential therapeutic targets for cancer intervention because elevated PC activity has been observed in many different cancer types and because many of the PC substrates, such as pro-IGF-1R, pro-TGF-beta, pro-VEGF, are involved in signaling pathways related to tumor development. Curcumin, reported to possess anticancer activity, also affects many of these pathways. We therefore investigated the effect of curcumin on PC activity. Our results show that curcumin inhibits PC activity in a cell lysate-based assay but not in vitro. PC zymogen maturation in the endoplasmic reticulum appears to be inhibited by curcumin. Treating cells with thapsigargin or cyclopiazonic acid, two structurally unrelated inhibitors of the sarco- and endoplasmic reticulum Ca(2+)ATPase (SERCA), also hampered both the PC zymogen maturation and the PC activity. Importantly, curcumin, like the SERCA inhibitors, impaired ATP-driven (45)Ca(2+) uptake in the endoplasmic reticulum. These results indicate that curcumin likely restrains PC activity by inhibiting SERCA-mediated Ca(2+)-uptake activity. Experiments in three colon cancer cell lines confirm that curcumin inhibits both the (45)Ca(2+) uptake and PC activity, notably the processing of pro-IGF-1R. Both curcumin and thapsigargin inhibit the anchorage-independent growth of these three colon carcinoma cell lines. In conclusion, our findings indicate that curcumin inhibits PC zymogen maturation and consequently PC activity and that its inhibitory effect on Ca(2+) uptake into the ER allows and is sufficient to explain this phenomenon.

Exploring the membrane topology of prohormone convertase 1 in AtT20 Cells: in situ analysis by immunofluorescence microscopy

Prohormone convertase 1 (PC1) was previously characterized as a partially transmembrane protein in purified chromaffin granules of bovine adrenal medulla1. This was challenged with experiments on transfected PC1 in COS1 cells, a non-endocrine cell line2. To address this issue, we undertook to analyze its extraction properties  in vitro and its immunocytochemical localization  in situ in AtT20 cells, an endocrine cell line that expresses PC1. Most of the 87 kDa form of PC1 was resistant to carbonate extraction suggesting that it had properties of a transmembrane protein. Under semi-permeabilized conditions whereby only the plasma membrane was permeabilized, the carboxy-terminus of PC1 was specifically immunostained whereas the amino-terminus was not. These results indicate that the amino-terminus of PC1 was within the lumen of the Golgi and granules, and some of the C-terminus was exposed to the cytosol. Thus, endogenous PC1 can assume a transmembrane orientation  in situ in AtT20 cells.

Molecular cloning, ontogeny and tissue distribution of zebrafish (Danio rerio) prohormone convertases: pcsk1 and pcsk2

Prohormone convertase subtilisin/kexin (PCSK) enzymes are a family of nine related serine proteases, found in a multitude of tissues, and responsible for the maturation of a variety of protein and peptide precursors. Pcsk1 and Pcsk2 are found within dense core secretory granules in endocrine and neuroendocrine cells and are responsible for cleaving several hormones and neuropeptide precursors. In this work, we cloned and sequenced the cDNA of pcsk1 and pcsk2 from zebrafish (Danio rerio). pcsk1 is a 2268bp ORF, whose 755 amino acid protein product is identical to that predicted from the genome sequence. pcsk2 is a 1941bp ORF, encoding a 646 amino acid peptide. Both Pcsk1 and Pcsk2 display high degrees of similarity to their counterparts in other species, including the conservation of the catalytic triad and other essential residues. The brain contained the highest expression levels of both pcsk1 (1.49+/-0.21) (displayed as ratio to EF-1a), and pcsk2 (0.23+/-0.04). Both transcripts were also detectable in the fore, mid and distal gut. pcsk1 and 2 were detectable at 4.5h post-fertilization, and while pcsk1 expression increased throughout development (0.12+/-0.01 maximum at 3 days post-fertilization), pcsk2 expression was highest at day 5 post-fertilization (0.03+/-0.01), and decreased prior. For the first time, we have identified and characterized a pcsk1 transcript in fish. We have also identified and characterized the pcsk2 transcript in zebrafish, and have assessed the tissue distribution and ontogeny of both.

