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Biomedical subjects

B L Tang

Publications and source records attributed to B L Tang.

At least 37 records · Page 2Linked to original sources

Hsec22c: a homolog of yeast Sec22p and mammalian rsec22a and msec22b/ERS-24.

We have cloned a new member of a family of mammalian proteins homologous to Sec22p, a v-SNARE in Saccharomyces cerevisiae required for transport between the endoplasmic reticulum (ER) and the Golgi apparatus. The open reading frame encodes a polypeptide of 250 amino acids which is homologous to, but obviously different from, the recently reported mammalian Sec22p homologs rat sec22a, mouse sec22b, and hamster ERS-24. Northern blot analysis revealed two transcripts of about 1 and 5 kb respectively which are ubiquitously expressed. myc-epitope tagged sec22c is localized to the ER. Overexpression of the myc-tagged protein resulted in an anomalous staining pattern of SNARE molecules participating in ER-Golgi transport such as syntaxin 5 and mammalian bet1, but not the endosomal SNARE syntaxin 7. The presence of multiple forms of sec22 protein in the mammalian early secretory pathway is in-line with task specification in a highly elaborate transport machinery.

Amino Acid Sequence↗

Molecular cloning and localization of human syntaxin 16, a member of the syntaxin family of SNARE proteins.

We have cloned a new member of the syntaxin family of proteins, designated human syntaxin 16 (hsyn16). The open reading frame encodes a polypeptide of 307 amino acids with potential coiled-coil domains and a carboxy-terminal hydrophobic tail, which is characteristic of other members of the syntaxin family. The encoded polypeptide bears sequence homology to known syntaxin molecules. Northern blot analysis revealed a single transcript that is fairly ubiquitous, being slightly more enriched in heart and pancreas. Indirect immunofluorescence localised myc-tagged hsyn16 (myc-hsyn16) to the Golgi apparatus, colocalizing well with lens culinaris agglutinin, an established Golgi marker, as well as with other Golgi SNAREs such as GS28 and syntaxin 5. Myc-hsyn16 is redistributed to the endoplasmic reticulum upon brefeldin A treatment, indicating that it is localised to the Golgi stack. The ubiquitous expression and Golgi localization of hsy16 suggest that it is involved in a vesicular transport step within the organelle.

Amino Acid Sequence↗

Syntaxin 10: a member of the syntaxin family localized to the trans-Golgi network.

We have cloned a new member of the syntaxin family of proteins, designated human syntaxin 10 (hsyn10). The open reading frame encodes a polypeptide of 249 amino acids with potential coiled-coil domains and a carboxy-terminal hydrophobic tail. hsyn10 is particularly homologous to the recently reported rat syntaxin 6 (about 60% identity). Northern blot analysis showed that the transcript is enriched in the heart, skeletal muscles and pancreas. Indirect immunofluorescence studies using polyclonal antibodies raised against recombinant protein showed that the protein is localized to intracellular membrane structures, with perinuclear staining patterns colocalising well with the Golgi SNARE GS28. Morphological alterations of the staining pattern of the protein with brefeldin A but not wortmannin treatment indicate that the protein is localize to the trans-Golgi network.

Amino Acid Sequence↗

A novel synaptobrevin/VAMP homologous protein (VAMP5) is increased during in vitro myogenesis and present in the plasma membrane.

cDNA clones encoding a novel protein (VAMP5) homologous to synaptobrevins/VAMPs are detected during database searches. The predicted 102-amino acid VAMP5 harbors a 23-residue hydrophobic region near the carboxyl terminus and exhibits an overall amino acid identity of 33% with synaptobrevin/VAMP1 and 2 and cellubrevin. Northern blot analysis reveals that the mRNA for VAMP5 is preferentially expressed in the skeletal muscle and heart, whereas significantly lower levels are detected in several other tissues but not in the brain. During in vitro differentiation (myogenesis) of C2C12 myoblasts into myotubes, the mRNA level for VAMP5 is increased approximately 8- to 10-fold. Immunoblot analysis using antibodies specific for VAMP5 shows that the protein levels are also elevated approximately 6-fold during in vitro myogenesis of C2C12 cells. Indirect immunofluorescence microscopy and immunoelectron microscopy reveal that VAMP5 is associated with the plasma membrane as well as intracellular perinuclear and peripheral vesicular structures of myotubes. Epitope-tagged versions of VAMP5 are similarly targeted to the plasma membrane.

