The secretory lectin ZG16p mediates sorting of enzyme proteins to the zymogen granule membrane in pancreatic acinar cells

Human oral lectin ZG16B acts as a cell wall polysaccharide probe to decode host-microbe interactions with oral commensals

The oral microbiome is critical to human health and disease, yet the role that host salivary proteins play in maintaining oral health is unclear. A highly expressed gene in human salivary glands encodes the lectin zymogen granule protein 16 homolog B (ZG16B). Despite the abundance of this protein, its interaction partners in the oral microbiome are unknown. ZG16B possesses a lectin fold, but whether it binds carbohydrates is unclear. We postulated that ZG16B would bind microbial glycans to mediate recognition of oral microbes. To this end, we developed a microbial glycan analysis probe (mGAP) strategy based on conjugating the recombinant protein to fluorescent or biotin reporter functionality. Applying the ZG16B-mGAP to dental plaque isolates revealed that ZG16B predominantly binds to a limited set of oral microbes, including Streptococcus mitis, Gemella haemolysans, and, most prominently, Streptococcus vestibularis. S. vestibularis is a commensal bacterium widely distributed in healthy individuals. ZG16B binds to S. vestibularis through the cell wall polysaccharides attached to the peptidoglycan, indicating that the protein is a lectin. ZG16B slows the growth of S. vestibularis with no cytotoxicity, suggesting that it regulates S. vestibularis abundance. The mGAP probes also revealed that ZG16B interacts with the salivary mucin MUC7. Analysis of S. vestibularis and MUC7 with ZG16B using super-resolution microscopy supports ternary complex formation that can promote microbe clustering. Together, our data suggest that ZG16B influences the compositional balance of the oral microbiome by capturing commensal microbes and regulating their growth using a mucin-assisted clearance mechanism.

Intestinal mucus components and secretion mechanisms: what we do and do not know

Damage to the colon mucus barrier, the first line of defense against microorganisms, is an important determinant of intestinal diseases such as inflammatory bowel disease and colorectal cancer, and disorder in extraintestinal organs. The mucus layer has attracted the attention of the scientific community in recent years, and with the discovery of new mucosal components, it has become increasingly clear that the mucosal barrier is a complex system composed of many components. Moreover, certain components are jointly involved in regulating the structure and function of the mucus barrier. Therefore, a comprehensive and systematic understanding of the functional components of the mucus layer is clearly warranted. In this review, we summarize the various functional components of the mucus layer identified thus far and describe their unique roles in shaping mucosal structure and function. Furthermore, we detail the mechanisms underlying mucus secretion, including baseline and stimulated secretion. In our opinion, baseline secretion can be categorized into spontaneous Ca²⁺ oscillation-mediated slow and continuous secretion and stimulated secretion, which is mediated by massive Ca²⁺ influx induced by exogenous stimuli. This review extends the current understanding of the intestinal mucus barrier, with an emphasis on host defense strategies based on fortification of the mucus layer.

Conformational switches and redox properties of the colon cancer-associated human lectin ZG16

Zymogen granule membrane protein 16 (ZG16) is produced in organs that secrete large quantities of enzymes and other proteins into the digestive tract. ZG16 binds microbial pathogens, and lower ZG16 expression levels correlate with colorectal cancer, but the physiological function of the protein is poorly understood. One prominent attribute of ZG16 is its ability to bind glycans, but other aspects of the protein may also contribute to activity. An intriguing feature of ZG16 is a CXXC motif at the carboxy terminus. Here, we describe crystal structures and biochemical studies showing that the CXXC motif is on a flexible tail, where it contributes little to structure or stability but is available to engage in redox reactions. Specifically, we demonstrate that the ZG16 cysteine thiols can be oxidized to a disulfide by quiescin sulfhydryl oxidase 1, which is a sulfhydryl oxidase present together with ZG16 in the Golgi apparatus and in mucus, as well as by protein disulfide isomerase. ZG16 crystal structures also draw attention to a nonproline cis peptide bond that can isomerize within the protein and to the mobility of glycine‐rich loops in the glycan‐binding site. An understanding of the properties of the ZG16 CXXC motif and the discovery of internal conformational switches extend existing knowledge relating to the glycan‐binding activity of the protein.

ZG16p, an Animal Homologue of Plant β-Prism Fold Lectins: Purification Methods of Natural and Recombinant ZG16p and Inhibition Assay of Cancer Cell Growth Using ZG16p

ZG16p is a soluble 16-kDa protein abundantly expressed in the pancreas and gut, and has a β-prism fold structure similar to that of mannose-binding Jacalin-related lectins (mJRLs) such as BanLec, Heltuba, and Artocarpin. ZG16p binds to mannose via the well-conserved GXXXD loop among mJRLs and sulfated glycosaminoglycans (e.g., heparin and heparan sulfate) via the basic patch of molecular surface. In addition to the above binding activities, ZG16p has inhibitory activity against proliferation of colon cancer cells. This manuscript describes purification of rat pancreatic ZG16p and recombinant ZG16p expressed in Escherichia coli expression system, and cell growth inhibition assay using ZG16p as an inhibitor.

