Detection of endocytic vesicle fusion in vitro, using assay based on avidin-biotin association reaction.
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Animals↗
Biomedical subjects
W A Braell
Publications and source records attributed to W A Braell.
Characterization of the mechanism of endocytic vesicle fusion in vitro.
A cell-free assay to monitor receptor-mediated endocytic processes has been developed that uses biotinylated transferrin and avidin-linked beta-galactosidase as receptor-associated and fluid-phase probes, respectively (Wessling-Resnick, M., and Braell, W. A. (1990) J. Biol. Chem. 265, 690-699). The fusion of vesicles from heterologous sources can be detected in this assay: endocytic vesicles from K562 cells (a human cell line) will fuse with vesicles from Chinese hamster ovary cells. Fusion between endocytic vesicles is inhibited upon treatment with N-ethylmaleimide but can be restored by the addition of untreated cytosol from either cell type. The in vitro fusion reaction is also inhibited by the nonhydrolyzable nucleotide analogs guanosine 5'-(3-thiotriphosphate) (GTP gamma S) and adenosine 5'-(3-thiotriphosphate) (ATP gamma S). Other nonhydrolyzable guanine nucleotides are found to inhibit the in vitro reaction in the following order of potency: GTP gamma S greater than 5'-guanylyl imidodiphosphate (GTP-PNP) greater than alpha,beta-methylene GTP (GTP-PCP). The inhibitory effects of the nonhydrolyzable analogs of GTP and ATP are not additive. Moreover, excess GTP relieves the inhibition by GTP gamma S more than it relieves the inhibition by ATP gamma S, while excess ATP preferentially alleviates ATP gamma S (not GTP gamma S) inhibition. These properties suggest that the two nucleotides exert their effects at distinct points in the fusion process. Although micromolar levels of excess Ca2+ also inhibit vesicle fusion, the inhibition exerted by GTP gamma S appears to proceed via a pathway independent of the divalent cation. The GTP gamma S-sensitive step in endocytic vesicle fusion is found to occur at a mechanistic stage prior to and distinct from the N-ethylmaleimide-sensitive step of the reaction. This situation permits the accumulation of a membrane vesicle intermediate in the presence of GTP gamma S; subsequent incubation of these vesicles with cytosol and GTP restores their fusion competence. Characteristics of in vitro endocytic vesicle fusion suggest that similarities exist with steps of the fusion mechanism involved with membrane traffic events of the secretory pathway.
Animals↗
The sorting and segregation mechanism of the endocytic pathway is functional in a cell-free system.
A cell-free system which reconstitutes early stages of receptor-mediated endocytosis has been developed, based on detection of the association between avidin-beta-galactosidase (Av beta Gal) and biotin-transferrin (B-Tf). Initially, Av beta Gal (a fluid-phase marker) and B-Tf (receptor-bound) are internalized and delivered to a common endosomal compartment in vivo and in vitro. Subsequently, these two probes enter divergent intracellular pathways: Av beta Gal is sorted from the endosome and directed for delivery to lysosomes, whereas B-Tf is segregated away from the fluid-phase marker, remaining bound to the transferrin receptor for return to the cell surface. Using the avidin-biotin association reaction to monitor the co-localization of these two probes, we have been able to reconstruct this sorting and segregation process in a cell-free system. The in vitro reaction is time-, temperature-, and ATP-dependent, and is not affected by NH4Cl; cell-free segregation of the two probes is also sensitive to N-ethylmaleimide. As these characteristics are also properties of in vitro endocytic vesicle fusion, it is likely that the latter event is a prerequisite for the sorting and segregation process. Both the in vivo and in vitro sorting of Av beta Gal and B-Tf to their respective and distinct destinations can be followed by subcellular fractionation on Percoll gradients. Our observations provide the first evidence that the cellular mechanism to identify, sort, and sequester endocytosed material can be reconstituted in a cell-free system.
Avidin↗
Two sensitive, convenient, and widely applicable assays for marker enzyme activities specific to endoplasmic reticulum.
