Molecular dissection of early stages of the eukaryotic secretory pathway

Intermediate compartment (IC): from pre-Golgi vacuoles to a semi-autonomous membrane system

Despite its discovery more than three decades ago and well-established role in protein sorting and trafficking in the early secretory pathway, the intermediate compartment (IC) has remained enigmatic. The prevailing view is that the IC evolved as a specialized organelle to mediate long-distance endoplasmic reticulum (ER)–Golgi communication in metazoan cells, but is lacking in other eukaryotes, such as plants and fungi. However, this distinction is difficult to reconcile with the high conservation of the core machineries that regulate early secretory trafficking from yeast to man. Also, it has remained unclear whether the pleiomorphic IC components—vacuoles, tubules and vesicles—represent transient transport carriers or building blocks of a permanent pre-Golgi organelle. Interestingly, recent studies have revealed that the IC maintains its compositional, structural and spatial properties throughout the cell cycle, supporting a model that combines the dynamic and stable aspects of the organelle. Moreover, the IC has been assigned novel functions, such as cell signaling, Golgi-independent trafficking and autophagy. The emerging permanent nature of the IC and its connections with the centrosome and the endocytic recycling system encourage reconsideration of its relationship with the Golgi ribbon, role in Golgi biogenesis and ubiquitous presence in eukaryotic cells.

Disruptions of the novel KIAA1202 gene are associated with X-linked mental retardation

The extensive heterogeneity underlying the genetic component of mental retardation (MR) is the main cause for our limited understanding of the aetiology of this highly prevalent condition. Hence we set out to identify genes involved in MR. We investigated the breakpoints of two balanced X;autosome translocations in two unrelated female patients with mild/moderate MR and found that the Xp11.2 breakpoints disrupt the novel human KIAA1202 (hKIAA1202) gene in both cases. We also identified a missense exchange in this gene, segregating with the Stocco dos Santos XLMR syndrome in a large four-generation pedigree but absent in >1,000 control X-chromosomes. Among other phenotypic characteristics, the affected males in this family present with severe MR, delayed or no speech, seizures and hyperactivity. Molecular studies of hKIAA1202 determined its genomic organisation, its expression throughout the brain and the regulation of expression of its mouse homologue during development. Transient expression of the wild-type KIAA1202 protein in HeLa cells showed partial colocalisation with the F-actin based cytoskeleton. On the basis of its domain structure, we argue that hKIAA1202 is a new member of the APX/Shroom protein family. Members of this family contain a PDZ and two ASD domains of unknown function and have been shown to localise at the cytoskeleton, and play a role in neurulation, cellular architecture, actin remodelling and ion channel function. Our results suggest that hKIAA1202 may be important in cognitive function and/or development.

Involvement of beta-COP in membrane traffic through the Golgi complex

Non-clathrin-coated vesicles mediate membrane traffic through the Golgi complex. The proteins that constitute the coats of these vesicles have similar molecular weights to the clathrin coat proteins. A major component of the coat of non-clathrin-coated vesicles, beta-COP, has significant homology with the clathrin coat protein beta-adaptin, indicating that the coats of the two different classes of vesicles may be structurally and functionally homologous.

Phosphorylation of components of the ER translocation site

In many eukaryotic cells, protein secretion is regulated by extracellular signalling molecules giving rise to increased intracellular Ca2+ and activation of kinases and phosphatases. To test whether components involved in the first step of secretion, the translocation of proteins across the endoplasmic reticulum (ER) membrane, are regulated by Ca2+-dependent phosphorylation and dephosphorylation, we have investigated the effect of Ca2+ on kinases associated with the rough ER. Using purified rough microsomes from dog pancreas we found that Ca2+-dependent isoforms of protein kinase C (PKC) are associated with the rough ER and phosphorylate essential components of the protein translocation machinery. Phosphorylation of microsomal proteins by PKCs increased protein translocation efficiency in vitro. We also found that proteins of the translocation machinery became phosphorylated in intact cells. This suggests a further level of regulation of protein translocation across the ER membrane.