Lectin ZG16p inhibits proliferation of human colorectal cancer cells via its carbohydrate-binding sites

Zymogen granule protein 16 (ZG16p) is a soluble lectin that binds to both mannose and heparin/heparan sulfate. It is highly expressed in the human digestive tract and is secreted into the mucus. In this study, we investigated the effect of ZG16p on the proliferation of human colorectal cancer cells. Overexpression of ZG16p in Caco-2 cells decreased cell growth. Recombinant ZG16p markedly inhibited proliferation of Caco-2, LS174T, HCT116 and HCT15 cells. Caco-2 cell growth was not inhibited by two mutated ZG16p proteins, D151A and M5 (K36A, R37A, R53A, R55A and R79A) lacking mannose- and heparin-binding activities, respectively. Immunofluorescent cell staining revealed that ZG16p-D151A maintained its binding to the Caco-2 cell surface, whereas ZG16p-M5 failed to bind to the cells. These results suggest that ZG16p interacts with the cell surface via basic amino acids substituted in ZG16p-M5 and inhibits Caco-2 cell proliferation via Asp151. In addition, growth of patient-derived colorectal tumor organoids in a 3D intestinal stem cell system was suppressed by ZG16p but not by ZG16p-M5. Taken together, our findings indicate that ZG16p inhibits the growth of colorectal cancer cells via its carbohydrate-binding sites in vitro and ex vivo. In this study, a novel pathway in cancer cell growth regulation through cell surface carbohydrate chains is suggested.

Rearing Water Treatment Induces Microbial Selection Influencing the Microbiota and Pathogen Associated Transcripts of Cod (Gadus morhua) Larvae

We have previously shown that K-selection and microbial stability in the rearing water increases survival and growth of Atlantic cod (Gadus morhua) larvae, and that recirculating aquaculture systems (RAS) are compatible with this. Here, we have assessed how water treatment influenced the larval microbiota and host responses at the gene expression level. Cod larvae were reared with two different rearing water systems: a RAS and a flow-through system (FTS). The water microbiota was examined using a 16S rDNA PCR/DGGE strategy. RNA extracted from larvae at 8, 13, and 17 days post hatching was used for microbiota and microarray gene expression analysis. Bacterial cDNA was synthesized and used for 16S rRNA amplicon 454 pyrosequencing of larval microbiota. Both water and larval microbiota differed significantly between the systems, and the larval microbiota appeared to become more dissimilar between systems with time. In total 4 phyla were identified for all larvae: Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. The most profound difference in larval microbiota was a high abundance of Arcobacter (Epsilonproteobacteria) in FTS larvae (34 ± 9% of total reads). Arcobacter includes several species that are known pathogens for humans and animals. Cod larval transcriptome responses were investigated using an oligonucleotide gene expression microarray covering approximately 24,000 genes. Interestingly, FTS larvae transcriptional profiles revealed an overrepresentation of upregulated transcripts associated with responses to pathogens and infections, such as c1ql3-like, pglyrp-2-like and zg16, compared to RAS larvae. In conclusion, distinct water treatment systems induced differences in the larval microbiota. FTS larvae showed up-regulation of transcripts associated with responses to microbial stress. These results are consistent with the hypothesis that RAS promotes K-selection and microbial stability by maintaining a microbial load close to the carrying capacity of the system, and ensuring long retention times for both bacteria and water in the system.

Evolutionary analysis of the jacalin-related lectin family genes in 11 fishes

Jacalin-related lectins are a type of carbohydrate-binding proteins, which are distributed across a wide variety of organisms and involved in some important biological processes. The evolution of this gene family in fishes is unknown. Here, 47 putative jacalin genes in 11 fish species were identified and divided into 4 groups through phylogenetic analysis. Conserved gene organization and motif distribution existed in each group, suggesting their functional conservation. Some fishes have eleven jacalin genes, while others have only one or zero gene in their genomes, suggesting dynamic changes in the number of jacalin genes during the evolution of fishes. Intragenic recombination played a key role in the evolution of jacalin genes. Synteny analyses of jacalin genes in some fishes implied conserved and dynamic evolution characteristics of this gene family and related genome segments. Moreover, a few functional divergence sites were identified within each group pairs. Divergent expression profiles of the zebra fish jacalin genes were further investigated in different stresses. The results provided a foundation for exploring the characterization of the jacalin genes in fishes and will offer insights for additional functional studies.

Sugar recognition and protein-protein interaction of mammalian lectins conferring diverse functions

Recent advances in structural analyses of mammalian lectins reveal atomic-level details of their fine specificities toward diverse endogenous and exogenous glycans. Local variations on a common scaffold can enable certain lectins to recognize complex carbohydrate ligands including branched glycans and O-glycosylated peptides. Simultaneous recognition of both glycan and the aglycon moieties enhances the affinity and specificity of lectins such as CLEC-2 and PILRα. Attention has been paid to the roles of galectin and RegIII family of proteins in protein-protein interactions involved in critical biological functions including signal transduction and bactericidal pore formation.

Defining the Interaction of Human Soluble Lectin ZG16p and Mycobacterial Phosphatidylinositol Mannosides

ZG16p is a soluble mammalian lectin that interacts with mannose and heparan sulfate. Here we describe detailed analysis of the interaction of human ZG16p with mycobacterial phosphatidylinositol mannosides (PIMs) by glycan microarray and NMR. Pathogen-related glycan microarray analysis identified phosphatidylinositol mono- and di-mannosides (PIM1 and PIM2) as novel ligand candidates of ZG16p. Saturation transfer difference (STD) NMR and transferred NOE experiments with chemically synthesized PIM glycans indicate that PIMs preferentially interact with ZG16p by using the mannose residues. The binding site of PIM was identified by chemical-shift perturbation experiments with uniformly (15)N-labeled ZG16p. NMR results with docking simulations suggest a binding mode of ZG16p and PIM glycan; this will help to elucidate the physiological role of ZG16p.