The transmembrane domain of the prohormone convertase PC3: a key motif for targeting to the regulated secretory pathway

The biosynthesis of hormones and neuropeptides involves post-translational cleavage of precursors at basic amino acids by prohormone convertases (PCs) predominantly in secretory granules that bud from the trans-Golgi Network. This study reports that the amino acid sequence of PC3 (aa617-638), previously identified as a novel transmembrane (TM) domain, confers lipid raft association and facilitates sorting of the enzyme to the secretory granules of Neuro2A cells for prohormone cleavage. Floatation analysis on sucrose density gradients showed that a proportion of full length (PC3-FL) and carboxyl terminus-truncated PC3(1-638) (PC3-638) containing the TM domain were associated with lipid rafts in Neuro2A cells, while PC3(1-616) (PC3-616) and PC3-DeltaTM lacking the TM domain were not. Secondly, PC3-FL and PC3-638 underwent stimulated secretion and were shown to be colocalized with a secretory granule marker, chromogranin A, by immunocytochemistry. In contrast, PC3-616 and PC3-DeltaTM were constitutively secreted and primarily localized in the Golgi. These data indicate that the transmembrane domain of PC3 plays a key role in sorting the enzyme to the regulated secretory pathway.

Single amino acid substitution in the PC1/3 propeptide can induce significant modifications of its inhibitory profile toward its cognate enzyme

The proprotein convertase PC1/3 is synthesized as a large precursor that undergoes proteolytic processing of the signal peptide, the propeptide and ultimately the COOH-terminal tail, to generate the mature form. The propeptide is essential for protease folding, and, although cleaved by an autocatalytic process, it remains associated with the mature form acting as an auto-inhibitor of PC1/3. To further assess the role of certain residues in its interaction with its cognate enzyme, we performed an alanine scan on two PC1/3 propeptide potential cleavable sites ((50)RRSRR(54) and (61)KR(62)) and an acidic region (65)DDD(67) conserved among species. Upon incubation with PC1/3, the ensuing peptides exhibit equal inhibitory potency, lower potency, or higher potency than the wild-type propeptide. The K(i) values calculated varied between 0.15 and 16.5 nm. All but one mutant exhibited a tight binding behavior. To examine the specificity of mutants, we studied their reactivity toward furin, a closely related convertase. The mutation of certain residues also affects the inhibition behavior toward furin yielding propeptides exhibiting K(i) ranging from 0.2 to 24 nm. Mutant propeptides exhibited against each enzyme either different mode of inhibition, enhanced selectivity in the order of 40-fold for one enzyme, or high potency with no discrimination. Hence, we demonstrate through single amino acid substitution that it is feasible to modify the inhibitory behavior of propeptides toward convertases in such a way as to increase or decrease their potency, modify their inhibitory mechanisms, as well as increase their selectivity.

Discovery of the Proprotein Convertases and their Inhibitors

The members of the convertase family play a central role in the processing of various protein precursors ranging from hormones and growth factors to viral envelope proteins and bacterial toxins. The proteolysis of these precursors that occurs at basic residues is mediated by the proprotein convertases (PCs), namely: PC1, PC2, Furin, PACE4, PC4, PC5 and PC7. The proteolysis at non-basic residues is performed by subtilisin/kexin-like isozyme-1 (S1P/SKI-1) and the newly identified neural apoptosis-regulated convertase-1 (NARC-1/PCSK9). These proteases have key roles in many physiological processes and various pathologies including cancer, obesity, diabetes, neurodegenerative diseases and autosomal dominant hypercholesterolermia. Here we summarize the discovery of the proprotein convertases and their inhibitors, discuss their properties, roles, resemblance and differences

Expression and transient nuclear translocation of proprotein convertase 1 (PC1) during mouse preimplantation embryonic development

Preimplantation embryos express a number of hormones, neuropeptides, and membrane receptors known to derive from proteolytic activation of their precursors by the seven-member family of subtilisin-like, calcium-dependent serine proteinases known as proprotein convertases (PCs). The goal of this study was to determine the pattern of PC expression in mouse preimplantation embryos. Transcripts for all PCs, except PC2, were detected by reverse transcription-polymerase chain reaction (RT-PCR) in unfertilized and fertilized eggs. Furin, PACE4, PC1, and PC7 transcripts remained present at subsequent stages of preimplantation embryonic development, whereas the levels of transcripts for PC4 and PC5 gradually disappeared after the 2-cell stage. Proprotein convertase 1 (PC1) expression was further examined at the protein level. Immunoblotting revealed the presence of the zymogen and mature forms of this enzyme in eggs and embryos. Immunofluorescence laser confocal microscopy showed PC1-specific staining throughout the cytoplasm of unfertilized eggs. After fertilization, surprisingly, the staining was concentrated in pronuclei. It relocated to the cytoplasm at postzygotic stages and was particularly strong at junctions between blastomeres. The nuclear translocation of PC1 in fertilized eggs is probably mediated by its prodomain. Indeed, when transduced in human colon carcinoma LoVo cells, a mutant proPC1 incapable of cleaving off its prodomain was shown to accumulate in the nucleus. Furthermore, when N-terminally fused to green fluorescent protein, this domain was able to direct the reporter protein to the nucleus of these cells. Collectively, these data establish that eggs and preimplantation embryos express various PCs necessary for proteolytic activation of precursors of hormones and growth factors. They also raise the possibility of a nuclear function for PC1 during zygote formation.