Immunocytochemical localization of beta-COP to the ER-Golgi boundary and the TGN

Recent data strongly suggest that the coatomer (COP) complex is involved in membrane transport between the ER and Golgi complex. This vesicular coat has been implicated in ER to Golgi, in intra Golgi as well as in Golgi to ER traffic. In this study we present a detailed immunocytochemical analysis of the distribution of beta-COP in different tissue culture cells. Our results extend previous studies by showing, using electron microscopy, that beta-COP accumulates on vesicular profiles and buds in the intermediate compartment (IC) under conditions that block ER to Golgi transport (15 degrees C). Importantly, under these conditions beta-COP co-localizes on these structures with a passenger protein, the membrane glycoprotein of vesicular stomatis virus (ts-O45-G). Furthermore, quantitative immunofluorescence microscopy of cells with ts-045-G accumulated in the ER, IC and trans-Golgi network, shifted briefly to the permissive temperature, showed that beta-COP was associated with many of the putative transport intermediates containing the viral glycoprotein which is in transit between the ER/IC and the cis-Golgi. The simplest interpretation of these data is that COP-coated vesicles are involved in anterograde transport of ts-045-G from the IC to the Golgi complex. Since many putative COP vesicle lacked the G protein following release of the 15 degrees C block this pool could be involved in retrograde transport. We also show that beta-COP is present on the membranes of the trans-Golgi network. However, in contrast to the ER-Golgi boundary, we could find no convincing evidence that this pool of beta-COP is associated with buds or trans-Golgi network-derived transport vesicles.

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.

Disruptions in Golgi structure and membrane traffic in a conditional lethal mammalian cell mutant are corrected by epsilon-COP

The CHO cell temperature-sensitive mutant ldlF exhibits two defects in membrane traffic at the nonpermissive temperature (39.5 degrees C): rapid degradation of LDL receptors, possibly caused by endocytic missorting, and disruption of ER-through-Golgi transport. Here, we show that at 39.5 degrees C, the Golgi in ldlF cells dissociated into vesicles and tubules. This dissociation was inhibited by AlF4-, suggesting trimeric G proteins are involved in the dissociation mechanism. This resembled the effects of brefeldin A on wild-type cells. We isolated a hamster cDNA that specifically corrected the ts defects of ldlF cells, but not those of other similar ts mutants (ldlE, ldlG, ldlH, and End4). Its predicted protein sequence is conserved in humans, rice, Arabidopsis, and Caenorhabditis elegans, and is virtually identical to that of bovine epsilon-COP, a component of the coatomer complex implicated in membrane transport. This provides the first genetic evidence that coatomers in animal cells can play a role both in maintaining Golgi structure and in mediating ER-through-Golgi transport, and can influence normal endocytic recycling of LDL receptors. Thus, along with biochemical and yeast genetics methods, mammalian somatic cell mutants can provide powerful tools for the elucidation of the mechanisms underlying intracellular membrane traffic.

Characterization of a novel 63 kDa membrane protein. Implications for the organization of the ER-to-Golgi pathway

Owing to the lack of appropriate markers the structural organization of the ER-to-Golgi pathway and the dynamics of its membrane elements have been elusive. To elucidate this organization we have taken a monoclonal antibody (mAb) approach. A mAb against a novel 63 kDa membrane protein (p63) was produced that identifies a large tubular network of smooth membranes in the cytoplasm of primate cells. The distribution of p63 overlaps with the ER-Golgi intermediate compartment, defined by a previously described 53 kDa marker protein (here termed ERGIC-53), as visualized by confocal laser scanning immunofluorescence microscopy and immunoelectron microscopy. The p63 compartment mediates protein transport from the ER to Golgi apparatus, as indicated by partial colocalization of p63 and vesicular stomatitis virus G protein in Vero cells cultured at 15 degrees C. Low temperatures and brefeldin A had little effect on the cellular distribution of p63, suggesting that this novel marker is a stably anchored resident protein of these pre-Golgi membranes. p63 and ERGIC-53 were enriched to a similar degree by the same subcellular fractionation procedure. These findings demonstrate an unanticipated complexity of the ER-Golgi interface and suggest that the ER-Golgi intermediate compartment defined by ERGIC-53 may be part of a greater network of smooth membranes.