Proteoglycans support proper granule formation in pancreatic acinar cells

Zymogen granules (ZG) are specialized organelles in the exocrine pancreas which allow digestive enzyme storage and regulated secretion. The molecular mechanisms of their biogenesis and the sorting of zymogens are still incompletely understood. Here, we investigated the role of proteoglycans in granule formation and secretion of zymogens in pancreatic AR42J cells, an acinar model system. Cupromeronic Blue cytochemistry and biochemical studies revealed an association of proteoglycans primarily with the granule membrane. Removal of proteoglycans by carbonate treatment led to a loss of membrane curvature indicating a supportive role in the maintenance of membrane shape and stability. Chemical inhibition of proteoglycan synthesis impaired the formation of normal electron-dense granules in AR42J cells and resulted in the formation of unusually small granule structures. These structures still contained the zymogen carboxypeptidase, a cargo molecule of secretory granules, but migrated to lighter fractions after density gradient centrifugation. Furthermore, the basal secretion of amylase was increased in AR42J cells after inhibitor treatment. In addition, irregular-shaped granules appeared in pancreatic lobules. We conclude that the assembly of a proteoglycan scaffold at the ZG membrane is supporting efficient packaging of zymogens and the proper formation of stimulus-competent storage granules in acinar cells of the pancreas.

Discovery, primary, and crystal structures and capacitation-related properties of a prostate-derived heparin-binding protein WGA16 from boar sperm

Mammalian sperm acquire fertility through a functional maturation process called capacitation, where sperm membrane molecules are drastically remodeled. In this study, we found that a wheat germ agglutinin (WGA)-reactive protein on lipid rafts, named WGA16, is removed from the sperm surface on capacitation. WGA16 is a prostate-derived seminal plasma protein that has never been reported and is deposited on the sperm surface in the male reproductive tract. Based on protein and cDNA sequences for purified WGA16, it is a homologue of human zymogen granule protein 16 (ZG16) belonging to the Jacalin-related lectin (JRL) family in crystal and primary structures. A glycan array shows that WGA16 binds heparin through a basic patch containing Lys-53/Lys-73 residues but not the conventional lectin domain of the JRL family. WGA16 is glycosylated, contrary to other ZG16 members, and comparative mass spectrometry clearly shows its unique N-glycosylation profile among seminal plasma proteins. It has exposed GlcNAc and GalNAc residues without additional Gal residues. The GlcNAc/GalNAc residues can work as binding ligands for a sperm surface galactosyltransferase, which actually galactosylates WGA16 in situ in the presence of UDP-Gal. Interestingly, surface removal of WGA16 is experimentally induced by either UDP-Gal or heparin. In the crystal structure, N-glycosylated sites and a potential heparin-binding site face opposite sides. This geography of two functional sites suggest that WGA16 is deposited on the sperm surface through interaction between its N-glycans and the surface galactosyltransferase, whereas its heparin-binding domain may be involved in binding to sulfated glycosaminoglycans in the female tract, enabling removal of WGA16 from the sperm surface.

Structural insights into Bacillus thuringiensis Cry, Cyt and parasporin toxins

Since the first X-ray structure of Cry3Aa was revealed in 1991, numerous structures of B. thuringiensis toxins have been determined and published. In recent years, functional studies on the mode of action and resistance mechanism have been proposed, which notably promoted the developments of biological insecticides and insect-resistant transgenic crops. With the exploration of known pore-forming toxins (PFTs) structures, similarities between PFTs and B. thuringiensis toxins have provided great insights into receptor binding interactions and conformational changes from water-soluble to membrane pore-forming state of B. thuringiensis toxins. This review mainly focuses on the latest discoveries of the toxin working mechanism, with the emphasis on structural related progress. Based on the structural features, B. thuringiensis Cry, Cyt and parasporin toxins could be divided into three categories: three-domain type α-PFTs, Cyt toxin type β-PFTs and aerolysin type β-PFTs. Structures from each group are elucidated and discussed in relation to the latest data, respectively.

Structural basis for multiple sugar recognition of Jacalin-related human ZG16p lectin

ZG16p is a soluble mammalian lectin, the first to be described with a Jacalin-related β-prism-fold. ZG16p has been reported to bind both to glycosaminoglycans and mannose. To determine the structural basis of the multiple sugar-binding properties, we conducted glycan microarray analyses of human ZG16p. We observed that ZG16p preferentially binds to α-mannose-terminating short glycans such as Ser/Thr-linked O-mannose, but not to high mannose-type N-glycans. Among sulfated glycosaminoglycan oligomers examined, chondroitin sulfate B and heparin oligosaccharides showed significant binding. Crystallographic studies of human ZG16p lectin in the presence of selected ligands revealed the mechanism of multiple sugar recognition. Manα1–3Man and Glcβ1–3Glc bound in different orientations: the nonreducing end of the former and the reducing end of the latter fitted in the canonical shallow mannose binding pocket. Solution NMR analysis using ¹⁵N-labeled ZG16p defined the heparin-binding region, which is on an adjacent flat surface of the protein. On-array competitive binding assays suggest that it is possible for ZG16p to bind simultaneously to both types of ligands. Recognition of a broad spectrum of ligands by ZG16p may account for the multiple functions of this lectin in the formation of zymogen granules via glycosaminoglycan binding, and in the recognition of pathogens in the digestive system through α-mannose-related recognition.

Modulating zymogen granule formation in pancreatic AR42J cells

Zymogen granules (ZG) are specialized organelles in the exocrine pancreas which allow digestive enzyme storage and regulated secretion. To investigate ZG biogenesis, cargo sorting and packaging, suitable cellular model systems are required. Here, we demonstrate that granule formation in pancreatic AR42J cells, an acinar model system, can be modulated by altering the growth conditions in cell culture. We find that cultivation of AR42J cells in Panserin™ 401, a serum-free medium, enhances the induction of granule formation in the presence or absence of dexamethasone when compared to standard conditions including serum. Biochemical and morphological studies revealed an increase in ZG markers on the mRNA and protein level, as well as in granule size compared to standard conditions. Our data indicate that this effect is related to pronounced differentiation of AR42J cells. To address if enhanced expression of ZG proteins promotes granule formation, we expressed several zymogens and ZG membrane proteins in unstimulated AR42J cells and in constitutively secreting COS-7 cells. Neither single expression nor co-expression was sufficient to initiate granule formation in AR42J cells or the formation of granule-like structures in COS-7 cells as described for neuroendocrine cargo proteins. The importance of our findings for granule formation in exocrine cells is discussed.