Amino Acid Sequence↗

Alpha-SNAP but not gamma-SNAP is required for ER-Golgi transport after vesicle budding and the Rab1-requiring step but before the EGTA-sensitive step.

N-ethylmaleimide-sensitive factor (NSF) and soluble NSF attachment proteins (SNAPs) have been implicated in diverse vesicular transport events; yet their exact role and site of action remain to be established. Using an established in vitro system, we show that antibodies against alpha-SNAP inhibit vesicle transport from the ER to the cis-Golgi and that recombinant alpha-SNAP enhances/stimulates the process. Cytosol immunodepleted of alpha-SNAP does not support normal transport unless supplemented with recombinant alpha-SNAP but not gamma-SNAP. In marked contrast, cytosol immunodepleted of gamma-SNAP supports ER-Golgi transport to the normal level. Neither antibodies against gamma-SNAP nor recombinant gamma-SNAP have any effect on ER-Golgi transport. These results clearly establish an essential role for alpha-SNAP but not gamma-SNAP in ER-Golgi transport. When the transport assay is performed with cytosol immunodepleted of alpha-SNAP, followed by incubation with cytosol immunodepleted of a COPII subunit, normal transport is achieved. In marked contrast, no transport is detected when the assay is first performed with cytosol depleted of the COPII subunit followed by alpha-SNAP-depleted cytosol, suggesting that alpha-SNAP is required after a step that requires COPII (the budding step). In combination with cytosol immunodepleted of Rab1, it is seen that alpha-SNAP is required after a Rab1-requiring step. It has been shown previously that EGTA blocks ER-Golgi transport at a step after vesicle docking but before fusion and we show here that alpha-SNAP acts before the step that is blocked by EGTA. Our results suggest that alpha-SNAP may be involved in the pre-docking or docking but not the fusion process.

Antibodies↗

The recycling pathway of protein ERGIC-53 and dynamics of the ER-Golgi intermediate compartment.

To establish recycling routes in the early secretory pathway we have studied the recycling of the ER-Golgi intermediate compartment (ERGIC) marker ERGIC-53 in HepG2 cells. Immunofluorescence microscopy showed progressive concentration of ERGIC-53 in the Golgi area at 15 degreesC. Upon rewarming to 37 degreesC ERGIC-53 redistributed into the cell periphery often via tubular processes that largely excluded anterograde transported albumin. Immunogold labeling of cells cultured at 37 degreesC revealed ERGIC-53 predominantly in characteristic beta-COP-positive tubulo-vesicular clusters both near the Golgi apparatus and in the cell periphery. Concentration of ERGIC-53 at 15 degreesC resulted from both accumulation of ERGIC-53 in the ERGIC and movement of ERGIC membranes closer to the Golgi apparatus. Upon rewarming to 37 degreesC the labeling of ERGIC-53 in the ERGIC rapidly returned to normal levels whereas ERGIC-53's labeling in the cis-Golgi was unchanged. Temperature manipulations had no effect on the average number of ERGIC-53 clusters. Density gradient centrifugation indicated that the surplus ERGIC-53 accumulating in the ERGIC at 15 degreesC was rapidly transported to the ER upon rewarming. These results suggest that the ERGIC is a dynamic membrane system composed of a constant average number of clusters and that the major recycling pathway of ERGIC-53 bypasses the Golgi apparatus.

Albumins↗

The mammalian protein (rbet1) homologous to yeast Bet1p is primarily associated with the pre-Golgi intermediate compartment and is involved in vesicular transport from the endoplasmic reticulum to the Golgi apparatus.