Expression of proprotein convertase 2 mRNA in the ovarian follicles of the medaka, Oryzias latipes

Proprotein convertases (PCs) are enzymes responsible for processing the precursors of many bioactive peptides in vertebrates and invertebrates. In the present study, a cDNA for proprotein convertase 2 (PC2) was cloned for the first time from a fish. The clone, which was isolated from the ovary of the medaka, Oryzias latipes, by a combination of RT-PCR cloning and 5'- and 3'-rapid amplification of cDNA ends, codes for a protein of 641 amino acid residues highly homologous to other vertebrate PC2. The medaka preproPC2 consists of a signal sequence, a propeptide with sites for autocatalytic activation, a Kex2-like catalytic domain, and a P-domain. The catalytic triad residues (Asp-169, His-210, and Ser-386) were all conserved. Northern blot analysis revealed that PC2 was expressed in the brain, ovary, and kidney of the fish. The size of PC2 mRNA expressed in the ovary was 2.3 kb, whereas those of the brain and kidney were 2.8 kb. This size difference was attributed to the lack of an approximately 300-bp nucleotide sequence just before the poly(A)+ tail of the ovarian PC2 mRNA. Ovarian expression of the PC2 gene was found in the medaka but not in the mouse, and therefore further analysis was conducted for the fish ovary. The greatest expression of PC2 mRNA in the oocytes of small growing follicles in the mature medaka was demonstrated by Northern blotting, RT-PCR and in situ hybridization analysis. These results suggest that PC2 may play a role in the processing of proproteins and/or pro-hormones expressed in the growing oocytes.

The C-terminus of prohormone convertase 2 is sufficient and necessary for Raft association and sorting to the regulated secretory pathway

Prohormone convertase 2 (PC2) is a member of the subtilisin family of proteases involved in prohormone maturation in the granules of the regulated secretory pathway (RSP). It has been suggested that targeting of this enzyme to the RSP is dependent on its association with lipid rafts in membranes at the trans-Golgi network. Here, we investigate the orientation of PC2 in granule membranes and the role of the C-terminus in sorting of the enzyme to the RSP. Molecular modeling and circular dichroism showed that this domain of PC2 forms an alpha-helix and inserts into artificial membranes. Furthermore, we show that the C-terminus of PC2 can be biotinylated at the C-terminus in intact chromaffin granules, indicating that it is a transmembrane protein. To determine if the PC2 C-terminus is necessary for raft association and sorting, we transfected a chimera of CPEDelta15 (carboxypeptidase E without the last 15 residues) and the last 25 residues of PC2 (CPEDelta15-PC2), and a truncated PC2 mutant with the last 6 residues deleted (PC2Delta6) into Neuro2a cells. Whereas CPEDelta15 was not raft-associated or sorted to the RSP, addition of the 25 residues of PC2 C-terminus to CPEDelta15 restored raft association and localization to the RSP granules, as determined by immunocytochemistry. Deletion of the last 6 residues of PC2 eliminated lipid raft association and sorting of PC2Delta6 to the RSP. These results showed that the PC2 C-terminus confers raft association and is sufficient and necessary for sorting PC2 to the RSP.

Furin and prohormone convertase 1/3 are major convertases in the processing of mouse pro-growth hormone-releasing hormone

We investigated the proteolytic processing of mouse pro-GHRH [84 amino acids (aa)] by furin, PC1/3, PC2, and PC5/6A. We created six point mutations in the N- and C-terminal cleavage sites, RXXR decreased and RXRXXR decreased, respectively. The following results were obtained after transient transfection/cotransfection and metabolic pulse-chase labeling studies in several neuroendocrine cells. 1) Furin was the most efficient convertase in cleaving the N-terminal RXXR/RXRR site to generate intermediate I, 12-84aa, whereas PC1/3 was the most potent in processing the C-terminal RXRXXR site to yield mature GHRH, 12-53aa. 2) Both PC1/3 and PC5/6A also processed the N-terminal site but less efficiently than furin. 3) PC2 was much weaker in cleaving the C-terminal site relative to PC1/3 to generate mature GHRH. 4) The Q10R mutant was significantly more susceptible to furin cleavage at the N-terminal site than the wild-type pro-GHRH. And 5) the N- and C-terminal P1 Arg residues, R11 and R54, respectively, were essential for mature GHRH production. We also showed localization of the GHRH immunoreactive peptides in Golgi and secretory granules in neuroendocrine cells by an immunofluorescence assay. We conclude that the efficient production of mature GHRH from pro-GHRH is a stepwise process mediated predominantly by furin at the N-terminal cleavage site followed by PC1/3 at the C terminus.