Crystal structures of human secretory proteins ZG16p and ZG16b reveal a Jacalin-related β-prism fold

ZG16p is a secretory protein that mediates condensation-sorting of pancreatic enzymes to the zymogen granule membrane in pancreatic acinar cells. ZG16p interacts with glycosaminoglycans and the binding is considered to be important for condensation-sorting of pancreatic enzymes. ZG16b/PAUF, a paralog of ZG16p, has recently been found to play a role in gene regulation and cancer metastasis. However, the detailed functions of ZG16p and ZG16b remain to be clarified. Here, in order to obtain insights into structure-function relationships, we conducted crystallographic studies of human ZG16p lectin as well as its paralog, ZG16b, and determined their crystal structures at 1.65 and 2.75 Å resolution, respectively. ZG16p has a Jacalin-related β-prism fold, the first to be reported among mammalian lectins. The putative sugar-binding site of ZG16p is occupied by a glycerol molecule, mimicking the mannose bound to Jacalin-related mannose-binding-type plant lectins such as Banlec. ZG16b also has a β-prism fold, but some amino acid residues of the putative sugar-binding site differ from those of the mannose-type binding site suggesting altered preference. A positively charged patch, which may bind sulfated groups of the glycosaminoglycans, is located around the putative sugar-binding site of ZG16p and ZG16b. Taken together, we suggest that the sugar-binding site and the adjacent basic patch of ZG16p and ZG16b cooperatively form a functional glycosaminoglycan-binding site.

Analysis of low abundance membrane-associated proteins from rat pancreatic zymogen granules

Zymogen granules (ZG) are specialized storage organelles in the exocrine pancreas that allow the sorting, packaging, and regulated apical secretion of digestive enzymes. As there is a critical need for further understanding of the key processes in regulated secretion to develop new therapeutic options in medicine, we applied a suborganellar proteomics approach to identify peripheral membrane-associated ZG proteins. We focused on the analysis of a "basic" group (pH range 6.2-11) with about 46 spots among which 44 were identified by tandem mass spectrometry. These spots corresponded to 16 unique proteins, including rat mast cell chymase (RMCP-1) and peptidyl-prolyl cis-trans isomerase B (PpiB; cyclophilin B), an ER-resident protein. To confirm that these proteins were specific to zymogen granules and not contaminants of the preparation, we conducted a series of validation experiments. Immunoblotting of ZG subfractions revealed that chymase and PpiB behaved like bona fide peripheral membrane proteins. Their expression in rat pancreas was regulated by feeding behavior. Ultrastructural and immunofluorescence studies confirmed their ZG localization. Furthermore, a chymase-YFP fusion protein was properly targeted to ZG in pancreatic AR42J cells. Interestingly, for both proteins, proteoglycan-binding properties have been reported. The importance of our findings for sorting and packaging during ZG formation is discussed.

Replacement of Rbpj with Rbpjl in the PTF1 complex controls the final maturation of pancreatic acinar cells

BACKGROUND & AIMS

The mature pancreatic acinar cell is dedicated to the production of very large amounts of digestive enzymes. The early stages of pancreatic development require the Rbpj form of the trimeric Pancreas Transcription Factor 1 complex (PTF1-J). As acinar development commences, Rbpjl gradually replaces Rbpj; in the mature pancreas, PTF1 contains Rbpjl (PTF1-L). We investigated whether PTF1-L controls the expression of genes that complete the final stage of acinar differentiation.

METHODS

We analyzed acinar development and transcription in mice with disrupted Rbpjl (Rbpjl(ko/ko) mice). We performed comprehensive analyses of the messenger RNA population and PTF1 target genes in pancreatic acinar cells from these and wild-type mice.

RESULTS

In Rbpjl(ko/ko) mice, acinar differentiation was incomplete and characterized by decreased expression (as much as 99%) of genes that encode digestive enzymes or proteins of regulated exocytosis and mitochondrial metabolism. Whereas PTF1-L bound regulatory sites of genes in normal adult pancreatic cells, the embryonic form (PTF1-J) persisted in the absence of Rbpjl and replaced PTF1-L; the extent of replacement determined gene expression levels. Loss of PTF1-L reduced expression (>2-fold) of only about 50 genes, 90% of which were direct targets of PTF1-L. The magnitude of the effects on individual digestive enzyme genes correlated with the developmental timing of gene activation. Absence of Rbpjl increased pancreatic expression of liver-restricted messenger RNA.

CONCLUSIONS

Replacement of Rbpj by Rbpjl in the PTF1 complex drives acinar differentiation by maximizing secretory protein synthesis, stimulating mitochondrial metabolism and cytoplasmic creatine-phosphate energy stores, completing the packaging and secretory apparatus, and maintaining acinar-cell homeostasis.

Isolation of zymogen granules from rat pancreas

This unit describes methods for preparing zymogen granules from rat pancreas. Zymogen granules are storage organelles in pancreatic acinar cells containing digestive enzymes that are released into the pancreatic duct. The protocols in this unit take advantage of the large size (up to 1 microm diameter) and high density (>1.20 g/cm(3) on sucrose gradients) of the granules as compared to other cellular organelles. They use a combination of differential sedimentation and density gradient separation to accomplish the purification. Similar procedures can be used to isolate zymogen granules from mouse pancreas and canine pancreas. A protocol for preparing zymogen granules from dog pancreas is also included.