Yeast Bet1p participates in vesicular transport from the endoplasmic reticulum to the Golgi apparatus and functions as a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) associated with ER-derived vesicles. A mammalian protein (rbet1) homologous to Bet1p was recently identified, and it was concluded that rbet1 is associated with the Golgi apparatus based on the subcellular localization of transiently expressed epitope-tagged rbet1. In the present study using rabbit antibodies raised against the cytoplasmic domain of rbet1, we found that the majority of rbet1 is not associated with the Golgi apparatus as marked by the Golgi mannosidase II in normal rat kidney cells. Rather, rbet1 is predominantly associated with vesicular spotty structures that concentrate in the peri-Golgi region but are also present throughout the cytoplasm. These structures colocalize with the KDEL receptor and ERGIC-53, which are known to be enriched in the intermediate compartment. When the Golgi apparatus is fragmented by nocodazole treatment, a significant portion of rbet1 is not colocalized with structures marked by Golgi mannosidase II or the KDEL receptor. Association of rbet1 in cytoplasmic spotty structures is apparently not altered by preincubation of cells at 15 degrees C. However, upon warming up from 15 to 37 degrees C, rbet1 concentrates into the peri-Golgi region. Furthermore, rbet1 colocalizes with vesicular stomatitis virus G-protein en route from the ER to the Golgi. Antibodies against rbet1 inhibit in vitro transport of G-protein from the ER to the Golgi apparatus in a dose-dependent manner. This inhibition can be neutralized by preincubation of antibodies with recombinant rbet1. EGTA is known to inhibit ER-Golgi transport at a stage after vesicle docking but before the actual fusion event. Antibodies against rbet1 inhibit ER-Golgi transport only when they are added before the EGTA-sensitive stage. These results suggest that rbet1 may be involved in the docking process of ER-derived vesicles with the cis-Golgi membrane.

Amino Acid Sequence↗

The mammalian homolog of yeast Sec13p is enriched in the intermediate compartment and is essential for protein transport from the endoplasmic reticulum to the Golgi apparatus.

The role of COPII components in endoplasmic reticulum (ER)-Golgi transport, first identified in the yeast Saccharomyces cerevisiae, has yet to be fully characterized in higher eukaryotes. A human cDNA whose predicted amino acid sequence showed 70% similarity to the yeast Sec13p has previously been cloned. Antibodies raised against the human SEC13 protein (mSEC13) recognized a cellular protein of 35 kDa in both the soluble and membrane fractions. Like the yeast Sec13p, mSEC13 exist in the cytosol in both monomeric and higher-molecular-weight forms. Immunofluorescence microscopy localized mSEC13 to the characteristic spotty ER-Golgi intermediate compartment (ERGIC) in cells of all species examined, where it colocalized well with the KDEL receptor, an ERGIC marker, at 15 degrees C. Immunoelectron microscopy also localized mSEC13 to membrane structures close to the Golgi apparatus. mSEC13 is essential for ER-to-Golgi transport, since both the His6-tagged mSEC13 recombinant protein and the affinity-purified mSEC13 antibody inhibited the transport of restrictive temperature-arrested vesicular stomatitis virus G protein from the ER to the Golgi apparatus in a semi-intact cell assay. Moreover, cytosol immunodepleted of mSEC13 could no longer support ER-Golgi transport. Transport could be restored in a dose-dependent manner by a cytosol fraction enriched in the high-molecular-weight mSEC13 complex but not by a fraction enriched in either monomeric mSEC13 or recombinant mSEC13. As a putative component of the mammalian COPII complex, mSEC13 showed partially overlapping but mostly different properties in terms of localization, membrane recruitment, and dynamics compared to that of beta-COP, a component of the COPI complex.

Animals↗

Endoplasmic reticulum retention mediated by the transmembrane domain of type II membrane proteins Sec12p and glucosidase 1.

The yeast Sec12p, a type II protein localized to the yeast endoplasmic reticulum (ER), is similarly localized to the ER when expressed in mammalian cells. Replacing the transmembrane domain of the plasma membrane molecule dipeptidyl peptidase IV (D4) with that of Sec12p or the ER-localized enzyme glucosidase 1 resulted in the ER retention of the chimeric molecules, as assessed by immunocytochemical localization and the persistence of pulse-labeled proteins in the endoglycosidase H-sensitive form. Retention is not due to gross misfolding as these chimeras remained enzymatically active. Density gradient analysis revealed that the ER-localized chimeric molecules form high molecular weight oligomers quickly after synthesis. The type II transmembrane domain of ER proteins could therefore mediate retention in the ER.

Animals↗

Cytotoxicity of KDEL-terminated ricin toxins correlates with distribution of the KDEL receptor in the Golgi.

