Targeting of frog prodermorphin to the regulated secretory pathway by fusion to proenkephalin

Gene-encoded antibiotics made in bacteria

Production of antimicrobial peptides and proteins is very common among bacteria and a variety of such substances has been described. In general Gram-negative bacteria produce protein bacteriocins (e.g. colicins) with narrow action spectra based on receptor-mediated activity. They produce comparatively few peptides, such as the post-translationally modified microcin B17. In contrast Gram-positive bacteria tend to produce peptide bacteriocins smaller than 10 kDa and of wider activity spectra. These show particular potential for application. They can be divided into unmodified peptides (e.g. lactococcins, lactacins, pediocins) and lanthionine-containing peptides (lantibiotics, e.g. nisin, epidermin, Pep5). The unmodified peptides are mostly hydrophobic or amphiphilic and act by disturbing the function of the cytoplasmic membrane. They are synthesized as prepeptides with a characteristic N-terminal leader peptide. In some cases genes for immunity peptides were found in close proximity to structural genes; furthermore, two-component response regulators seem to be involved in the regulation of their synthesis. The biosynthetic genes for lantibiotics are also organized in operons. Lantibiotic gene clusters include genes encoding the unique enzymes which dehydrate serine and threonine and form the characteristic thioether-bridged lanthionines. Three types of lantibiotics are currently distinguished on the basis of structural features and functional aspects: type A, which include elongated, amphiphilic, pore-forming peptides (e.g. nisin); type B, which are of globular shape and inhibit phospholipases (e.g. duramycins); and type C (e.g. actagardine) with intermediate features which act by inhibiting bacterial cell wall biosynthesis.

Protein sorting and secretion granule formation in regulated secretory cells

Formation of secretion granules in regulated secretory cells involves packaging a subject of proteins undergoing intracellular transport into specific vesicular carriers that function in stimulus-dependent exocytosis. Recent findings suggest that immature granules are a site of passive sorting, involving condensation of regulated secretory proteins. Proteins that are not condensed are stored to a lesser degree and are enriched in unstimulated, constitutive-like secretion. While these observations have helped to distinguish possible mechanisms of secretory protein sorting, there are only recent hints about the sorting processes that may be required to create the regulated secretory carrier membranes.

Inhibitory effect of the alpha1-antitrypsin Pittsburgh type-mutant (alpha1-PIM/R) on proinsulin processing in the regulated secretory pathway of the pancreatic beta-cell line MIN6

To elucidate its effect on proinsulin processing, we introduced the expression of a Pittsburgh type-mutant, alpha1-protease inhibitor M/R (alpha1-PIM/R) and its chimera protein with growth hormone (GH) (GHalpha1-PIM/R) into MIN6 cells. In metabolic labeling and chasing experiments with [3H]-Leu and [35S]-Met, proinsulin appeared in the medium during stimulatory secretion only from MIN6 clones expressing GHalpha1-PIM/R and, surprisingly, alpha1-PIM/R, but not from the clones of either the control or alpha1-PI. The major part of alpha1-PIM/R was secreted through the constitutive pathway and about 10% of total secreted alpha1-PIM/R in the chase periods entered the regulated pathway. On the other hand, GHalpha1-PIM/R was mainly transported to the secretory granules and about 80% of the total secreted GHalpha1-PIM/R in the chase periods was secreted during stimulatory secretion. In the first 3 h chase periods without stimulation, only alpha1-PIM/R and no GHalpha1-PIM/R appeared in the medium, thus suggesting that alpha1-PIM/R might be transported through a constitutive-like pathway for those periods. The alpha1-PI, which had no inhibitory effect on proinsulin processing, showed similar secretion pathways to those of alpha1-PIM/R. This implies that some part of alpha1-PIM/R and alpha1-PI entered the regulated pathway, not due to any specific interaction between the processing endoproteases and serine protease inhibitors, but due to some type of passive transport in a nonselective manner. The inhibitory effect of alpha1-PIM/R in the regulated secretory pathway was slightly but clearly evident when it was expressed in MIN6 beta-cells.

