Cholesterol-dependent interaction of syncollin with the membrane of the pancreatic zymogen granule

ESAT-6 from Mycobacterium tuberculosis dissociates from its putative chaperone CFP-10 under acidic conditions and exhibits membrane-lysing activity

The 6-kDa early secreted antigenic target ESAT-6 and the 10-kDa culture filtrate protein CFP-10 of Mycobacterium tuberculosis are secreted by the ESX-1 system into the host cell and thereby contribute to pathogenicity. Although different studies performed at the organismal and cellular levels have helped to explain ESX-1-associated phenomena, not much is known about how ESAT-6 and CFP-10 contribute to pathogenesis at the molecular level. In this study we describe the interaction of both proteins with lipid bilayers, using biologically relevant liposomal preparations containing dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol, and cholesterol. Using flotation gradient centrifugation, we demonstrate that ESAT-6 showed strong association with liposomes, and in particular with preparations containing DMPC and cholesterol, whereas the interaction of CFP-10 with membranes appeared to be weaker and less specific. Most importantly, binding to the biomembranes no longer occurred when the proteins were present as a 1:1 ESAT-6.CFP-10 complex. However, lowering of the pH resulted in dissociation of the protein complex and subsequent protein-liposome interaction. Finally, cryoelectron microscopy revealed that ESAT-6 destabilized and lysed liposomes, whereas CFP-10 did not. In conclusion, we propose that one of the main features of ESAT-6 in the infection process of M. tuberculosis is the interaction with biomembranes that occurs after dissociation from its putative chaperone CFP-10 under acidic conditions typically encountered in the phagosome.

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.

Syncollin is required for efficient zymogen granule exocytosis

Syncollin is a 13 kDa protein that is present in the exocrine pancreas, where the majority of the protein is tightly attached to the luminal surface of the zymogen granule membrane. We have addressed the physiological role of syncollin by studying the phenotype of syncollin KO (knockout) mice. These mice show pancreatic hypertrophy and elevated pancreatic amylase levels. Further, secretagogue-stimulated amylase release from pancreatic lobules of syncollin KO mice was found to be reduced by about 45% compared with wild-type lobules, and the delivery of newly synthesized protein to zymogen granules was delayed, indicating that the mice have a pancreatic secretory defect. As determined by two-photon imaging, the number of secretagogue-stimulated exocytotic events in acini from syncollin KO mice was reduced by 50%. This reduction was accounted for predominantly by a loss of later, 'secondary' fusion events between zymogen granules and other granules that had already fused with the plasma membrane. We conclude that syncollin is required for efficient exocytosis in the pancreatic acinar cell, and that it plays a particularly important role in compound exocytosis.

Targeting of Helicobacter pylori vacuolating toxin to lipid raft membrane domains analysed by atomic force microscopy

The Helicobacter pylori vacuolating toxin VacA causes several effects on mammalian cells in vitro, including intracellular vacuolation, formation of pores in the plasma membrane and apoptosis. When added to cells, VacA becomes associated with detergent-resistant membranes, indicating that it binds preferentially to lipid rafts. In the present study, we have used atomic force microscopy to examine directly the association of VacA with lipid domains in supported lipid bilayers. VacA did not bind to lipid bilayers at pH 7.6. In contrast, at pH 4.0, VacA associated with the bilayers in the form of 26-nm oligomeric complexes. VacA bound to bilayers produced from either brain lipids or SM (sphingomyelin) plus cholesterol, each of which lacked detectable lipid domains. Bilayers composed of DOPC (dioleoylphosphatidylcholine), SM and cholesterol contained clearly visible raft-like domains, and VacA preferentially associated with these rafts. VacA bound poorly to raft-like domains in DOPC/SM bilayers, indicating that cholesterol is required for efficient association of VacA with lipid domains. When PS (phosphatidylserine), an anionic phospholipid that does not partition significantly into rafts, was added to the mixture of DOPC, SM and cholesterol, VacA was excluded from the rafts, indicating that it binds more avidly to PS than to the raft components. A typical plasma membrane exhibits pronounced lipid asymmetry, with SM enriched in the outer leaflet and PS in the inner leaflet. Therefore it is probable that the association of VacA with rafts in DOPC/SM/cholesterol bilayers represents a useful model for understanding the interactions of VacA with membranes in vivo.