DNAs encoding ricin toxin A chain (RTA), with or without a C-terminal endoplasmic reticulum retention signal KDEL, were subcloned into pGEX2T bacterial expression plasmid. After transformation of JM105 E. coli cells and induction with isopropylthio-beta-galactoside (IPTG), fusion proteins were bound to an immobilized glutathione matrix and recombinant ricin A chains released with thrombin. Both recombinant wild-type RTA and RTA with KDEL had immunological reactivity and catalytic activity indistinguishable from plant RTA. The bacterial RTA products reassociated with plant ricin B chain (RTB) similarly to plant RTA. Cell cytotoxicities were measured on seven cell lines for each A-chain and heterodimer. Although KDEL sequences enhanced cytotoxicity in most cases, significant variability was observed. In each case, addition of KDEL enhanced A-chain cytotoxicity more than holotoxin cytotoxicity. Three cell lines showed reduced KDEL enhancement of both RTA and ricin cytotoxicity. The concentration of KDEL receptor was examined on each cell line by immunofluorescence microscopy with an antireceptor monoclonal antibody. Differences in sensitivity to KDEL-containing toxins correlated with altered distribution of KDEL receptor between endoplasmic reticulum (ER) and Golgi compartments.

Amino Acid Sequence↗

Retardation of a surface protein chimera at the cis Golgi.

Dipeptidyl peptidase IV (D4) and the alpha subunit of human chorionic gonadotrophin (alpha hcg) are plasma membrane and secretory proteins, respectively. In the course of studies to understand mechanisms involved in transport along the exocytotic pathway, the ectoplasmic domain of D4 was replaced by the mature polypeptide of alpha hcg, resulting in the membrane anchored chimera, D4 alpha hcg. Surprisingly, when transfected into Chinese hamster ovary (CHO) and Madin-Darby canine kidney (MDCK) cells, strong perinuclear Golgi staining was predominant, in addition to the expected surface staining. By following the biogenesis and transport of the molecule, it was established that newly synthesized D4 alpha hdg is eventually transported to the cell surface but only after a significant retardation in the Golgi apparatus. The compartment of retardation was identified as the early or cis Golgi, before the medial Golgi, where resistance to endoglycosidase (endo) H is conferred. As a result of the transport retardation of the chimera, we were able to document the appearance of an endo D sensitive intermediate, which is usually too transient to be apparent in normal cells. The retardation of this chimera in the cis Golgi complements our previous report in which the D4 molecule with its transmembrane domain replaced by that of aminopeptidase N resulted in retardation in the trans Golgi/trans Golgi network in MDCK and CHO cells [Low, S. H., Tang, B. L., Wong, S. H., & Hong, W. (1994) J. Biol. Chem. 269, 1985-1994).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

Monoclonal antibody HFD9 identifies a novel 28 kDa integral membrane protein on the cis-Golgi.

We have raised a monoclonal antibody (mAb) (HFD9) that detects a 28 kDa protein (p28) enriched in the Golgi membrane. p28 was localized to the perinuclear Golgi region in all cell lines thus far examined. Its Golgi localization was confirmed by its colocalization with Golgi markers using indirect immunofluorescence microscopy. Immunogold labelling demonstrates that the majority of p28 was localized on the cis-Golgi and its associated structures. Two independent experiments demonstrate that the p28 epitope recognized by mAb HFD9 is exposed to the cytosol. Extraction of Golgi membranes with a variety of reagents revealed that p28 behaves like an integral membrane protein. mAb HFD9 thus defines a novel 28 kDa integral membrane protein on the cis-Golgi. To our knowledge, p28 represents the first integral membrane protein of the Golgi system identified via the antibody approach whose epitope is cytoplasmically-oriented and highly-conserved. Monoclonal antibody HFD9 will thus provide a useful tool for further studies on the cis side of the Golgi, which is not well characterised due to the lack of good markers.

Antibodies, Monoclonal↗

Cell type differences in Golgi retention signals for transmembrane proteins.

