Interaction of the HIV-1 rev cofactor eukaryotic initiation factor 5A with ribosomal protein L5

It has previously been shown that interaction of eukaryotic initiation factor 5A (eIF-5A) with the Rev trans-activator protein of HIV-1 mediates the transport of unspliced or incompletely spliced viral mRNAs across the nuclear envelope. Consequently, mutants of eIF-5A block Rev function and thereby replication of HIV-1 in trans, indicating that eIF-5A is a crucial protein that connects the viral Rev regulator with cellular RNA transport systems. Here we show that the ribosomal protein L5, which is the central protein component of the 5S rRNA export system, is a cellular interaction partner of eIF-5A. Functional studies demonstrate that overexpression of L5 protein significantly enhances Rev activity. Furthermore, Rev nuclear export activity is inhibited in human somatic cells by antibodies that recognize eIF-5A or L5. Our data suggest that the Rev export pathway shares components of a cellular transport system involved in the intracellular trafficking of polymerase III (5S rRNA) transcripts.

Role of vesicle-associated syntaxin 5 in the assembly of pre-Golgi intermediates

Syntaxins are thought to function during vesicular transport as receptors on the target membrane and to contribute to the specificity of membrane docking and fusion by interacting with vesicle-associated receptors. Here, syntaxin 5 (Syn5) was shown to be an integral component of endoplasmic reticulum-derived transport vesicles. This pool, but not the target, Golgi-associated Syn5 pool, was essential for the assembly of vesicular-tubular pre-Golgi intermediates and the delivery of cargo to the Golgi. The requirement for vesicle-associated Syn5 in transport suggests a reevaluation of the basis for operation of the early secretory pathway.

Mss4 does not function as an exchange factor for Rab in endoplasmic reticulum to Golgi transport

Mss4 and its yeast homologue, Dss4, have been proposed to function as guanine nucleotide exchange factors (GEFs) for a subset of Rab proteins in the secretory pathway. We have previously shown that Rab1A mutants defective in GTP-binding potently inhibit endoplasmic reticulum to Golgi transport, presumably by sequestering an unknown GEF regulating its function. We now demonstrate that these mutants stably associate with Mss4 both in vivo and in vitro and that Mss4 effectively neutralizes the inhibitory activity of the Rab1A mutants. An equivalent Rab3A mutant (Rab3A[N135I]), a Rab protein specifically involved in regulated secretion at the cell surface, associates with Mss4 as efficiently as the Rab1A[N124I] mutant. Although Rab3A[N135I] prevents the ability of Mss4 to neutralize the inhibitory effects of Rab1A mutants on transport, it has no effect on Rab1 function or endoplasmic reticulum to Golgi transport. Furthermore, quantitative immunodepletion of Mss4 fails to inhibit transport in vitro. We conclude that Mss4 and its yeast homologue, Dss4, are not GEFs mediating activation of Rab, but rather, they interact with the transient guanine nucleotide-free state, defining a new class of Ras-superfamily GTPase effectors that function as guanine nucleotide-free chaperones (GFCs).

Mammalian Sly1 regulates syntaxin 5 function in endoplasmic reticulum to Golgi transport

Members of the syntaxin gene family are components of protein complexes which regulate vesicle docking and/or fusion during transport of cargo through the secretory pathway of eukaryotic cells. We have previously demonstrated that syntaxin 5 is specifically required for endoplasmic reticulum to Golgi transport (Dascher, C., Matteson, J., and Balch, W. E.(1994) J. Biol. Chem. 269, 29363-29366). To extend these observations we have now cloned a protein from rat liver membranes which forms a native complex with syntaxin 5. We demonstrate that this protein is the mammalian homologue to yeast Sly1p, previously identified as a protein which genetically and biochemically interacts with the small GTPase Ypt1p and Sed5p, proteins involved in docking/fusion in the early secretory pathway of yeast. Using transient expression we find that overexpression of rat liver Sly1 (rSly1) can neutralize the dominant negative effects of excess syntaxin 5 on endoplasmic reticulum to Golgi transport. These results suggest that rSly1 functions to positively regulate syntaxin 5 function.