Two assay protocols are described for enzyme activities known to reside in the endoplasmic reticulum of a wide variety of species and tissue types, with the intent that they be used as marker enzyme assays in subcellular fractionations. The enzyme activities assayed are choline phosphotransferase and dolichol-P-mannosyl synthase, both of which result in synthesis of lipid products. The assays are constructed to make them easy to perform and sensitive enough to detect enzyme activity even using microgram quantities of cell protein. The assay methodologies are effective not only in vertebrate cells, but in insect cells and yeast cells as well. This implies that these assays should be useful as marker enzyme assays for a wide variety of eukaryotic cells.
Animals↗
Fusion between endocytic vesicles in a cell-free system.
Cell-free fusion between endocytic vesicles has been obtained in a sensitive assay based on the avidin-biotin binding reaction. Chinese hamster ovary cells are allowed to endocytose either avidin-linked beta-galactosidase or biotinylated IgG. Postnuclear supernatant extracts prepared from these cells are incubated at 37 degrees C in the presence of an ATP-regenerating system. Fusion between vesicles from the two extracts permits the avidin-biotin association to occur, so that the amount of avidin-enzyme-biotinylated IgG complex produced is proportional to the amount of fusion between vesicles. The amount of complex formed is measured, after detergent lysis of the vesicles, by an ELISA technique, using a fluorogenic substrate for beta-galactosidase. The fusion process requires ATP hydrolysis and specific cytosolic proteins. The vesicles that fuse appear to be endocytic vesicles populated by the endocytosed proteins within 5 min of their internalization at 37 degrees C. Ionophores and weak bases do not inhibit fusion, suggesting that a pH gradient across the vesicle membrane is not crucial for the fusion process.
Animals↗
ATP catalyzes the sequestration of clathrin during enzymatic uncoating.
ATP facilitates the sequestration of displaced triskelions by uncoating protein. In so doing, ATP is not hydrolyzed; nor does the concentration of ATP affect the equilibrium of this binding. However, the rates of both the binding of uncoating protein to clathrin and of their dissociation are greatly accelerated by ATP. These properties suggest that ATP acts catalytically to speed the capture of displaced triskelions by uncoating protein, as well as stoichiometrically in its hydrolysis to drive the displacement of triskelions from cages. The nucleotide specificity of this "catalytic" site for ATP on the uncoating protein is much less strict than that of the distinct "hydrolytic" site that drives the ATP-dependent displacement of triskelions from cages.
Adenosine Triphosphate↗
Reconstitution of the transport of protein between successive compartments of the Golgi measured by the coupled incorporation of N-acetylglucosamine.
Transport of the VSV-encoded glycoprotein (G protein) between successive compartments of the Golgi has been reconstituted in a cell-free system and is measured, in a rapid and sensitive new assay, by the coupled incorporation of 3H-N-acetylglucosamine (GlcNAc). This glycosylation occurs when G protein is transported during mixed incubations from the "donor" compartment in Golgi from VSV-infected CHO clone 15B cells (missing a key Golgi GlcNAc transferase) to the next, successive "acceptor" compartment (containing the GlcNAc transferase) in Golgi from wild-type CHO cells. Golgi fractions used in this assay have been extensively purified, and account for all of the donor and acceptor activity in the cells. Together with several other lines of evidence, this indicates that the cell-free system is highly specific, measuring only transport between sequential compartments in the Golgi stack. Transport in vitro is almost as efficient as in the cell, and requires ATP and the cytosol fraction in addition to protein components on the cytoplasmic surface of the Golgi membranes.
Acetylglucosamine↗
The glycoprotein that is transported between successive compartments of the Golgi in a cell-free system resides in stacks of cisternae.
Electron microscope autoradiography has been used to localize the glycoprotein transported between successive compartments of the Golgi in a cell-free system. Both donor and acceptor Golgi fractions contain stacks of cisternae, which remain as discrete populations even after prolonged incubations together. The glycosylated VSV G protein, having received 3H-GlcNAc residues following its transport in vitro, is entirely within the population of acceptor stacks from the uninfected wild-type Golgi population (those housing GlcNAc transferase l). Quantitation of 3H grains reveals between 6,000 and 12,000 molecules of G protein introduced into each acceptor cisterna as a result of transport in the cell-free system, amounting to approximately 5% of its total membrane protein. This represents about the full complement of transported protein normally contained in a Golgi cisterna in vivo. Transport in the cell-free system is efficient and specific, preserving the integrity of the Golgi stack and its individual cisternae.