Proteomic analysis of pancreatic zymogen granules: identification of new granule proteins

The composition of zymogen granules from rat pancreas was determined by LC-MS/MS. Enriched intragranular content, peripheral membrane, and integral membrane protein fractions were analyzed after one-dimensional SDS-PAGE and tryptic digestion of gel slices. A total of 371 proteins was identified with high confidence, including 84 previously identified granule proteins. The 287 remaining proteins included 37 GTP-binding proteins and effectors, 8 tetraspan membrane proteins, and 22 channels and transporters. Seven proteins, pantophysin, cyclic nucleotide phosphodiesterase, carboxypeptidase D, ecto-nucleotide phosphodiesterase 3, aminopeptidase N, ral, and the potassium channel TWIK-2, were confirmed by immunofluorescence microscopy or by immunoblotting to be new zymogen granule membrane proteins.

hZG16, a novel human secreted protein expressed in liver, was down-regulated in hepatocellular carcinoma

Here we reported a novel human secreted protein named as hZG16, with a Jacalin domain. Evolution analysis through comparing with the orthologs of other organisms suggested that ZG16 is a conserved gene under the purifying selection (d(N)/d(s)<1) in the evolution. Interestingly, Northern and dot blot analyses showed that hZG16 were highly expressed in adult liver, not in fetal liver, and moderately in gut, including jejunum, ileum, and colon, in which the tissue expression pattern of hZG16 was significantly dissimilar to that of mouse and rat orthologs that were uniquely expressed in spleen and pancreas, respectively. Unexpectedly, hZG16 was markedly down-regulated in hepatocellular carcinoma (HCC) as indicated by RT-PCR, Northern blot analysis and immunohistochemistry staining. However, the tunicamicin treatment and pulse-chase experiments showed that hZG16 protein had a similar molecular function with rZG16 that take part in glycoproteins' secretion in a bus mode.

Screening of hepatocyte proteins binding to F protein of hepatitis C virus by yeast two-hybrid system

AIM: To investigate the biological function of F protein by yeast two-hybrid system.

METHODS: We constructed F protein bait plasmid by cloning the gene of F protein into pGBKT7, then recombinant plasmid DNA was transformed into yeast AH109 (a type). The transformed yeast AH109 was mated with yeast Y187 (α type) containing liver cDNA library plasmid in 2×YPDA medium. Diploid yeast was plated on synthetic dropout nutrient medium (SD/-Trp-Leu-His-Ade) containing X-α-gal for selection and screening. After extracting and sequencing plasmids from positive (blue) colonies, we underwent sequence analysis by bioinformatics.

RESULTS: Thirty-six colonies were selected and sequenced. Among them, 11 colonies were zymogen granule protein, 5 colonies were zinc finger protein, 4 colonies were zinc-α-2-glycoprotein, 1 colony was sialyltransferase, 1 colony was complement control protein factor I, 1 colony was vitronectin, and 2 colonies were new genes with unknown function.

CONCLUSION: The yeast two-hybrid system is an effective method for identifying hepatocyte proteins interacting with F protein of hepatitis C virus. F protein may bind to different proteins.

Protein sorting to the storage vacuoles of plants: a critical appraisal

The vacuole of plant cells is no longer considered to be a single compartment with multifunctional properties. A lot of evidence now points to the presence of multiple functionally distinct vacuolar compartments, some existing side by side in the same cell. As a consequence, the plant Golgi apparatus is faced with the problem of recognizing proteins destined for lytic and storage vacuoles and segregating them individually from the flow of secretory proteins to the cell surface. In contrast to acid hydrolases, which are sorted by BP-80-like receptors at the trans-Golgi of plant cells, the identification of receptors for storage proteins has in many ways resembled 'the search for the Holy Grail'. There are several candidates for storage protein receptors, but in no single case is the evidence entirely convincing. Much of the problem lies in the lack of consensus, sorting sequences in the proteins investigated. Other difficulties stem from 'out-of-context' heterologous expression studies. Evidence is now accumulating for the participation of hydrophobic sequences in inducing the formation of protein aggregates in the early Golgi apparatus, for which classical sorting receptors do not appear to be necessary. This review critically examines the current situation and contrasts the differences between data obtained in situ and data obtained transgenically. It highlights the so-called 'dense-vesicle' pathway and culminates with a discussion on the hitherto neglected problem of the intracellular transport of storage protein processing enzymes.

Differential expression of the trypsin inhibitor SPINK3 mRNA and the mouse ortholog of secretory granule protein ZG-16p mRNA in the mouse pancreas after repetitive injury

A mouse model using repetitive acinar cell injury caused by supraphysiologic doses of cerulein to induce the characteristic fibrosis and loss of acinar cell mass found in human chronic pancreatitis was employed to identify early changes in gene expression. A gene array was used to detect changes in 18,000 expressed sequence tags; changes in specific transcripts were confirmed by RNase protection assays. These methods identified SPINK3, the mouse homologue of human and rat protease inhibitor genes, as being highly expressed in the pancreas and induced after pancreatic injury. Because SPINK3 may be an important serine protease inhibitor, its up-regulation may reflect an important endogenous cytoprotective mechanism in preventing further injury. The up-regulation of SPINK3 was specific; the mouse homologue of the zymogen-processing protein ZG-16p was also highly expressed in the pancreas but sharply down-regulated early in the course of injury. These findings suggest that the pancreatic acinar cell may respond to injury with a program of self-preservation and loss of normal function.