Ricin cytotoxicity is sensitive to recycling between the endoplasmic reticulum and the Golgi complex

Cytotoxic proteins that kill mammalian cells by catalytically inhibiting protein synthesis must enter the cytosol in order to reach their substrates. With the exception of diphtheria toxin, which enters the cytosol from acidified endosomes, the intracellular site of translocation of other toxins including ricin, Escherichia coli Shiga-like toxin-1, and Pseudomonas exotoxin A is likely to involve early compartments of the secretory pathway. We have used a molecular approach to identify the site and mechanism of toxin delivery to the cytosol by transiently expressing mutant GTPases that inhibit the assembly of biochemical complexes mediating anterograde and retrograde transport in the exocytic and endocytic pathways. The results provide evidence to suggest that receptors actively recycling between the endoplasmic reticulum and terminal Golgi compartments are essential for toxin translocation to the cytosol from the endoplasmic reticulum. The rapid kinetics of intoxication demonstrate a substantial level of bidirectional membrane flow and sorting through the early secretory pathway.

Systemic immunization with Hsp60 alters the development of chlamydial ocular disease

PURPOSE

To determine whether immunization with recombinant Hsp60 would exacerbate ocular pathology on challenge with viable chlamydial elementary bodies.

METHODS

Guinea pigs were immunized either subcutaneously with recombinant Hsp60 or both subcutaneously with recombinant Hsp60 and ocularly with attenuated Salmonella typhimurium expressing the guinea pig inclusion conjunctivitis (GPIC) Hsp60 antigen. All animals were challenged in the conjunctiva with the agent of GPIC, and the degree of gross ocular pathology was determined. Immunoglobulin G (IgG) and immunoglobulin A (IgA) antibody titers to Hsp60 were measured in ocular secretions as a measure of the degree of immunization.

RESULTS

In primary and challenge GPIC infection, the degree of gross ocular pathology was lower in the immunized group. The presence of high IgA and IgG antibody titers to Hsp60 in tears suggested that the response may have been modified by the presence of blocking antibodies that either may have removed the antigen quickly or prevented interaction with sensitized T cells. In contrast to subcutaneous immunization, the combined immunization regimen, consisting of subcutaneous recombinant Hsp60 followed by ocular inoculation of the attenuated Salmonella, resulted in no difference in gross pathology after reinfection of guinea pigs with GPIC.

CONCLUSIONS

These data indicated that the immunization with Hsp60 did not produce exacerbated disease on challenge with viable organisms; however, the data suggested that the route of administration, form of antigen, or both may be critical in the disease process.

Syntaxin 5 regulates endoplasmic reticulum to Golgi transport

Syntaxins are a family of vesicular transport receptors that are involved in membrane traffic through both the constitutive and regulated secretory pathways. Syntaxins 1A/B,2,3, and 4 are principally associated with the plasma membrane. Two of the syntaxins, 1A and 1B, have been suggested to be the docking receptors for synaptic vesicles with the presynaptic membrane. The most distant member of the family, syntaxin 5, has been found in the Golgi region and has significant homology (35% identity) with Sed5p, an essential protein in yeast which is required for vesicular transport from the endoplasmic reticulum (ER) to the Golgi stack. Here we present evidence that syntaxin 5 performs an analogous function in ER to Golgi transport in mammalian cells. Transient expression of an hemagglutinin-tagged full-length clone of syntaxin 5 and a truncated mutant lacking the transmembrane domain inhibited the transport of vesicular stomatitis virus glycoprotein to the Golgi stack. Under these conditions, vesicular stomatitis virus glycoprotein accumulated in pre-Golgi intermediates, which were strongly enriched in syntaxin 5. Our results suggest that syntaxin 5 is the functional mammalian homologue of Sed5p and provides evidence for its role in regulating the potential targeting and/or fusion of carrier vesicles following export from the ER.