Acetylglucosamine↗
An enzyme that removes clathrin coats: purification of an uncoating ATPase.
Uncoating ATPase, an abundant 70,000-mol-wt polypeptide mediating the ATP-dependent dissociation of clathrin from coated vesicles and empty clathrin cages, has been purified to virtual homogeneity from calf brain cytosol. Uncoating protein is present in cells in amounts roughly stoichiometric with clathrin. This enzyme is isolated as a mixture of monomers and dimers, both forms being active. ATP can support protein-facilitated dissociation of clathrin at micromolar levels; all other ribotriphosphates as well as deoxy-ATP are inactive. The clathrin that is released from cages consists of trimers (triskelions) in a stoichiometric complex with uncoating ATPase. These complexes with clathrin have little tendency to self-associate at neutral pH, and at acidic pH they interfere with the assembly of free clathrin. The possible existence and function of these complexes as clathrin carriers in cells would explain why uncoating protein is made in quantities equivalent to clathrin.
Adenosine Triphosphatases↗
Dissociation of clathrin coats coupled to the hydrolysis of ATP: role of an uncoating ATPase.
ATP hydrolysis was used to power the enzymatic release of clathrin from coated vesicles. The 70,000-mol-wt protein, purified on the basis of its ATP-dependent ability to disassemble clathrin cages, was found to possess a clathrin-dependent ATPase activity. Hydrolysis was specific for ATP; neither dATP nor other ribonucleotide triphosphates would either substitute for ATP or inhibit the hydrolysis of ATP in the presence of clathrin cages. The ATPase activity is elicited by clathrin in the form of assembled cages, but not by clathrin trimers, the product of cage disassembly. The 70,000-mol-wt polypeptide, but not clathrin, was labeled by ATP in photochemical cross-linking, indicating that the hydrolytic site for ATP resides on the uncoating protein. Conditions of low pH or high magnesium concentration uncouple ATP hydrolysis from clathrin release, as ATP is hydrolyzed although essentially no clathrin is released. This suggests that the recognition event triggering clathrin-dependent ATP hydrolysis occurs in the absence of clathrin release, and presumably precedes such release.
Adenosine Triphosphatases↗
Methods for study of the synthesis and maturation of the erythrocyte anion transport protein.
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Animals↗
Ovalbumin utilizes an NH2-terminal signal sequence.
In synchronized translation experiments in the wheat germ and reticulocyte lysate systems, ovalbumin (385 amino acids) was glycosylated by and segregated in dog pancreatic microsomes only if microsomes were added before the nascent ovalbumin polypeptide contained less than 150 amino acids. This would place the "signal" sequence of ovalbumin prior to residue 150, in contrast to a previous report.
Amino Acid Sequence↗
The erythrocyte anion transport protein is contranslationally inserted into microsomes.
The biosynthesis of the erythrocyte anion transport protein, Band III (molecular weight 100,000), is of interest, as its NH2-terminal half is hydrophilic and faces the cytoplasmic surface, and its COOH-terminal half spans the phospholipid bilayer several times. To investigate the problem of insertion of Band III into membranes, we used erythroid precursor cells from the spleens of anemic mice as a source of messenger RNA for in vitro studies in the wheat germ and reticulocyte lysate cell-free system containing dog pancreatic microsomes. Immediately after synthesis, Band III was found to be inserted into intracellular membranes in its mature configuration, with the NH2-terminal portion exposed to the cytoplasm and its hydrophobic COOH-terminal portion spanning the lipid bilayer. The newly synthesized Band III was provided with a high mannose asparagine-linked oligosaccharide, which was sensitive to cleavage by endoglycosidase H; this is presumably the precursor of the very large and complex oligosaccharide found on the finished molecule. Band III was found to insert into dog pancreatic microsomes in a cotranslational manner; in synchronized translation studies microsomes could be added as late as the time when the hydrophilic NH2-terminal half of the protein had been synthesized and still allow normal transmembrane insertion and glycosylation. There is no cleavage of any NH2-terminal peptide during membrane insertion. The results suggest that Band III contains a sequence near the middle of the protein that directs its insertions into endoplasmic reticulum membranes.