The prohormone processing enzyme PC3 is a lipid raft-associated transmembrane protein

The biosynthesis of most biologically active peptides involves the action of prohomone convertases, including PC3 (also known as PC1), that catalyze limited proteolysis of precursor proteins. Proteolysis of prohormones occurs mainly in the granules of the regulated secretory pathway. It has been proposed that the targeting of these processing enzymes to secretory granules involves their association with lipid rafts in granule membranes. We now provide evidence for the interaction of the 86 and 64 kDa forms of PC3 with secretory granule membranes. Furthermore, both forms of PC3 were resistant to extraction with TX-100, were floated to low-density fractions in sucrose gradients, and were partially extracted upon cholesterol depletion by methyl-beta-cyclodextrin, indicating that they were associated with lipid rafts in the membranes. Protease protection assays, immunolabeling, and biotinylation of proteins in intact secretory granules identified an approximately 115-residue cytoplasmic tail for 86 kDa PC3. Using two-dimensional gel electrophoresis and a specific antibody, a novel, raft-associated form of 64 kDa PC3 that contains a transmembrane domain consisting of residues 619-638 was identified. This form was designated as 64 kDa PC3-TM, and differs from the 64 kDa mature form of PC3. We present a model of the membrane topology of PC3, where it is anchored to lipid rafts in secretory granule membranes via the transmembrane domain. We demonstrate that the transmembrane domain of PC3 alone was sufficient to target the extracellular domain of the IL2 receptor alpha-subunit (Tac) to secretory granules.

Neuroendocrine secretory protein 7B2: structure, expression and functions

7B2 is an acidic protein residing in the secretory granules of neuroendocrine cells. Its sequence has been elucidated in many phyla and species. It shows high similarity among mammals. A Pro-Pro-Asn-Pro-Cys-Pro polyproline motif is its most conserved feature, being carried by both vertebrate and invertebrate sequences. It is biosynthesized as a precursor protein that is cleaved into an N-terminal fragment and a C-terminal peptide. In neuroendocrine cells, 7B2 functions as a specific chaperone for the proprotein convertase (PC) 2. Through the sequence around its Pro-Pro-Asn-Pro-Cys-Pro motif, it binds to an inactive proPC2 and facilitates its transport from the endoplasmic reticulum to later compartments of the secretory pathway where the zymogen is proteolytically matured and activated. Its C-terminal peptide can inhibit PC2 in vitro and may contribute to keep the enzyme transiently inactive in vivo. The PC2-7B2 model defines a new neuroendocrine paradigm whereby proteolytic activation of prohormones and proneuropeptides in the secretory pathway is spatially and temporally regulated by the dynamics of interactions between converting enzymes and their binding proteins. Interestingly, unlike PC2-null mice, which are viable, 7B2-null mutants die early in life from Cushing's disease due to corticotropin ('ACTH') hypersecretion by the neurointermediate lobe, suggesting a possible involvement of 7B2 in secretory granule formation and in secretion regulation. The mechanism of this regulation is yet to be elucidated. 7B2 has been shown to be a good marker of several neuroendocrine cell dysfunctions in humans. The possibility that anomalies in its structure and expression could be aetiological causes of some of these dysfunctions warrants investigation.

Impaired prohormone convertases in Cpe(fat)/Cpe(fat) mice

A spontaneous point mutation in the coding region of the carboxypeptidase E (CPE) gene results in a loss of CPE activity that correlates with the development of late onset obesity (Nagert, J. K., Fricker, L. D., Varlamov, O., Nishina, P. M., Rouille, Y., Steiner, D. F., Carroll, R. J., Paigen, B. J., and Leiter, E. H. (1995) Nat. Genet. 10, 135-142). Examination of the level of neuropeptides in these mice showed a decrease in mature bioactive peptides as a result of a decrease in both carboxypeptidase and prohormone convertase activities. A defect in CPE is not expected to affect endoproteolytic processing. In this report we have addressed the mechanism of this unexpected finding by directly examining the expression of the major precursor processing endoproteases, prohormone convertases PC1 and PC2 in Cpe(fat) mice. We found that the levels of PC1 and PC2 are differentially altered in a number of brain regions and in the pituitary. Since these enzymes have been implicated in the generation of neuroendocrine peptides (dynorphin A-17, beta-endorphin, and alpha- melanocyte-stimulating hormone) involved in the control of feeding behavior and body weight, we compared the levels of these peptides in Cpe(fat) and wild type animals. We found a marked increase in the level of dynorphin A-17, a decrease in the level of alpha-melanocyte-stimulating hormone, and an alteration in the level of C-terminally processed beta-endorphin. These results suggest that the impairment in the level of these and other peptides involved in body weight regulation is mainly due to an alteration in carboxypeptidase and prohormone convertase activities and that this may lead to the development of obesity in these animals.