Membrane targeting in secretion

Regulated secretion and exocytosis require the selective packaging of regulated secretory proteins in secretory storage organelles and the controlled docking and fusion of these organelles with the plasma membrane. Secretory granule biogenesis involves sorting of secretory proteins and membrane components both at the level of the trans-Golgi network and the immature secretory granule. Sorting is thought to be mediated by selective protein aggregation and the interaction of these proteins with specific membrane domains. There is now considerable interest in the understanding of the complex lipid-protein and protein-protein interactions at the trans-Golgi network and the granule membrane. A role for lipid microdomains and associated sorting receptors in membrane targeting and granule formation is vividly discussed for (neuro)endocrine cells. In exocrine cells, however, little has been known of granule membrane composition and membrane protein function. With the cloning and characterization of granule membrane proteins and their interactions at the inner leaflet of zymogen granules of pancreatic acinar cells, it is now possible to elucidate their function in membrane targeting and sorting of zymogens at the molecular level.

Serglycin proteoglycan is sorted into zymogen granules of rat pancreatic acinar cells

Serglycin is known as a secretory granule proteoglyean in hematopoietic cells. In this study we identified a high-molecular-weight molecule in aggregated content proteins of zymogen granules of pancreatic acinar cells. The amino acid composition of the isolated protein showed high similarity to serglycin proteoglycan core protein. To confirm the expression of serglycin proteoglycan in pancreatic acinar cells we cloned the rat pancreas cDNA of serglycin core protein and detected the serglycin mRNA in pancreas tissue and AR4-2J cells by reverse transcription-PCR. In AR4-2J cells, transfected with serglycin fused to green fluorescent protein (EGFP), serglycin localized within a web-like pattern in the perinuclear space as well as with a punctate pattern distributed in the cytoplasm. The perinuclear structures colocalized with the Golgi membrane-associated protein p115 and the punctate structures with the secretory enzyme procarboxypeptidase A, indicating that the serglycin-EGFP fusion protein travels through compartments of the secretory pathway and is sorted into secretory granules. Using an antiserum against serglycin core protein immunofluorescence as well as immunogold electron microscopy analysis conrirmed the subcellular distribution of serglycin proteoglycan in zymogen granules of pancreatic acinar cells. To prevent glycosylation of serglycin core protein we incubated AR4-2J cells with 2 mM p-nitrophenyl-beta-D-xylopyranoside (PNP-xyloside), which serves as alternate substrate for glycosaminoglycan chain attachment. Furthermore, we deleted the serine/glycine repeat region in the serglycin core protein. In both approaches the transfected serglycin-EGFP fusion protein could be detected predominantly in perinuclear Golgi membrane structures, while in control cells the serglycin fusion protein was mostly sorted into the secretory granules. Additionally, we show that sorting of secretory enzymes like amylase

Role of sulfated O-linked glycoproteins in zymogen granule formation

Packaging of proteins into regulated secretory granules is mediated by the mildly acidic pH of the trans Golgi network and immature secretory granules. This need for an acidic pH indicates that ionic interactions are important. The mouse pancreatic acinar cell contains four major sulfated glycoproteins,including the zymogen granule structural component Muclin. I tested the hypothesis that sulfation and the O-linked glycosylation to which the sulfates are attached are required for normal formation of zymogen granules in the exocrine pancreas. Post-translational processing was perturbed with two chemicals: sodium chlorate was used to inhibit sulfation and benzyl-N-acetyl-α-galactosaminide was used to inhibit O-linked oligosaccharide elongation. Both chemicals resulted in the accumulation in the Golgi region of the cell of large vacuoles that appear to be immature secretory granules, and the effect was much more extensive with benzyl-N-acetyl-α-galactosaminide than chlorate. Both chemical treatments inhibited basal secretion at prolonged chase times, and again benzyl-N-acetyl-α-galactosaminide had a greater effect than chlorate. In addition, benzyl-N-acetyl-α-galactosaminide, but not chlorate, totally inhibited stimulated secretion of newly synthesized proteins. These data provide evidence for a role of sulfated O-linked glycoproteins in protein condensation and maturation of zymogen granules. Under maximal inhibition of O-linked oligosaccharide biosynthesis, anterograde post-Golgi traffic in the regulated pathway is almost totally shut down, demonstrating the importance of these post-translational modifications in progression of secretory proteins through the regulated pathway and normal granule formation in the pancreatic acinar cell.

Loss of the zymogen granule protein syncollin affects pancreatic protein synthesis and transport but not secretion

Syncollin is a small protein that is abundantly expressed in pancreatic acinar cells and that is tightly associated with the lumenal side of the zymogen granule membrane. To shed light on the hitherto unknown function of syncollin, we have generated syncollin-deficient mice. The mice are viable and show a normal pancreatic morphology as well as normal release kinetics in response to secretagogue stimulation. Although syncollin is highly enriched in zymogen granules, no change was found in the overall protein content and in the levels of chymotrypsin, trypsin, and amylase. However, syncollin-deficient mice reacted to caerulein hyperstimulation with a more severe pancreatitis. Furthermore, the rates of both protein synthesis and intracellular transport of secretory proteins were reduced. We conclude that syncollin plays a role in maturation and/or concentration of zymogens in zymogen granules.