Animals↗
Synthesis and maturation of the erythrocyte anion transport protein--an internal sequence for membrane insertion.
The biosynthesis of the erythrocyte anion transport glycoprotein, Band III (Mr 100,000), is of interest, as its N-terminal half is hydrophilic and faces the cytoplasmic surface; the C-terminal half spans the phospholipid bilayer several times. Band III is synthesized by erythroid precursor cells obtained from the spleens of anaemic mice. Newly synthesized Band III was inserted into rough endoplasmic reticulum membranes with an asymmetric orientation which resembled that of mature Band III in erythrocyte membranes: the N-terminal portion of the molecule facing the cytoplasm. Newly made Band III contained a high-mannose asparagine-linked oligosaccharide, which was susceptible to cleavage by endoglycosidase H. During the next 20-30 min, this oligosaccharide was processed to a form resistant to endoglycosidase H degradation, presumably in the Golgi complex. The processed Band III was subsequently expressed on the cell surface, at about 30-45 min after synthesis. To study the mechanism of insertion of Band III into microsomes, we used erythroid precursor cells from the spleens of anaemic mice as a source of messenger RNA for studies in vitro in the wheat germ and reticulocyte lysate cell-free system containing dog pancreatic microsomes. Immediately after synthesis, Band III was found to be inserted into microsomal membranes in its mature configuration, with the N-terminal portion exposed to the cytoplasm and its hydrophobic C-terminal portion spanning the lipid bilayer. The newly-synthesized Band III was also provided with a high-mannose asparagine-linked oligosaccharide. Band III was found to be inserted into dog pancreatic microsomes in a co-translational manner; in synchronized translation studies microsomes could be added as late as the time when the hydrophilic N-terminal half of the protein had been synthesized and still allow normal trans-membrane insertion and glycosylation. There is no cleavage of any N-terminal peptide during membrane insertion. In many respects, therefore, the biosynthesis of Band III resembles that of co-translationally-inserted proteins whose N-terminal portions are exposed on the exterior of the cell, like vesicular stomatitis virus glycoprotein, HLA-A antigens, and glycophorin. However, our results suggest that Band III contains a sequence near the middle of the protein which directs its insertion into endoplasmic reticulum membranes.
Acetylglucosaminidase↗
Biosynthesis of the erythrocyte anion transport protein.
The biosynthesis of the erythrocyte anion transport protein (Band III) was studied in erythroid precursor cells obtained from the spleens of anemic mice. Newly synthesized Band III was inserted during or immediately after translation into rough endoplasmic reticulum membranes. The asymmetric orientation of Band III in these membranes resembled that of mature Band III in erythrocyte membranes, with the NH2-terminal portion of the molecule facing the cytoplasm. At this stage Band III contained a high mannose core oligosaccharide, which was susceptible to cleavage by endoglycosidase H. During the next 20 to 30 min, this oligosaccharide was processed to a form resistant to endoglycosidase H degradation, presumably in the Golgi complex. The processed Band III was subsequently expressed on the cell surface, at about 30 to 45 min after synthesis. In many respects, therefore, the biosynthesis of Band III resembles that of cotranslationally inserted proteins whose NH2-terminal portions are exposed on the exterior of the cell, like VSV glycoprotein, HLA-A antigens, and glycophorin.
Animals↗
Synthesis and assembly of membrane and organelle proteins.
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Animals↗
A new procedure for the preparation of GDP-[U-14C]mannose.
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Alkaline Phosphatase↗
A role for clathrin light chains in the recognition of clathrin cages by 'uncoating ATPase'.
A cycle of clathrin assembly and disassembly drives the formation of coated vesicles, intermediates in intracellular protein transport. The heavy chain of clathrin is needed for assembly, but the function of the clathrin light chains has remained obscure. An enzyme has now been purified which uses ATP hydrolysis to power the release of clathrin from coated vesicles, presumably recycling the coat protein for repeated rounds of vesicle budding. This 'uncoating ATPase' requires clathrin light chains for its action.
Adenosine Triphosphatases↗