A predicted alpha -helix mediates targeting of the proprotein convertase PC1 to the regulated secretory pathway

The proprotein convertase PC1 is a protease whose activity is largely confined to the dense core secretory granules of neuroendocrine cells. Efficient processing of PC1 substrates in granules requires a mechanism that will both limit the activity of the enzyme to these organelles and promote its targeting to the nascent secretory granules. In the current study, we provide evidence that targeting of PC1 to secretory granules is mediated by alpha-helical structures in its C-terminal tail and, at least in part, is dependent on interactions with specific components of the secretory granule membrane.

Defective prodynorphin processing in mice lacking prohormone convertase PC2

Prodynorphin, a multifunctional precursor of several important opioid peptides, is expressed widely in the CNS. It is processed at specific single and paired basic sites to generate various biologically active products. Among the prohormone convertases (PCs), PC1 and PC2 are expressed widely in neuroendocrine tissues and have been proposed to be the major convertases involved in the biosynthesis of hormonal and neural peptides. In this study we have examined the physiological involvement of PC2 in the generation of dynorphin (Dyn) peptides in mice lacking active PC2 as a result of gene disruption. Enzymological and immunological assays were used to confirm the absence of active PC2 in these mice. The processing profiles of Dyn peptides extracted from brains of these mice reveal a complete lack of Dyn A-8 and a substantial reduction in the levels of Dyn A-17 and Dyn B-13. Thus, PC2 appears to be involved in monobasic processing, leading to the generation of Dyn A-8, Dyn A-17, and Dyn B-13 from prodynorphin under physiological conditions. Brains of heterozygous mice exhibit only half the PC2 activity of wild-type mice; however, the levels of Dyn peptides in these mice are similar to those of wild-type mice, suggesting that a 50% reduction in PC2 activity is not sufficient to significantly reduce prodynorphin processing. The disruption of the PC2 gene does not lead to compensatory up-regulation in the levels of other convertases with similar substrate specificity because we find no significant changes in the levels of PC1, PC5/PC6, or furin in these mice as compared with wild-type mice. Taken together, these results support a critical role for PC2 in the generation of Dyn peptides.

Inhibition of the vacuolar H+-ATPase perturbs the transport, sorting, processing and release of regulated secretory proteins

Vacuolar H+-ATPases (V-ATPases) are multisubunit enzymes that acidify various intracellular organelles, including secretory pathway compartments. We have examined the effects of the specific V-ATPase inhibitor bafilomycin A1 (Baf) on the intracellular transport, sorting, processing and release of a number of neuroendocrine secretory proteins in primary Xenopus intermediate pituitary cells. Ultrastructural examination of Baf-treated intermediate pituitary cells revealed a reduction in the amount of small dense-core secretory granules and the appearance of vacuolar structures in the trans-Golgi area. Pulse-chase incubations in combination with immunoprecipitation analysis showed that in treated cells, the proteolytic processing of the newly synthesized prohormone proopiomelanocortin, prohormone convertase PC2 and secretogranin III (SgIII) was inhibited, and an intracellular accumulation of intact precursor forms and intermediate cleavage products became apparent. Moreover, we found that treated cells secreted considerable amounts of a PC2 processing intermediate and unprocessed SgIII in a constitutive fashion. Collectively, these data indicate that in the secretory pathway, V-ATPases play an important role in creating the microenvironment that is essential for proper transport, sorting, processing and release of regulated secretory proteins.

Processing of synthetic pro-islet amyloid polypeptide (proIAPP) 'amylin' by recombinant prohormone convertase enzymes, PC2 and PC3, in vitro

Islet amyloid polypeptide (IAPP), amylin, is the constituent peptide of pancreatic islet amyloid deposits which form in islets of Type 2 diabetic subjects. Human IAPP is synthesized as a 67-residue propeptide in islet beta-cells and colocalized with insulin in beta-cell granules. The mature 37-amino acid peptide is produced by proteolysis at pairs of basic residues at the C- and N-termini of the mature peptide. To determine the enzymes responsible for proteolysis and their activity at the potential cleavage sites, synthetic human proIAPP was incubated (0.5-16 h) with recombinant prohormone convertases, PC2 or PC3 at appropriate conditions of calcium and pH. The products were analysed by MS and HPLC. Proinsulin was used as a control and was cleaved by both recombinant enzymes resulting in intermediates. PC3 was active initially at the N-terminal-IAPP junction and later at the C-terminus, whereas initial PC2 activity was at the IAPP-C-terminal junction. Processing at the basic residues within the C-terminal flanking peptide rarely occurred. There was no evidence for substantial competition for the processing enzymes when the combined substrates proinsulin and proIAPP were incubated with both PC2 and PC3. As proinsulin cleavage is sequential in vivo (PC3 active at the B-chain-C-peptide junction, followed by PC2 at A chain-C-peptide junction), these data suggest that proteolysis of proIAPP and proinsulin is coincident in secretory granules and increased proinsulin secretion in diabetes could be accompanied by increased production of proIAPP.