Syncollin inhibits regulated corticotropin secretion from AtT-20 cells through a reduction in the secretory vesicle population

Syncollin is a 13 kDa protein that is highly expressed in the exocrine pancreas. Syncollin normally exists as a doughnut-shaped homo-oligomer (quite probably a hexamer) in close association with the luminal surface of the zymogen granule membrane. In the present study, we examine the effect of expression of syncollin in AtT-20 neuroendocrine cells, which do not normally express this protein. Efficient expression was achieved by infection of the cells with adenoviral constructs encoding either untagged or GFP (green fluorescent protein)-tagged syncollin. Both forms of the protein were sorted into corticotropin (ACTH)-positive secretory vesicles present mainly at the tips of cell processes. Neither form affected basal corticotropin secretion or the constitutive secretion of exogenously expressed secreted alkaline phosphatase. In contrast, regulated secretion of corticotropin was inhibited (by 49%) by untagged but not by GFP-tagged syncollin. In parallel, untagged syncollin caused a 46% reduction in the number of secretory vesicles present at the tips of the cell processes. Syncollin-GFP was without effect. We could also show that native syncollin purified from rat pancreas was capable of permeabilizing erythrocytes. We suggest that syncollin may induce uncontrolled permeabilization of corticotropin-containing vesicles and subsequently destabilize them. Both forms of syncollin were tightly membrane-associated and appeared to exist as homooligomers. Hence, the lack of effect of syncollin-GFP on regulated exocytosis suggests that the GFP tag interferes in a subtler manner with the properties of the assembled protein.

Syntaxin is efficiently excluded from sphingomyelin-enriched domains in supported lipid bilayers containing cholesterol

Formation of a trans-complex between the three SNARE proteins syntaxin, synaptobrevin and SNAP-25 drives membrane fusion. The structure of the core SNARE complex has been studied extensively. Here we have used atomic force microscopy to study the behavior of recombinant syntaxin 1A both in detergent extracts and in a lipid environment. Full-length syntaxin in detergent extracts had a marked tendency to aggregate, which was countered by addition of munc-18. In contrast, syntaxin lacking its transmembrane region was predominantly monomeric. Syntaxin could be integrated into liposomes, which formed lipid bilayers when deposited on a mica support. Supported bilayers were decorated with lipid vesicles in the presence, but not the absence, of full-length syntaxin, indicating that formation of syntaxin complexes in trans could mediate vesicle docking. Syntaxin complexes remained at the sites of docking following detergent solubilization of the lipids. Raised lipid domains could be seen in bilayers containing sphingomyelin, and these domains were devoid of syntaxin and docked vesicles in the presence, but not the absence, of cholesterol. Our results demonstrate that syntaxin is excluded from sphingomyelin-enriched domains in a cholesterol-dependent manner.

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.

Plasma membrane cholesterol modulates cellular vacuolation induced by the Helicobacter pylori vacuolating cytotoxin

The Helicobacter pylori vacuolating cytotoxin (VacA) induces the degenerative vacuolation of mammalian cells both in vitro and in vivo. Here, we demonstrate that plasma membrane cholesterol is essential for vacuolation of mammalian cells by VacA. Vacuole biogenesis in multiple cell lines was completely blocked when cholesterol was extracted selectively from the plasma membrane by using beta-cyclodextrins. Moreover, increasing plasma membrane cholesterol levels strongly potentiated VacA-induced vacuolation. In contrast, inhibiting de novo biosynthesis of cholesterol with lovastatin or compactin had no detectable effect on vacuolation. While depletion of plasma membrane cholesterol has been shown to interfere with both clathrin-mediated endocytosis and caveola-dependent endocytosis, neither of these two internalization pathways was found to be essential for vacuolation of cells by VacA. Depleting plasma membrane cholesterol attenuated the entry of VacA into HeLa cells. In addition, beta-cyclodextrin reagents blocked vacuolation of cells that were either preloaded with VacA or had VacA directly expressed within the cytosol. Collectively, our results suggest that plasma membrane cholesterol is important for both the intoxication mechanism of VacA and subsequent vacuole biogenesis.

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.

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.