The regulation of exocytosis in the pancreatic acinar cell

The pancreatic acinar cell synthesises a variety of digestive enzymes. In transit through the secretory pathway, these enzymes are separated from constitutively secreted proteins and packaged into zymogen granules, which are localised in the apical pole of the cell. Stimulation of the cell by secretagogues such as acetylcholine and cholecystokinin, acting at receptors on the basolateral plasma membrane, causes the generation of an intracellular Ca(2+) signal. This signal, in turn, triggers the fusion of the zymogen granules with the apical plasma membrane, leading to the polarised secretion of the enzymes. This review describes recent advances in our understanding of the control of secretion in the acinar cell. In particular, we discuss the mechanisms underlying the sorting of digestive enzymes into the zymogen granules, the molecular components of the exocytotic "membrane fusion machine," the generation and propagation of the Ca(2+ signal and the development of new techniques for the visualisation of single granule fusion events.

Interaction of syncollin with GP-2, the major membrane protein of pancreatic zymogen granules, and association with lipid microdomains

Syncollin, a novel pancreatic zymogen granule protein, is present on the luminal side of the granule membrane. To address the function of syncollin, we searched for putative binding partners. Cross-linking experiments with purified syncollin, and granule content and membrane proteins revealed a direct interaction between syncollin and GP-2, a major glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein. An interaction was also observed when cross-linking was performed with recombinant GP-2. In addition, syncollin could be cross-linked to itself, supporting the suggestion that it exists as a homo-oligomer. Cleavage of the GPI anchor of GP-2 by treatment of granule membranes with phosphatidylinositol-specific phospholipase C had no effect on the membrane attachment of syncollin, indicating that it is not mediated exclusively via an interaction with GP-2. Syncollin was found to be associated with detergent-insoluble cholesterol/glycolipid-enriched complexes. These complexes floated to the lighter fractions of sucrose-density gradients and also contained GP-2, the lectin ZG16p, sulphated matrix proteoglycans and the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) syntaxin 3 and synaptobrevin 2. Our results indicate that membrane-associated syncollin is a component of lipid rafts, where it interacts both with GP-2 and membrane lipids. We suggest that the syncollin-GP-2 complex might play a role in signal transduction across the granule membrane.

Vacuolar sorting determinants within a plant storage protein trimer act cumulatively

The mechanism for vacuolar sorting of seed storage proteins is as yet poorly understood and no receptor has been identified to date. The homotrimeric glycoprotein phaseolin, which is the major storage protein of the common bean, requires a transient tetrapeptide at the C-terminus for its vacuolar sorting. A mutated construct without the tetrapeptide is secreted. We show here that coexpression of wild-type phaseolin and the mutated, secreted form in transgenic tobacco results in the formation of mixed trimers and partial vacuolar delivery of the mutated polypeptides and partial secretion of wild-type polypeptides. This indicates that the sorting signal has a cumulative effect within a phaseolin trimer. The result is discussed in the light of the hypothesized mechanisms for vacuolar sorting of seed storage proteins.

Regulated apical secretion of zymogens in rat pancreas. Involvement of the glycosylphosphatidylinositol-anchored glycoprotein GP-2, the lectin ZG16p, and cholesterol-glycosphingolipid-enriched microdomains

We examined the role of glycosphingolipid- and cholesterol-enriched microdomains, or rafts, in the sorting of digestive enzymes into zymogen granules destined for apical secretion and in granule formation. Isolated membranes of zymogen granules from pancreatic acinar cells showed an enrichment in cholesterol and sphingomyelin and formed detergent-insoluble glycolipid-enriched complexes. These complexes floated to the lighter fractions of sucrose density gradients and contained the glycosylphosphatidylinositol (GPI)-anchored glycoprotein GP-2, the lectin ZG16p, and sulfated matrix proteoglycans. Morphological and pulse-chase studies with isolated pancreatic lobules revealed that after inhibition of GPI-anchor biosynthesis by mannosamine or the fungal metabolite YW 3548, granule formation was impaired leading to an accumulation of newly synthesized proteins in the Golgi apparatus and the rough endoplasmic reticulum. Furthermore, the membrane attachment of matrix proteoglycans was diminished. After cholesterol depletion or inhibition of glycosphingolipid synthesis by fumonisin B1, the formation of zymogen granules as well as the formation of detergent-insoluble complexes was reduced. In addition, cholesterol depletion led to constitutive secretion of newly synthesized proteins, e.g. amylase, indicating that zymogens were missorted. Together, these data provide first evidence that in polarized acinar cells of the exocrine pancreas GPI-anchored proteins, e.g. GP-2, and cholesterol-sphingolipid-enriched microdomains are required for granule formation as well as for regulated secretion of zymogens and may function as sorting platforms for secretory proteins destined for apical delivery.

Processing of pro-Muclin and divergent trafficking of its products to zymogen granules and the apical plasma membrane of pancreatic acinar cells

Proteins are sorted and packaged into regulated secretory granules at the trans Golgi network but how such granules form is poorly understood. We are studying Muclin, the major sulfated protein of the mouse pancreatic acinar cell, and what its role may be in zymogen granule formation. Muclin behaves as a peripheral membrane protein localized to the lumen of the zymogen granule but the cDNA for this protein predicts it is a type I membrane protein with a short, 16-amino-acid, cytosolic tail (C-Tail). Using domain-specific antibodies, we demonstrate that Muclin is derived from a precursor, pro-Muclin, which is cleaved to produce Muclin and an approximately 80-kDa membrane glycoprotein (p80). Incubation of pulse-labeled cells at < or = 22 degrees C to block exit from the trans Golgi network also blocks cleavage of pro-Muclin but not sulfation, a trans Golgi network event, suggesting that cleavage occurs in a post-Golgi compartment. After cleavage the two products of pro-Muclin diverge with Muclin remaining in the regulated secretory pathway and p80 trafficking to the apical plasma membrane, presumably via the constitutive-like pathway. When transfected into exocrine AR42J cells, Muclin labeling is perinuclear and in large sub-plasma membrane puncta. Transiently transfected AR42J cells have greater immunolabeling for amylase than nontransfected cells, suggesting a role for Muclin in cargo accumulation in the regulated secretory pathway. A construct with the C-Tail deleted targets to small diffusely-distributed puncta and without the large sub-plasma membrane structures. Thus, the C-Tail is required for proper Muclin targeting. When transfected into neuroendocrine AtT-20 cells Muclin is not colocalized with ACTH in cell processes, and it appears to be constitutively trafficked to the plasma membrane, suggesting that Muclin has exocrine-specific information. We present a working model for pro-Muclin as a Golgi cargo receptor for exocrine secretory granule formation at the trans Golgi network.