Pharmacology of amphibian opiate peptides

In 1980 the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists, and named dermorphins. Since 1987 deltorphins, a family of highly selective delta-opioid peptides were identified either by cloning of the cDNA from frog skins or isolation of the peptides. The distinctive feature of opioid peptides is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe. The discovery of the amphibian opiate peptides, provided new insights into the functional role of the mu- and delta-opiate systems. It also provided models for novel analgesics with enhanced therapeutic benefits and reduced toxicity.

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.

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.

Essential role of the disulfide-bonded loop of chromogranin B for sorting to secretory granules is revealed by expression of a deletion mutant in the absence of endogenous granin synthesis

Sorting of regulated secretory proteins in the TGN to immature secretory granules (ISG) is thought to involve at least two steps: their selective aggregation and their interaction with membrane components destined to ISG. Here, we have investigated the sorting of chromogranin B (CgB), a member of the granin family present in the secretory granules of many endocrine cells and neurons. Specifically, we have studied the role of a candidate structural motif implicated in the sorting of CgB, the highly conserved NH2-terminal disulfide- bonded loop. Sorting to ISG of full-length human CgB and a deletion mutant of human CgB (Deltacys-hCgB) lacking the 22-amino acid residues comprising the disulfide-bonded loop was compared in the rat neuroendocrine cell line PC12. Upon transfection, i.e., with ongoing synthesis of endogenous granins, the sorting of the deletion mutant was only slightly impaired compared to full-length CgB. To investigate whether this sorting was due to coaggregation of the deletion mutant with endogenous granins, we expressed human CgB using recombinant vaccinia viruses, under conditions in which the synthesis of endogenous granins in the infected PC12 cells was shut off. In these conditions, Deltacys-hCgB, in contrast to full-length hCgB, was no longer sorted to ISG, but exited from the TGN in constitutive secretory vesicles. Coexpression of full-length hCgB together with Deltacys-hCgB by double infection, using the respective recombinant vaccinia viruses, rescued the sorting of the deletion mutant to ISG. In conclusion, our data show that (a) the disulfide-bonded loop is essential for sorting of CgB to ISG and (b) the lack of this structural motif can be compensated by coexpression of loop-bearing CgB. Furthermore, comparison of the two expression systems, transfection and vaccinia virus-mediated expression, reveals that analyses under conditions in which host cell secretory protein synthesis is blocked greatly facilitate the identification of sequence motifs required for sorting of regulated secretory proteins to secretory granules.

Intracellular transport and secretion of salivary proteins

Intracellular transport and secretion of salivary proteins are major activities of salivary acinar cells. While the major intracellular pathway followed by salivary proteins following their synthesis has been described previously, there is only limited understanding of how this process is regulated at the molecular level. Studies of salivary proteins, especially proline-rich proteins, expressed in an endocrine cell line have begun to provide insight regarding intermolecular interactions during transport and the role played by structural signals during intracellular sorting. Analysis of the secretion of newly synthesized salivary proteins in parotid tissue has shown that there are multiple pathways of discharge from acinar cells. While granule exocytosis is the major pathway, at least two other pathways that export salivary proteins have been found to originate from maturing secretion granules. These pathways may contribute to other acinar cell functions, including secretion of proteins in the absence of acute stimulation and support of the secretory process for fluid and electrolytes.

Passive sorting in maturing granules of AtT-20 cells: the entry and exit of salivary amylase and proline-rich protein

Previous studies have suggested that salivary amylase and proline-rich protein are sorted differently when expressed in AtT-20 cells (Castle, A.M., L.E. Stahl, and J.D. Castle. 1992. J. Biol. Chem. 267:13093- 13100; Colomer, V., K. Lal, T.C. Hoops, and M.J. Rindler. 1994.EMBO (Eur. Mol. Biol. Organ.) J. 13:3711- 3719). We now show that both exocrine proteins behave similarly and enter the regulated secretory pathway as judged by immunolocalization and secretagogue- dependent stimulation of secretion. Analysis of stimulated secretion of newly synthesized proline-rich protein, amylase, and endogenous hormones indicates that the exogenous proteins enter the granule pool with about the same efficiency as the endogenous hormones. However, in contrast to the endogenous hormones, proline-rich protein and amylase are progressively removed from the granule pool during the process of granule maturation such that only small portions remain in mature granules where they colocalize with the stored hormones. The exogenous proteins that are not stored are recovered from the incubation medium and are presumed to have undergone constitutive-like secretion. These results point to a level of sorting for regulated secretion after entry of proteins into forming granules and indicate that retention is essential for efficient storage. Consequently, the critical role of putative sorting receptors for regulated secretion may be in retention rather than in granule entry.