Morphological analysis of protein transport from the ER to Golgi membranes in digitonin-permeabilized cells: role of the P58 containing compartment

The glycoside digitonin was used to selectively permeabilize the plasma membrane exposing functionally and morphologically intact ER and Golgi compartments. Permeabilized cells efficiently transported vesicular stomatitis virus glycoprotein (VSV-G) through sealed, membrane-bound compartments in an ATP and cytosol dependent fashion. Transport was vectorial. VSV-G protein was first transported to punctate structures which colocalized with p58 (a putative marker for peripheral punctate pre-Golgi intermediates and the cis-Golgi network) before delivery to the medial Golgi compartments containing alpha-1,2-mannosidase II and processing of VSV-G to endoglycosidase H resistant forms. Exit from the ER was inhibited by an antibody recognizing the carboxyl-terminus of VSV-G. In contrast, VSV-G protein colocalized with p58 in the absence of Ca2+ or the presence of an antibody which inhibits the transport component NSF (SEC18). These studies demonstrate that digitonin permeabilized cells can be used to efficiently reconstitute the early secretory pathway in vitro, allowing a direct comparison of the morphological and biochemical events involved in vesicular tafficking, and identifying a key role for the p58 containing compartment in ER to Golgi transport.

Immunocytochemical analysis of the transfer of vesicular stomatitis virus G glycoprotein from the intermediate compartment to the Golgi complex

We performed an immunocytochemical analysis to study the transfer of a marker protein (G glycoprotein coded by vesicular stomatitis virus ts 045 strain) from the intermediate compartment to the Golgi stacks in infected Vero cells. The intermediate compartment seemed to consist of about 30-40 separate units of clustered small vesicles and short tubules. The units contained Rab2 protein and were spread throughout the cytoplasm, with a ratio of about 6:4 in the peripheral versus perinuclear site. Time-course experiments revealed a progressive transfer of G glycoprotein from the intermediate compartment to the Golgi stacks, while the tubulo-vesicular units did not appear to change their intracellular distribution. Moreover, the labeling density of peripheral and perinuclear units decreased in parallel during the transfer. These results support the notion that the intermediate compartment is a station in the secretory pathway, and that a vesicular transport connects this station to the Golgi complex.

Use of two-stage incubations to define sequential intermediates in endoplasmic reticulum to Golgi transport

Identification of the temporal requirement for components through the use of two-stage incubations is valuable in dissecting the overall transport reaction into steps relevant to vesicle fission and those related to vesicle fusion. In the context of semiintact mammalian cells in which a functional vesicle intermediate has not been detected, components playing a role in targeting are presently difficult to identify. However, the two-stage incubations are particularly powerful when either the donor or acceptor compartments can be manipulated independently, as is the case for intra-Golgi transport using enriched Golgi fractions or in the case of ER-to-Golgi transport in perforated yeast, in which a vesicle intermediate can be physically isolated.

Structural domains and molecular lifestyles of insulin and its precursors in the pancreatic beta cell

Insulin is both produced and degraded within the pancreatic Beta cell. Production involves the synthesis of the initial insulin precursor preproinsulin, which is converted to proinsulin shortly after (or during) translocation into the lumen of the rough endoplasmic reticulum. Proinsulin is then transported to the trans-cisternae of the Golgi complex where it is directed towards nascent secretory granules. Conversion of proinsulin to insulin and C-peptide arises within secretory granules, and is dependent upon their acidification. Granule contents are discharged by exocytosis in response to an appropriate stimulus. This represents the regulated secretory pathway to which more than 99% of proinsulin is directed in Beta cells of a healthy individual. An alternative route also exists in the Beta cell, the constitutive secretory pathway. It involves the rapid transfer of products from the Golgi complex to the plasma membrane for immediate release, with, it is supposed, little occasion for prohormone conversion. Even if delivered appropriately to secretory granules, not all insulin is released; some is degraded by fusion of granules with lysosomes (crinophagy). Each event in the molecular lifestyles of insulin and its precursors in the Beta cell will be seen to be governed by their own discrete functional domains. The identification and characterisation of these protein domains will help elucidate the steps responsible for delivery of proinsulin to secretory granules and conversion to insulin. Understanding the molecular mechanism of these steps may, in turn, help to explain defective insulin production in certain disease states including diabetes mellitus.

Proteins involved in vesicular transport and membrane fusion

In the past year, new information about proteins involved in vesicular transport has been plentiful. Particularly noteworthy are the complementary findings that Sec17p is required for vesicle consumption in endoplasmic reticulum-to-Golgi transport in yeast and that an analogous activity in mammalian cells, termed SNAP, is required for transport from the cis to the medial cisternae of the Golgi apparatus.

Mammalian glycosylation mutants as tools for the analysis and reconstitution of protein transport

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