Localization of alpha 1,3-fucosyltransferase VI in Weibel-Palade bodies of human endothelial cells

Surface glycosylation of endothelial cells is relevant to various processes including coagulation, inflammation, metastasis, and lymphocyte homing. One of the essential sugars involved in these processes is fucose linked alpha1-->3 to N-acetylglucosamine. A family of alpha1,3-fucosyltransferases (FucTs) called FucT-III, IV, V, VI, VII, and IX is able to catalyze such fucosylations. Reverse transcription-PCR analysis revealed that human umbilical vein endothelial cells express all of the FucTs except FucT-IX. The predominant activity, as inferred by acceptor specificity of enzyme activity in cell lysates, is compatible with the presence of FucT-VI. By using an antibody to recombinant soluble FucT-VI, the enzyme colocalized with beta4-galactosyltransferase-1 to the Golgi apparatus. By using a polyclonal antiserum raised against a 17-aa peptide of the variable (stem) region of the FucT-VI, immunocytochemical staining of FucT-VI was restricted to Weibel-Palade bodies, as determined by colocalization with P-selectin and von Willebrand factor. SDS/PAGE immunoblotting and amino acid sequencing of internal peptides confirmed the identity of the antigen isolated by the peptide-specific antibody as FucT-VI. Storage of a fucosyltransferase in Weibel-Palade bodies suggests a function independent of Golgi-associated glycosylation.

A submembranous matrix of proteoglycans on zymogen granule membranes is involved in granule formation in rat pancreatic acinar cells

The secretory lectin ZG16p mediated the binding of aggregated zymogens to the granule membrane in pancreatic acinar cells. Using a recently established in vitro condensation-sorting assay, we now show that pretreatment of zymogen granule membranes (ZGM) with either sodium bicarbonate at pH 10 or with phosphatidyl inositol-specific phospholipase C (PI-PLC) reduced the binding efficiency of zymogens to the same extent, as distinct components were liberated from ZGM. Analysis of the composition of the bicarbonate extract revealed the presence of the secretory lectin ZG16p, the serpin ZG46p and the GPI-linked glycoprotein GP-2, together with several unknown proteins, and small amounts of lipase and carboxylester lipase. The unknown proteins detected in 2-D gels represented a group of acidic and basic protein spots, which were positive in a glycan staining reaction and were soluble in methanol. One protein spot of the acidic group and several of the basic group reacted with a monoclonal antibody directed against chondroitin sulfate, indicating that the proteins represented proteoglycans. A staining pattern similar to the glycan reaction was observed in immunoblots using a polyclonal antibody directed against the whole bicarbonate extract. Immunogold electron microscopy revealed that this antibody reacted with components in the periphery of zymogen granules and strongly stained ZGM in the pellet fraction of a standard in vitro condensation-sorting assay. The amino acid composition of isolated components of both the acidic and basic group showed similarities to aggrecan, a cartilage-specific proteoglycan, and to glycine-rich glycoproteins, respectively. We therefore conclude that a submembranous matrix on the ZGM composed of proteoglycans and glycoproteins is involved in granule formation in pancreatic acinar cells.

A novel zymogen granule protein (ZG29p) and the nuclear protein MTA1p are differentially expressed by alternative transcription initiation in pancreatic acinar cells of the rat

Using a polyclonal antibody against purified zymogen granule membrane components from rat pancreas a cDNA coding for the 29 kDa protein (ZG29p) was identified by immunoscreening of a hormonally stimulated pancreas cDNA library. Western blot analysis suggests that ZG29p is a pancreas-specific protein and immunofluorescence shows that ZG29p is mainly associated with zymogen granules. Analysis of subcellular fraction applying immunoblotting revealed that ZG29p was localized mainly in the soluble fraction of zymogen granules and in a Golgi- and RER-enriched fraction, but was absent from the cytosol. In isolated zymogen granule content ZG29p was associated with protein complexes containing amylase as main constituent. The cDNA coding for ZG29p is homologous to the C-terminal region of the candidate metastasis-associated gene mta1. Northern blot analysis and RT-PCR showed that no MTA1 mRNA is present in pancreas from fasted rats and in the rat pancreas carcinoma cell line AR4-2J in its protodifferentiated state. Although no ZG29p specific mRNA was seen in the northern blot analysis, RT-PCR showed that ZG29p was expressed under both non-stimulated and stimulated conditions. The expression of MTA1 was up-regulated in the pancreas by endogenous cholecystokinin release and in AR4-2J after induction of cellular differentiation by dexamethasone. Western blotting and immunofluorescense studies indicated that MTA1p is localized in the nucleus in all tissues studied. Using genomic DNA in PCR analysis it was shown that two short introns are present flanking the sequences of the 5'end of ZG29p cDNA. One intron contains consensus elements required for pancreas specific transcription initiation, suggesting that MTA1 and ZG29 are differentially expressed by alternative transcription initiation in the pancreas. The localisation of MTA1p in the nucleus of most cell types could signify a general role in gene regulation, while the cell type specific and exclusive expression of ZG29p in pancreatic acinar cells could indicate a role in granule formation.