Involvement of the membrane lipid bilayer in sorting prohormone convertase 2 into the regulated secretory pathway

Prohormone convertase 2 (PC2) is a neuroendocrine-specific protease involved in the intracellular maturation of prohormones and proneuropeptides. PC2 is synthesised as a proprotein (proPC2) that undergoes proteolysis, aggregation and membrane association during its transit through the regulated secretory pathway. We have previously shown that the pro region of proPC2 plays a key role in its aggregation and membrane association. To investigate this further, we determined the binding properties of a peptide containing amino acids 45-84 of proPC2 (proPC2(45-84)) to trans-Golgi network/granule-enriched membranes from the AtT20 cell line. Removal of peripheral membrane proteins or hydrolysis of integral membrane proteins did not affect the binding properties of proPC2(45-84). Rather, proPC2(45-84) was shown to bind to protein-free liposomes in a pH- and Ca(2+)-dependent manner. To identify the component of the lipid bilayer involved in this membrane association, we used chromaffin-granule membranes and studied the binding properties of the endogenous PC2. Treatment of the membranes with saponin, a cholesterol-depleting detergent, failed to extract PC2 from the membranes, whereas chromogranin A (CgA) was removed. Treatment of the membranes with Triton X-100 yielded a low-density detergent-insoluble fraction enriched in PC2, but not CgA. The detergent-insoluble fraction also contained glycoprotein III, known to be part of the lipid rafts (membrane microdomains rich in sphingolipids). Finally, sphingolipid depletion of AtT20 cells resulted in the mis-sorting of PC2, suggestive of a link between the association of PC2 with lipid rafts in the membrane and its sorting into the regulated secretory pathway.

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.

Translational regulation of proinsulin biosynthesis and proinsulin conversion in the pancreatic beta-cell

Insulin secretion from the pancreatic beta -cell can be initiated in minutes, vary as much as 50-100-fold, and be sustained for several hours without need for changes in insulin gene transcription. Remarkably, the cellular content of the hormone and its molecular composition do not vary appreciably in the face of changes of insulin granule exocytosis. Minimal morphological changes are apparent, further indicating that the movement of lipids and membrane proteins between the granule storage pool, the plasma membrane, and Golgi are likewise tightly controlled. Such homeostasis is achieved by an interplay of signaling pathways originating from the metabolism of glucose with downstream targets at the level of translation of dense-core granule proteins, granule biogenesis, and membrane trafficking. Our scant knowledge in this area is confined mostly to a descriptive account of the fate of the major secreted components, principally insulin and the enzymes PC1, PC2, and CPH involved in the proteolytic conversion of proinsulin to insulin. A common theme seems to be the role of intracellular energy homeostasis in integrating the stimulus-secretion and stimulus-biosynthetic responses of this cell.

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.

LeSBT1, a subtilase from tomato plants. Overexpression in insect cells, purification, and characterization

The cDNA of a tomato subtilase designated LeSBT1 was cloned from a tomato flower cDNA library. The deduced amino acid sequence indicated for LeSBT1 the structure of a prepro-protein targeted to the secretory pathway by virtue of an amino-terminal signal peptide. LeSBT1 was expressed in the baculovirus/insect cell system and a processed 73-kDa form of LeSBT1, lacking both signal peptide and prodomain, was purified to homogeneity from culture supernatants. This 73-kDa LeSBT1, however, lacked proteolytic activity. Zymogen activation to yield 68-kDa LeSBT1 required the additional processing of an amino-terminal autoinhibitory peptide in a strictly pH-dependent manner. Mature 68-kDa LeSBT1 showed highest activity at acidic pH consistent with its presumed localization in the apoplast of the plant cell. In comparison to other plant subtilases, LeSBT1 exhibited a narrower substrate specificity in that it cleaves only polypeptide substrates preferentially but not exclusively carboxyl-terminal of glutamine residues. The possible involvement of LeSBT1 in selective proprotein processing is discussed with reference to the related mammalian proprotein convertases.

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 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.

