Chlamydia trachomatis hijacks intra-Golgi COG complex-dependent vesicle trafficking pathway

Chlamydia spp. are obligate intracellular bacteria that replicate inside the host cell in a bacterial modified unique compartment called the inclusion. As other intracellular pathogens, chlamydiae exploit host membrane trafficking pathways to prevent lysosomal fusion and to acquire energy and nutrients essential for their survival and replication. The Conserved Oligomeric Golgi (COG) complex is a ubiquitously expressed membrane-associated protein complex that functions in a retrograde intra-Golgi trafficking through associations with coiled-coil tethers, SNAREs, Rabs and COPI proteins. Several COG complex-interacting proteins, including Rab1, Rab6, Rab14 and Syntaxin 6 are implicated in chlamydial development. In this study, we analysed the recruitment of the COG complex and GS15-positive COG complex-dependent vesicles to Chlamydia trachomatis inclusion and their participation in chlamydial growth. Immunofluorescent analysis revealed that both GFP-tagged and endogenous COG complex subunits associated with inclusions in a serovar-independent manner by 8 h post infection and were maintained throughout the entire developmental cycle. Golgi v-SNARE GS15 was associated with inclusions 24 h post infection, but was absent on the mid-cycle (8 h) inclusions, indicating that this Golgi SNARE is directed to inclusions after COG complex recruitment. Silencing of COG8 and GS15 by siRNA significantly decreased infectious yield of chlamydiae. Further, membranous structures likely derived from lysed bacteria were observed inside inclusions by electron microscopy in cells depleted of COG8 or GS15. Our results showed that C. trachomatis hijacks the COG complex to redirect the population of Golgi-derived retrograde vesicles to inclusions. These vesicles likely deliver nutrients that are required for bacterial development and replication.

Identification of a human orthologue of Sec34p as a component of the cis-Golgi vesicle tethering machinery

The roles of the components of the Sec34p protein complex in intracellular membrane trafficking, first identified in the yeast Saccharomyces cerevisiae, have yet to be characterized in higher eukaryotes. We cloned a human cDNA whose predicted amino acid sequence showed 41% similarity to yeast Sec34p with homology throughout the entire coding region. Affinity-purified antibodies raised against the human SEC34 protein (hSec34p) recognized a cellular protein of 94 kDa in both soluble and membrane fractions. Like yeast Sec34p, cytosolic hSec34p migrated with an apparent molecular mass of 300 kDa on a glycerol velocity gradient, suggesting that it is part of a protein complex. Immunofluorescence microscopy localized hSec34p to the Golgi compartment in cells of all species examined, where it co-localized well with the cis/medial Golgi marker membrin and partially co-localized with cis-Golgi network marker p115 and trans-Golgi marker TGN38. The co-localization with membrin was maintained at 15 degrees C and after microtubule depolymerization with nocodazole. During transport of the tsO45 vesicular stomatitis virus G protein through the Golgi, there was significant overlap with the hSec34p compartment. Green fluorescent protein-hSec34 expressed in HeLa cells was restricted to Golgi cisternae, and its membrane association was sensitive to brefeldin A treatment. Taken together, our findings indicate that hSec34p is part of a peripheral membrane protein complex localized on cis/medial Golgi cisternae where it may participate in tethering intra-Golgi transport vesicles.

Sec35p, a novel peripheral membrane protein, is required for ER to Golgi vesicle docking

SEC35 was identified in a novel screen for temperature-sensitive mutants in the secretory pathway of the yeast Saccharomyces cerevisiae (. Genetics. 142:393-406). At the restrictive temperature, the sec35-1 strain exhibits a transport block between the ER and the Golgi apparatus and accumulates numerous vesicles. SEC35 encodes a novel cytosolic protein of 32 kD, peripherally associated with membranes. The temperature-sensitive phenotype of sec35-1 is efficiently suppressed by YPT1, which encodes the rab-like GTPase required early in the secretory pathway, or by SLY1-20, which encodes a dominant form of the ER to Golgi target -SNARE-associated protein Sly1p. Weaker suppression is evident upon overexpression of genes encoding the vesicle-SNAREs SEC22, BET1, or YKT6. The cold-sensitive lethality that results from deleting SEC35 is suppressed by YPT1 or SLY1-20. These genetic relationships suggest that Sec35p acts upstream of, or in conjunction with, Ypt1p and Sly1p as was previously found for Uso1p. Using a cell-free assay that measures distinct steps in vesicle transport from the ER to the Golgi, we find Sec35p is required for a vesicle docking stage catalyzed by Uso1p. These genetic and biochemical results suggest Sec35p acts with Uso1p to dock ER-derived vesicles to the Golgi complex.

Characterization of a novel yeast SNARE protein implicated in Golgi retrograde traffic

The protein trafficking machinery of eukaryotic cells is employed for protein secretion and for the localization of resident proteins of the exocytic and endocytic pathways. Protein transit between organelles is mediated by transport vesicles that bear integral membrane proteins (v-SNAREs) which selectively interact with similar proteins on the target membrane (t-SNAREs), resulting in a docked vesicle. A novel Saccharomyces cerevisiae SNARE protein, which has been termed Vti1p, was identified by its sequence similarity to known SNAREs. Vti1p is a predominantly Golgi-localized 25-kDa type II integral membrane protein that is essential for yeast viability. Vti1p can bind Sec17p (yeast SNAP) and enter into a Sec18p (NSF)-sensitive complex with the cis-Golgi t-SNARE Sed5p. This Sed5p/Vti1p complex is distinct from the previously described Sed5p/Sec22p anterograde vesicle docking complex. Depletion of Vti1p in vivo causes a defect in the transport of the vacuolar protein carboxypeptidase Y through the Golgi. Temperature-sensitive mutants of Vti1p show a similar carboxypeptidase Y trafficking defect, but the secretion of invertase and gp400/hsp150 is not significantly affected. The temperature-sensitive vti1 growth defect can be rescued by the overexpression of the v-SNARE, Ykt6p, which physically interacts with Vti1p. We propose that Vti1p, along with Ykt6p and perhaps Sft1p, acts as a retrograde v-SNARE capable of interacting with the cis-Golgi t-SNARE Sed5p.