The transmembrane domain of Golgi resident proteins such as beta-galactoside alpha 2,6-sialyltransferase (ST) and N-acetylglucosaminyltransferase 1 (NT) contain a Golgi retention signal which confers Golgi retention to reporter proteins appended to them in the appropriate context. Thus, chimeras of the cell surface protein dipeptidyl peptidase IV containing the transmembrane domain of ST and NT are retained in the Golgi apparatus in MDCK and COS cells, as assessed by indirect immunofluorescence microscopy. Transfection of these chimeric constructs into CHO cells, however, results in their transport to vesicular structures which do not colocalize with that of an endogenous Golgi marker, mannosidase II. Furthermore, the staining pattern of these structures are not affected by brefeldin A. Biochemical analysis of the transgene products in pulse-chase experiments revealed that the chimeric proteins eventually become resistant to endoglycosidase H, suggesting that they are transported beyond the medial Golgi and therefore the vesicular structures are likely to be post-Golgi. The vesicular structures colocalized well with a lysosomal marker, cathepsin D, and also with internalized FITC-dextran chased into the lysosomal compartment. Monitoring the cell surface appearance of the chimeric protein suggests that the majority is transported directly to the lysosomal compartment. Golgi retention can be completely restored for ST and improved for NT by the inclusion of sequences flanking the transmembrane domain. Our results reflect cell type differences in the interpretation of the transmembrane domain Golgi retention signal, established that general Golgi retention of type II glycosyltransferases requires the hydrophilic flanking sequence as well as the transmembrane domain, and demonstrate that proteins which escape Golgi retention may be channeled to the lysosomal pathway.

Animals↗

Segregation of ERGIC53 and the mammalian KDEL receptor upon exit from the 15 degrees C compartment.

Protein trafficking along the exocytotic pathway occurs by vesicular transport between successive membranous compartments. Transport from the endoplasmic reticulum (ER) to the Golgi apparatus has been proposed to be bridged by a morphologically defined ER-Golgi intermediate compartment (ERGIC). Using the subcellular dynamics of two markers for the ERGIC, the 53 kDa protein ERGIC53 and the mammalian KDEL receptor (KDEL-R), we have investigated the biochemical and physiological characteristics of ER-Golgi anterograde and retrograde transport. The KDEL-R at steady state is mainly confined to the perinuclear Golgi region while the ERGIC53 has a more elaborate distribution, including the ER. Both proteins can be colocalized to spotty structures distributed throughout the cytoplasm by incubating the cells at 15 degrees C. Upon returning the cells to 37 degrees C, the direction of transport for the two proteins diverged. KDEL-R was seen to emanate into tubular structures which eventually culminated in a focused, perinuclear staining. These dynamic changes are consistent with the anterograde transport process from the ER to the Golgi apparatus. ERGIC53, on the other hand, was distributed into an extended reticular network as well as the nuclear envelope, a staining pattern characteristic of the ER. With time, ERGIC53 was seen to return to the spotty structures again. The ER retrieval of ERGIC53 is consistent with the fact that the protein contains a dilysine motif which may function as an ER retrieval signal. The movement of ERGIC53 into the ER is not affected by microtubule disrupting agents, which inhibit the movement of KDEL-R to the Golgi. Both the processes are, however, sensitive to the alkylating agent N-ethylmaleimide. When reconstituted in vitro using digitonin permeabilized cells, the movement of ERGIC53 into the ER has a requirement for metabolic energy, is partially inhibited by the nonhydrolyzable guanine nucleotide analog GTP gamma S but could not be made to be cytosol dependent. These results documented the convergence of anterograde transport and retrograde transport at the 15 degrees C compartment and implied the existence of a segregation or a sorting process that would result in the segregation of proteins with different targeting signals in the structure.

Animals↗

Golgi retardation in Madin-Darby canine kidney and Chinese hamster ovary cells of a transmembrane chimera of two surface proteins.

Dipeptidyl peptidase IV (DDD) is a type II plasma membrane protein. Replacement of its transmembrane domain with that of another surface protein, aminopeptidase N, resulted in accumulation in the Golgi apparatus of Madin-Darby canine kidney cells and a delayed Golgi to surface transport in Chinese hamster ovary (CHO) cells. The compartment of retardation was identified as post medial-Golgi, most likely to be the trans-Golgi/trans Golgi network (TGN). Compared to native DDD, the rate of endoplasmic reticulum to Golgi transport for the chimera was largely unchanged in both cell types. On the other hand, Golgi to surface transport was delayed by more than 2 h in CHO cells and essentially undetectable up to 22 h of chase in Madin-Darby canine kidney cells. The decrease in the rate of Golgi to surface transport in CHO cells resulted in a significant accumulation of the fusion protein in the trans-Golgi/TGN. This phenomena is very unlikely to be due to any drastic conformational changes, as neither the enzyme activity nor the dimerization of the constructed molecule was affected. The findings of this study indicate that the transmembrane domain, in the context of its flanking sequences, is important for efficient Golgi to cell surface transport.