Sar1 promotes vesicle budding from the endoplasmic reticulum but not Golgi compartments

Two new members (Sar1a and Sar1b) of the SAR1 gene family have been identified in mammalian cells. Using immunoelectron microscopy, Sar1 was found to be restricted to the transitional region where the protein was enriched 20-40-fold in vesicular carriers mediating ER to Golgi traffic. Biochemical analysis revealed that Sar1 was essential for an early step in vesicle budding. A Sar1-specific antibody potently inhibited export of vesicular stomatitis virus glycoprotein (VSV-G) from the ER in vitro. Consistent with the role of guanine nucleotide exchange in Sar1 function, a trans-dominant mutant (Sar1a[T39N]) with a preferential affinity for GDP also strongly inhibited vesicle budding from the ER. In contrast, Sar1 was not found to be required for the transport of VSV-G between sequential Golgi compartments, suggesting that components active in formation of vesicular carriers mediating ER to Golgi traffic may differ, at least in part, from those involved in intra-Golgi transport. The requirement for novel components at different stages of the secretory pathway may reflect the recently recognized differences in protein transport between the Golgi stacks as opposed to the selective sorting and concentration of protein during export from the ER.

Dominant inhibitory mutants of ARF1 block endoplasmic reticulum to Golgi transport and trigger disassembly of the Golgi apparatus

Using three different trans dominant mutants of bovine ARF1 affecting GDP exchange or GTP hydrolysis we demonstrate the central role of ARF1 in controlling vesicular traffic from the endoplasmic reticulum (ER) to the Golgi apparatus and between successive Golgi compartments. Overexpression of ARF1(Q71L), a mutant likely to be restricted to the GTP-bound form, resulted in the accumulation of vesicular stomatitis virus glycoprotein in pre-Golgi intermediates, inhibited transport between successive Golgi compartments, and led to a striking association of beta-COP with pre-Golgi intermediates and the Golgi stack. In contrast, ARF1(T31N), a mutant which is likely to have a preferential affinity for GDP compared to the wild-type protein, inhibited export from the ER and triggered a brefeldin A-like phenotype, resulting in the redistribution of beta-COP from Golgi membranes to the cytosol and the collapse of the Golgi into the ER. This mutant, which may efficiently sequester an ARF-specific guanine nucleotide-exchange protein (ARF-GEF), suggests that ARF and ARF-GEF are essential for export from the ER. These results are discussed in the context of the GDP and GTP-bound forms of ARF in controlling both membrane structure and vesicular traffic through the early secretory pathway.

Expression and translocation of the chlamydial major outer membrane protein in Escherichia coli

The entire gene encoding the major outer membrane protein (MOMP) from Chlamydia psittaci strain GPIC has been cloned and expressed in Escherichia coli. A tightly regulated T7 promoter is used to control expression of the protein in Escherichia coli. Upon induction of expression, the precursor (pre-MOMP) is synthesized in the cell. This is followed by the appearance of a lower molecular weight protein that comigrates with mature MOMP from chlamydial elementary bodies by both one-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis. When E. coli cells expressing MOMP are converted to spheroplasts and subjected to protease treatment, MOMP is quantitatively degraded while cytoplasmic pre-MOMP is protected from degradation. Whole cells subjected to the same protease treatment show no degradation of MOMP. Furthermore, MOMP is not detected in surface-labeling experiments using several MOMP-specific antibodies. These data indicate that pre-MOMP is translocated to the periplasmic space and processed but is not surface exposed in E. coli. Expression of MOMP in this system causes a significant reduction in cell viability. In addition, coexpression in E. coli of MOMP or a MOMP-PhoA fusion with various chaperone proteins does not alter the level of MOMP translocation.

The yeast SLY gene products, suppressors of defects in the essential GTP-binding Ypt1 protein, may act in endoplasmic reticulum-to-Golgi transport

It has been shown previously that defects in the essential GTP-binding protein, Ypt1p, lead to a block in protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus in the yeast Saccharomyces cerevisiae. Here we report that four newly discovered suppressors of YPT1 deletion (SLY1-20, SLY2, SLY12, and SLY41) to a varying degree restore ER-to-Golgi transport defects in cells lacking Ypt1p. These suppressors also partially complement the sec21-1 and sec22-3 mutants which lead to a defect early in the secretory pathway. Sly1p-depleted cells, as well as a conditional lethal sly2 null mutant at nonpermissive temperatures, accumulate ER membranes and core-glycosylated invertase and carboxypeptidase Y. The sly2 null mutant under restrictive conditions (37 degrees C) can be rescued by the multicopy suppressor SLY12 and the single-copy suppressor SLY1-20, indicating that these three SLY genes functionally interact. Sly2p is shown to be an integral membrane protein.