Activation and routing of membrane-tethered prohormone convertases 1 and 2

Many peptide hormones and neuropeptides are processed by members of the subtilisin-like family of prohormone convertases (PCs), which are either soluble or integral membrane proteins. PC1 and PC2 are soluble PCs that are primarily localized to large dense core vesicles in neurons and endocrine cells. We examined whether PC1 and PC2 were active when expressed as membrane-tethered proteins, and how tethering to membranes alters the biosynthesis, enzymatic activity, and intracellular routing of these PCs. PC1 and PC2 chimeras were constructed using the transmembrane domain and cytoplasmic domain of the amidating enzyme, peptidylglycine alpha-amidating monooxygenase (PAM). The membrane-tethered PCs were rerouted from large dense core vesicles to the Golgi region. In addition, the chimeras were transiently expressed at the cell surface and rapidly internalized to the Golgi region in a fashion similar to PAM. Membrane-tethered PC1 and PC2 exhibited changes in pro-domain maturation rates, N-glycosylation, and in the pH and calcium optima required for maximal enzymatic activity against a fluorogenic substrate. In addition, the PC chimeras efficiently cleaved endogenous pro-opiomelanocortin to the correct bioactive peptides. The PAM transmembrane domain/cytoplasmic domain also prevented stimulated secretion of pro-opiomelanocortin products in AtT-20 cells.

Localization of metallocarboxypeptidase D in AtT-20 cells. Potential role in prohormone processing

Carboxypeptidase D (CPD) is a recently discovered metallocarboxypeptidase that is predominantly located in the trans-Golgi network (TGN), and also cycles between the cell surface and the TGN. In the present study, the intracellular distribution of CPD was examined in AtT-20 cells, a mouse anterior pituitary-derived corticotroph. CPD-containing compartments were isolated using antibodies to the CPD cytosolic tail. The immunopurified vesicles contained TGN proteins (TGN38, furin, syntaxin 6) but not lysosomal or plasma membrane proteins. The CPD-containing vesicles also contained neuropeptide-processing enzymes and adrenocorticotropic hormone, a product of proopiomelanocortin proteolysis. Electron microscopic analysis revealed that CPD is present within the TGN and immature secretory granules but is virtually absent from mature granules, suggesting that CPD is actively removed from the regulated pathway during the process of granule maturation. A second major finding of the present study is that a soluble truncated form of CPD is secreted mainly via the constitutive pathway in AtT-20 cells, indicating that the lumenal domain does not contain signals for the sorting of CPD to mature secretory granules. Taken together, these data are consistent with the proposal that CPD participates in the processing of proteins within the TGN and immature secretory vesicles.

Protein 7B2 is essential for the targeting and activation of PC2 into the regulated secretory pathway of rMTC 6-23 cells

Among the prohormone convertases, PC2 is unique in that it specifically binds to the neuroendocrine-specific protein 7B2 in the endoplasmic reticulum (ER) and is activated late in the regulated secretory pathway of neuroendocrine cells. Several roles, sometimes contradictory, have been suggested for 7B2 with regard to PC2 cellular fate. Thus, 7B2 was proposed to act as a PC2 chaperone in the ER, or to facilitate 7B2 transport from the ER to the trans-Golgi network and to be necessary for proPC2 activation, or to inhibit PC2 enzymatic activity until the latter reaches the secretory granules. To gain insight into the function of 7B2, we sought to block its expression in PC2-expressing endocrine cells using antisense strategies. We have previously shown that the endocrine rMTC 6-23 cell line expresses PC2 and that the enzyme is responsible for the processing of pro-neurotensin/neuromedin N (proNT/NN). Here, we show that rMTC 6-23 cells express 7B2 and that the protein was coordinately induced with PC2 and proNT/NN by dexamethasone. Stable transfection of rMTC 6-23 cells with 7B2 antisense cDNA led to a marked reduction (>90%) in 7B2 levels. ProPC2 was expressed to normal levels and cleaved to yield a PC2 form that was constitutively released, was not stored within secretory granules and was unable to process proNT/NN. We conclude that 7B2 is essential for the sorting and activation of PC2 into the regulated secretory pathway of endocrine cells.

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.

Differences in the autocatalytic cleavage of pro-PC2 and pro-PC3 can be attributed to sequences within the propeptide and Asp310 of pro-PC2