D-amino acids in animal peptides

D-amino acids have been detected in a variety of peptides synthesized by animal cells. These include opiate and antimicrobial peptides from amphibian skin, neuropeptides from snail ganglia, a hormone from crustaceans, and a constituent of a spider venom. cDNA cloning has shown that at those positions where a D-amino acid is found in the end-product, a normal codon for the corresponding L-amino acid is present. This implies that the D-residues are formed from L-amino acids by a posttranslational reaction. A prototype enzyme catalyzing such a reaction has recently been isolated from the venom of the funnel web spider.

The dermorphin peptide family

1. In 1980, the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists. Given the name dermorphins, these were the first members of a peptide family that in the past 15 years has grown to reach a total of seven naturally occurring peptides and nearly 30 synthetic analogs. 2. Dermorphin peptides are potent analgesics in rodents and primates, including man. Some dermorphins can enter the blood-brain barrier and produce central antinociception after peripheral administration. 3. The dermorphin family also includes mu 1-opioid receptor selective agonists that produce intense opioid analgesia, but stimulate pulmonary ventilation. 4. Experiments in rats and mice chronically exposed to dermorphins have shown that not only do they have higher antinociceptive efficacy and potency than morphine, but they are also less likely than morphine to produce tolerance, dependence and opiate side effects.

Coupled site-directed mutagenesis/transgenesis identifies important functional domains of the mouse agouti protein

The agouti locus encodes a novel paracrine signaling molecule containing a signal sequence, an N-linked glycosylation site, a central lysine-rich basic domain, and a C-terminal tail containing 10 cysteine (Cys) residues capable of forming five disulfide bonds. When overexpressed, agouti causes a number of pleiotropic effects including yellow coat and adult-onset obesity. Numerous studies suggest that agouti causes yellow coat color by antagonizing the binding of alpha-melanocyte-stimulating hormone (alpha-MSH) to the alpha-MSH-(Melanocortin-1) receptor. With the goal of identifying functional domains of agouti important for its diverse biological activities, we have generated 14 agouti mutations by in vitro site-directed mutagenesis and analyzed these mutations in transgenic mice for their effects on coat color and obesity. These studies demonstrate that the signal sequence, the N-linked glycosylation site, and the C-terminal Cys residues are important for full biological activity, while at least a portion of the lysine-rich basic domain is dispensable for normal function. They also show that the same functional domains of agouti important to coat color determination are important for inducing obesity, consistent with the hypothesis that agouti induces obesity by antagonizing melanocortin binding to other melanocortin receptors.

The propeptide of anglerfish preprosomatostatin-I rescues prosomatostatin-II from intracellular degradation

Polypeptide hormones and neuropeptides are initially synthesized as precursors possessing one or several domains that constitute the propeptide. Previous work from our laboratory demonstrated that expression of anglerfish prosomatostatin-I (proSRIF-I) in rat anterior pituitary GH3 cells resulted in efficient and accurate cleavage of the prohormone to generate the mature 14-amino acid peptide, SRIF-I. We also implicated the propeptide in mediating intracellular sorting to the trans Golgi network where proteolytic processing is initiated. In contrast, expression of a second form of the precursor, proSRIF-II in GH3 cells resulted in its intracellular degradation in an acidic, post-trans Golgi network compartment, most probably lysosomes. To further investigate the positive sorting signal present in proSRIF-I, we constructed a chimera comprising the signal peptide and proregion of SRIF-I fused to proSRIF-II and expressed the cDNA in GH3 cells. Here we demonstrate that the propeptide of SRIF-I rescued proSRIF-II from intracellular degradation quantitatively and diverted it to secretory vesicles. Furthermore, the chimera was processed to SRIF-28, an amino-terminally extended form of the hormone that is the physiological cleavage product of proSRIF-II processing in vivo. Most significantly, the SRIF-I propeptide functioned only in cis as part of the fusion protein and not in trans when expressed as a separate polypeptide. These data suggest that the SRIF-I propeptide may possess a sorting signal for sequestration into the secretory pathway rather than functioning as an intramolecular chaperone to promote protein folding.