Amino Acid Sequence↗

Localization of the Lys, Asp, Glu, Leu tetrapeptide receptor to the Golgi complex and the intermediate compartment in mammalian cells.

The carboxyl-terminal Lys-Asp-Glu-Leu (KDEL), or a closely-related sequence, is important for ER localization of both lumenal as well as type II membrane proteins. This sequence functions as a retrieval signal at post-ER compartment(s), but the exact compartment(s) where the retrieval occurs remains unresolved. With an affinity-purified antibody against the carboxyl-terminal sequence of the mammalian KDEL receptor, we have investigated its subcellular localization using immunogold labeling on thawed cryosections of different tissues, such as mouse spermatids and rat pancreas, as well as HeLa, Vero, NRK, and mouse L cells. We show that rab1 is an excellent marker of the intermediate compartment, and we use this marker, as well as budding profiles of the mouse hepatitis virus (MHV) in cells infected with this virus, to identify this compartment. Our results demonstrate that the KDEL receptor is concentrated in the intermediate compartment, as well as in the Golgi stack. Lower but significant labeling was detected in the rough ER. In general, only small amounts of the receptor were detected on the trans side of the Golgi stack, including the trans-Golgi network (TGN) of normal cells and tissues. However, some stress conditions, such as infection with vaccinia virus or vesicular stomatitis virus, as well as 20 degrees C or 43 degrees C treatment, resulted in a significant shift of the distribution towards the trans-TGN side of the Golgi stack. This shift could be quantified in HeLa cells stably expressing a TGN marker. No significant labeling was detected in structures distal to the TGN under all conditions tested. After GTP gamma S treatment of permeabilized cells, the receptor was detected in the beta-COP-containing buds/vesicles that accumulate after this treatment, suggesting that these vesicles may transport the receptor between compartments. We propose that retrieval of KDEL-containing proteins occurs at multiple post-ER compartments up to the TGN along the exocytotic pathway, and that within this pathway, the amounts of the receptor in different compartments varies according to physiological conditions.

Animals↗

Cytosolic factors block antibody binding to the C-terminal cytoplasmic tail of the KDEL receptor.

The mammalian KDEL receptor is an extremely hydrophobic membrane protein. One of the longest stretches of hydrophilic sequence resides at the C-terminus. Various antibodies against a synthetic peptide corresponding to this region confirmed that the C-terminus is exposed to the cytoplasm. It was observed that antibody binding to the C-terminus of the KDEL receptor was diminished during immunofluorescence microscopy procedures which involved fixation prior to permeabilization as compared to when cells were permeabilized before fixation. Binding of both polyclonal and monoclonal antibodies, as assessed by indirect immunofluorescence microscopy in digitonin permeabilized cells, was inhibited by preincubation with rat liver cytosol. This inhibition was not observed with antibody against another membrane protein (p28) with a cytoplasmically exposed epitope also residing in the Golgi/intermediate compartment. Rabbit reticulocyte lysate had a similar effect while Schizosaccharomyces pombe cytosol inhibited binding to a greater degree than Saccharomyces cerevisiae cytosol. This inhibition by cytosol was prevented by coincubation with the antibody and was dose-dependent on the cytosol. Inhibition did not occur on ice or at 15 degrees C, or when the cytosol was energy-depleted by apyrase treatment. Interestingly, pretreatment of permeabilized cells with N-ethylmaleimide or its addition into the incubation mixture abolished inhibition. N-ethylmaleimide-treated cytosol, however, remained inhibitory. The findings suggest the existence of cytosolic factor (s) which interacts specifically with the cytoplasmic C-terminus of the KDEL receptor, which are likely to be components of the KDEL protein retrieval machinery.

Animals↗