t-SNARE activation through transient interaction with a rab-like guanosine triphosphatase

Intracellular vesicle targeting involves the interaction of vesicle proteins, termed v-SNAREs, with target membrane proteins, termed t-SNAREs. Assembly of v-SNARE-t-SNARE targeting complexes is modulated by members of the Sec1-Sly1 protein family, and by small guanosine triphosphatases termed Rabs. The interactions of these proteins during assembly of the endoplasmic reticulum-to-Golgi targeting complex in Saccharomyces cerevisiae were studied. The data suggest that the Rab protein Ypt1p transiently interacts with the t-SNARE Sed5p and results in displacement of the negative regulator Sly1p, allowing subsequent formation of the v-SNARE-t-SNARE targeting complex.

Assembly of the ER to Golgi SNARE complex requires Uso1p

Uso1p, a Saccharomyces cerevisiae protein required for ER to Golgi transport, is homologous to the mammalian intra-Golgi transport factor p115. We have used genetic and biochemical approaches to examine the function of Uso1p. The temperature-sensitive phenotype of the uso1-1 mutant can be suppressed by overexpression of each of the known ER to Golgi v-SNAREs (Bet1p, Bos1p, Sec22p, and Ykt6p). Overexpression of two of them, BET1p and Sec22p, can also suppress the lethality of delta uso1, indicating that the SNAREs function downstream of Uso1p. In addition, overexpression of the small GTP-binding protein Ypt1p, or of a gain if function mutant (SLY1-20) of the t-SNARE associated protein Sly1p, also confers temperature resistance. Uso1p and Ypt1p appear to function in the same process because they have a similar set of genetic interactions with the v-SNARE genes, they exhibit a synthetic lethal interaction, and they are able to suppress temperature sensitive mutants of one another when overexpressed. Uso1p acts upstream of, or in conjunction with, Ypt1p because overexpression of Ypt1p allows a delta uso1 strain to grow, whereas overexpression of Uso1p does not suppress a delta ypt1 strain. Finally, biochemical analysis indicates that Uso1p, like Ypt1p, is required for assembly of the v-SNARE/t-SNARE complex. The implications of these findings, with respect to the mechanism of vesicle docking, are discussed.

Biochemical requirements for the targeting and fusion of ER-derived transport vesicles with purified yeast Golgi membranes

In order for secretion to progress, ER-derived transport vesicles must target to, and fuse with the cis-Golgi compartment. These processes have been reconstituted using highly enriched membrane fractions and partially purified soluble components. The functionally active yeast Golgi membranes that have been purified are highly enriched in the cis-Golgi marker enzymes alpha 1,6 mannosyltransferase and GDPase. Fusion of transport vesicles with these membranes requires both GTP and ATP hydrolysis, and depends on cytosolic and peripheral membrane proteins. At least two protein fractions from yeast cytosol are required for the reconstitution of ER-derived vesicle fusion. Soluble fractions prepared from temperature-sensitive mutants revealed requirements for the Ypt1p, Sec19p, Sly1p, Sec7p, and Uso1 proteins. A model for the sequential involvement of these components in the targeting and fusion reaction is proposed.

Identification of a novel secreted glycoprotein of the yeast Saccharomyces cerevisiae stimulated by heat shock

A new secreted yeast glycoprotein with an Mr of about 400 kDa (gp400) has been found. The glycoprotein is an O-mannosylated oligomer, whose synthesis and export into culture medium are stimulated by heat shock. Intracellular transport of gp400 is carried out by membrane vesicles distinct from the known constitutive secretory vesicles. Immunological analysis revealed gp400 only in Saccharomyces species.

[The nature of N-terminal signal sequence determines the type of intracellular distribution and effectiveness of export of human growth hormone in Saccharomyces cerevisiae]

Various N-terminal signal peptides (SP) were tested to investigate a human growth hormone (hGH) synthesis, processing and intracellular sorting in yeast. Maximal level of hGH was observed in the case when the mature hGH gene was placed under the control of PHO5 promoter. In this case about 90% of hGH was localized in the cytosol, but some portion was trustworthly detected in microsomes and periplasma in spite of the absence of SP. Addition of own or PHO5 SP resulted in lowering of the synthesis and a difficulty in the prehGH processing. In this case the immunoreactive products were localized mainly in periplasma and vacuoles and to a lesser degree in the cytosol. When hGH gene was placed under the control of the yeast MF alpha 1 promoter and alpha-factor preprosegment was used as SP more then a half (67%) of hGH processed correctly was exported in a medium, the rest was detected in vacuole (17%) and periplasma (8%).

[Cell wall permeability and export of proteins into the culture broth of Saccharomyces cerevisiae]

The upper limit of Saccharomyces cerevisiae cell wall permeability is such that only molecules with a mass not greater than 2-3 kDa can pass across the cell wall. However, proteins with a much higher molecular mass are exported into the cultural broth when the organism is cultivated in a liquid medium with peptone. Under certain conditions, exogenous proteins interact with the cytoplasmic membrane. These as well as some other findings imply that the cell wall may have microplots with an anomalous permeability, via which the regulated exchange of macromolecules is realized between the intracellular and outer media.