An N-terminal hydrophobic peak is the sorting signal of regulated secretory proteins

Endocrine and exocrine cells each contain a regulated and constitutive secretory pathway. The presence of two distinct secretory pathways in the same cell type requires a sorting step to direct secretory proteins to the correct pathway. It is thought that regulated secretory proteins contain a specific sorting signal. However, this signal has not been identified. Amino acid sequence comparisons have not revealed any significant similarity between different regulated secretory proteins, suggesting that the sorting signal does not consist of a conserved primary sequence. In the present report, we have analyzed the predicted secondary structures of regulated secretory proteins and identified an N-terminal hydrophobic peak (NHP) which is located approximately from amino acids 9-26, overlaps with a predicted alpha-helix and contains charged amino acid residues. This signal is present in regulated secretory proteins that exhibit an N-terminal sorting sequence, but it is absent from constitutively secreted proteins and proteins where the sorting sequence is not located near the N-terminus. It appears that the NHP is both necessary and sufficient for sorting of many secretory proteins to the regulated secretory pathway.

Intracellular degradation of prohormone-chloramphenicol-acetyl-transferase chimeras in a pre-lysosomal compartment

Small peptide hormones (less than 50 amino acids) are synthesized as larger inactive precursors. Work from several laboratories, including our own, has implicated the propeptide of various precursors in mediating intracellular transport and targeting to secretory granules. We previously demonstrated that the proregion of prosomatostatin, one of the simplest peptide hormone precursors, when fused to alpha-globin, enabled the globin polypeptide to be transported to the regulated secretory pathway. To identify sorting motifs in this propeptide, we have now constructed a chimera comprising the somatostatin signal peptide and proregion fused to chloramphenicol acetyl transferase (CAT) and a control protein consisting of the signal peptide fused to CAT, both of which were expressed in rat anterior-pituitary GH3 cells. Both molecules were translocated into the endoplasmic reticulum (ER) efficiently and core-glycosylated on the single cryptic N-linked glycosylation site present in CAT. Surprisingly, the glycosylated propeptide-CAT and signal without CAT were degraded intracellularly with half-lives of 30 min and 90 min, respectively. Based on the kinetics of degradation, temperature sensitivity, and resistance to lysosomotrophic agents, we suggest that degradation occurred in the ER. Our data imply that the pro-region is not an a priori universal sorter, but only directs heterologous peptides to the secretory pathway when the passenger peptide assumes a secretion-competent conformation.

Prohormone processing in the trans-Golgi network: endoproteolytic cleavage of prosomatostatin and formation of nascent secretory vesicles in permeabilized cells

Many peptide hormones are synthesized as larger precursors which undergo endoproteolytic cleavage at paired basic residues to generate a bioactive molecule. Morphological evidence from several laboratories has implicated either the TGN or immature secretory granules as the site of prohormone cleavage. To identify the site where prohormone cleavage is initiated, we have used retrovirally infected rat anterior pituitary GH3 cells which express high levels of prosomatostatin (proSRIF) (Stoller, T. J., and D. Shields. J. Cell Biol. 1988. 107:2087-2095). By incubating these cells at 20 degrees C, a temperature that prevents exit from the Golgi apparatus, proSRIF accumulated quantitatively in the TGN and no proteolytic processing was evident; processing resumed upon shifting the cells back to 37 degrees C. After the 20 degrees C block, the cells were mechanically permeabilized and pro-SRIF processing determined. Cleavage of proSRIF to the mature hormone was approximately 35-50% efficient, required incubation at 37 degrees C and ATP hydrolysis, but was independent of GTP or cytosol. The in vitro ATP-dependent proSRIF processing was inhibited by inclusion of chloroquine, a weak base, CCCP, a protonophore, or by preincubating the permeabilized cells with low concentrations of N-ethylmaleimide, an inhibitor of vacuolar-type ATP-dependent proton pumps. These data suggest that: (a) proSRIF cleavage is initiated in the TGN, and (b) this reaction requires an acidic pH which is facilitated by a Golgi-associated vacuolar-type ATPase. A characteristic feature of polypeptide hormone-producing cells is their ability to store the mature hormone in dense core secretory granules. To investigate the mechanism of protein sorting to secretory granules, the budding of nascent secretory vesicles from the TGN was determined. No vesicle formation occurred at 20 degrees C; in contrast, at 37 degrees C, the budding of secretory vesicles was approximately 40% efficient and was dependent on ATP, GTP, and cytosolic factors. Vesicle formation was inhibited by GTP gamma S suggesting a role for GTP-binding proteins in this process. Vesicle budding was dependent on cytosolic factors that were tightly membrane associated and could be removed only by treating the permeabilized cells with high salt. After high salt treatment, vesicle formation was dependent on added cytosol or the dialyzed salt extract. The formation of nascent secretory vesicles contrasts with prosomatostatin processing which required only ATP for efficient cleavage. Our results demonstrate that prohormone cleavage which is initiated in the TGN, precedes vesicle formation and that processing can be uncoupled from the generation of nascent secretory vesicles.