PC2 and PC3 are subtilisin-like proteases involved in the maturation of prohormones and proneuropeptides within neuroendocrine cells. They are synthesized as zymogens that undergo autocatalytic maturation within the secretory pathway. Maturation of pro-PC2 is slow (t12 >8 h), exhibits a pH optimum of 5.5 and is dependent on calcium (K0.5 2 mM), while pro-PC3 maturation is relatively rapid (t12 15 min), exhibits a neutral pH optimum and is not calcium dependent. These differences in the rates and optimal conditions for activation of the proteases may contribute to the diversity of products generated by these proteases in different cell types. Although highly similar, there are two major differences between pro-PC2 and pro-PC3: the presence of an aspartate at position 310 in pro-PC2 compared with asparagine at the equivalent position in pro-PC3 (and all other members of the subtilisin family), and the N-terminal propeptides, which exhibit low sequence identity (30%). With a view to establishing the structural features that might be responsible for these differences in the maturation of pro-PC2 and pro-PC3, Asp310 in pro-PC2 was mutated to Asn, and Asn309 in pro-PC3 was mutated to Asp. Chimaeric proteins were also made consisting of the pro-region of PC2 fused to the mature portion of PC3 and the pro-region of PC3 fused to the mature region of PC2. The wild-type and mutant DNA constructs were then transcribed and translated in an in vitro system capable of supporting maturation of pro-PC2 and pro-PC3. The results demonstrated that Asp310 of pro-PC2 is responsible for the acidic pH optimum for maturation. Thus changing Asp310 to Asn shifted the pH optimum for maturation to pH 7.0. However, changing Asn309 of pro-PC3 to Asp had no effect on the optimum pH for maturation of pro-PC3. A chimaeric construct containing the propeptide of pro-PC2 attached to PC3 shifted the pH optimum for maturation from pH 7.0 to 6.0 and slowed down the rate of maturation (t12 >8 h). When attached to PC2, the pro-region of pro-PC3 had no effect on the optimum pH for maturation (pH 5.5-6.0), but it did accelerate the rate of maturation (t12 2 h). These results demonstrate that Asp310 and the pro-region of pro-PC2 contribute to the acidic pH optimum and low rate of maturation of this zymogen relative to its closely related homologue PC3.

A subtilisin-like protein in secretory organelles of Plasmodium falciparum merozoites

In the vertebrate host, the malaria parasite invades and replicates asexually within circulating erythrocytes. Parasite proteolytic enzymes play an essential but poorly understood role in erythrocyte invasion. We have identified a Plasmodium falciparum gene, denoted pfsub-1, encoding a member of the subtilisin-like serine protease family (subtilases). The pfsub-1 gene is expressed in asexual blood stages of P. falciparum, and the primary gene product (PfSUB-1) undergoes post-translational processing during secretory transport in a manner consistent with its being converted to a mature, enzymatically active form, as documented for other subtilases. In the invasive merozoite, the putative mature protease (p47) is concentrated in dense granules, which are secretory organelles located toward the apical end of the merozoite. At some point following merozoite release and completion of erythrocyte invasion, p47 is secreted from the parasite in a truncated, soluble form. The subcellular location and timing of secretion of p47 suggest that it is likely to play a role in erythrocyte invasion. PfSUB-1 is a new potential target for antimalarial drug development.

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.

Sorting and storage during secretory granule biogenesis: looking backward and looking forward

Secretory granules are specialized intracellular organelles that serve as a storage pool for selected secretory products. The exocytosis of secretory granules is markedly amplified under physiologically stimulated conditions. While granules have been recognized as post-Golgi carriers for almost 40 years, the molecular mechanisms involved in their formation from the trans-Golgi network are only beginning to be defined. This review summarizes and evaluates current information about how secretory proteins are thought to be sorted for the regulated secretory pathway and how these activities are positioned with respect to other post-Golgi sorting events that must occur in parallel. In the first half of the review, the emerging role of immature secretory granules in protein sorting is highlighted. The second half of the review summarizes what is known about the composition of granule membranes. The numerous similarities and relatively limited differences identified between granule membranes and other vesicular carriers that convey products to and from the plasmalemma, serve as a basis for examining how granule membrane composition might be established and how its unique functions interface with general post-Golgi membrane traffic. Studies of granule formation in vitro offer additional new insights, but also important challenges for future efforts to understand how regulated secretory pathways are constructed and maintained.

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.

Prohormone and proneuropeptide processing. Recent progress and future challenges

Our knowledge of prohormone and proneuropeptide processing and its relationship to the secretory pathway has advanced significantly in the last five years. The recent discovery of the prohormone convertase family of proteolytic enzymes has provided new candidates for the prohormone and proneuropeptide convertases. The increasing appreciation of the role of proteolysis in diverse cellular processes has also brought the prohormone processing field closer to the fields of growth factor processing, the role of host proteases in viral and bacterial pathogenesis and toxicity, control of the cell cycle, inflammation, and apoptosis. The last five years have been very productive, but the most interesting questions are still unanswered. Which enzymes are actually responsible for prohormone cleavages in specific tissues? What structural features of the prohormones determine where it will be processed or how it is recognized as secretory material by the sorting machinery? How is tissue-specific processing determined and regulated? The availability of protease knockout mice and and a more detailed understanding of the complex biosynthetic activation of these enzymes will provide at least some of the answers.

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.