Secretory protein traffic. Chromogranin A contains a dominant targeting signal for the regulated pathway

Secretory proteins are targeted into either constitutive (secreted upon synthesis) or regulated (stored in vesicles and released in response to a secretagogue) pathways. To investigate mechanisms of protein targeting into catecholamine storage vesicles (CSV), we stably expressed human chromogranin A (CgA), the major soluble protein in human CSV, in the rat pheochromocytoma PC-12 cell line. Chromaffin cell secretagogues (0.1 mM nicotinic cholinergic agonist, 55 mM K+, or 2 mM Ba++) caused cosecretion of human CgA and catecholamines from human CgA-expressing cells. Sucrose gradients colocalized human CgA and catecholamines to subcellular particles of the same buoyant density. Chimeric proteins, in which human CgA (either full-length [457 amino acids] or truncated [amino-terminal 226 amino acids]) was fused in-frame to the ordinarily nonsecreted protein chloramphenicol acetyltransferase (CAT), were expressed transiently in PC-12 cells. Both constructs directed CAT activity into regulated secretory vesicles, as judged by secretagogue-stimulated release. These data demonstrate that human CgA expressed in PC-12 cells is targeted to regulated secretory vesicles. In addition, human CgA can divert an ordinarily non-secreted protein into the regulated secretory pathway, consistent with the operation of a dominant targeting signal for the regulated pathway within the peptide sequence of CgA.

Frog prodermorphin expressed in mammalian cells is partly converted to the hydroxyproline containing precursor

Using recombinant vaccinia virus, we have expressed in mammalian cells the cDNA coding for the precursor of dermorphin, a D-alanine containing opioid peptide from the skin of the South American frog Phyllomedusa sauvagei. HeLa cells and AtT-20 cells produced prodermorphin where proline-6 of dermorphin was partly hydroxylated. This was demonstrated by digesting the partially purified precursors with trypsin and carboxypeptidase B. After immunoprecipitation and separation by HPLC, two decapeptides were detected which differed by the presence of proline or hydroxy-proline at position 6. This demonstrates that HeLa cells as well as AtT-20 cells can perform the post-translational conversion of certain proline residues to hydroxyproline in a foreign hormone precursor expressed in these cells.

Protein targeting to dense-core secretory granules

Regulated secretory proteins are stored within specialized vesicles known as secretory granules. It is not known how proteins are sorted into these organelles. Regulated proteins may possess targeting signals which interact with specific sorting receptors in the lumen of the trans-Golgi network (TGN) prior to their aggregation to form the characteristic dense-core of the granule. Alternatively, sorting may occur as the result of specific aggregation of regulated proteins in the TGN. Aggregates may be directed to secretory granules by interaction of a targeting signal on the surface with a sorting receptor. Novel targeting signals which confer on regulated proteins a tendency to aggregate under certain conditions, and in so doing cause them to be incorporated into secretory granules, have been implicated. Specific targeting signals may also play a role in directing membrane proteins to secretory granules.

Biogenesis of secretory granules

The biogenesis of secretory granules in endocrine, neuroendocrine, and exocrine cells is thought to involve a selective aggregation of the regulated secretory proteins into a dense-cored structure. The dense-core is then enveloped by membrane in the trans-Golgi network and buds, forming an immature secretory granule. The immature secretory granule then undergoes a maturation process which gives rise to the mature secretory granule. The recent data on the processes of aggregation, budding and maturation are summarized here. In addition, the current knowledge about the mature secretory granule is reviewed with emphasis on the biogenesis of the membrane of this organelle.