Genes that control the fidelity of endoplasmic reticulum to Golgi transport identified as suppressors of vesicle budding mutations

The yeast p24 complex is required for the formation of COPI retrograde transport vesicles from the Golgi apparatus

The p24 family members are transmembrane proteins assembled into heteromeric complexes that continuously cycle between the ER and the Golgi apparatus. These cargo proteins were assumed to play a structural role in COPI budding because of their major presence in mammalian COPI vesicles. However, this putative function has not been proved conclusively so far. Furthermore, deletion of all eight yeast p24 family members does not produce severe transport phenotypes, suggesting that the p24 complex is not essential for COPI function. In this paper we provide direct evidence that the yeast p24 complex plays an active role in retrograde transport from Golgi to ER by facilitating the formation of COPI-coated vesicles. Therefore, our results demonstrate that p24 proteins are important for vesicle formation instead of simply being a passive traveler, supporting the model in which cargo together with a small GTPase of the ARF superfamily and coat subunits act as primer for vesicle formation.

Mammalian GPI-anchored proteins require p24 proteins for their efficient transport from the ER to the plasma membrane

The GPI (glycosylphosphatidylinositol) moiety is attached to newly synthesized proteins in the lumen of the ER (endoplasmic reticulum). The modified proteins are then directed to the PM (plasma membrane). Less well understood is how nascent mammalian GPI-anchored proteins are targeted from the ER to the PM. In the present study, we investigated mechanisms underlying membrane trafficking of the GPI-anchored proteins, focusing on the early secretory pathway. We first established a cell line that stably expresses inducible temperature-sensitive GPI-fused proteins as a reporter and examined roles of transport-vesicle constituents called p24 proteins in the traffic of the GPI-anchored proteins. We selectively suppressed one of the p24 proteins, namely p23, employing RNAi (RNA interference) techniques. The suppression resulted in pronounced delays of PM expression of the GPI-fused reporter proteins. Furthermore, maturation of DAF (decay-accelerating factor), one of the GPI-anchored proteins in mammals, was slowed by the suppression of p23, indicating delayed trafficking of DAF from the ER to the Golgi. Trafficking of non-GPI-linked cargo proteins was barely affected by p23 knockdown. This is the first to demonstrate direct evidence for the transport of mammalian GPI-anchored proteins being mediated by p24 proteins.

Identification of yeast proteins necessary for cell-surface function of a potassium channel

Inwardly rectifying potassium (Kir) channels form gates in the cell membrane that regulate the flow of K(+) ions into and out of the cell, thereby influencing the membrane potential and electrical signaling of many cell types, including neurons and cardiomyocytes. Kir-channel function depends on other cellular proteins that aid in the folding of channel subunits, assembly into tetrameric complexes, trafficking of quality-controlled channels to the plasma membrane, and regulation of channel activity at the cell surface. We used the yeast Saccharomyces cerevisiae as a model system to identify proteins necessary for the functional expression of a mammalian Kir channel at the cell surface. A screen of 376 yeast strains, each lacking one nonessential protein localized to the early secretory pathway, identified seven deletion strains in which functional expression of the Kir channel at the plasma membrane was impaired. Six deletions were of genes with known functions in trafficking and lipid biosynthesis (sur4Delta, csg2Delta, erv14Delta, emp24Delta, erv25Delta, and bst1Delta), and one deletion was of an uncharacterized gene (yil039wDelta). We provide genetic and functional evidence that Yil039wp, a conserved, phosphoesterase domain-containing protein, which we named "trafficking of Emp24p/Erv25p-dependent cargo disrupted 1" (Ted1p), acts together with Emp24p/Erv25p in cargo exit from the endoplasmic reticulum (ER). The seven yeast proteins identified in our screen likely impact Kir-channel functional expression at the level of vesicle budding from the ER and/or the local lipid environment at the plasma membrane.

Disparate effects of p24alpha and p24delta on secretory protein transport and processing

Background

The p24 family is thought to be somehow involved in endoplasmic reticulum (ER)-to-Golgi protein transport. A subset of the p24 proteins (p24α₃, -β₁, -γ₃ and -δ₂) is upregulated when Xenopus laevis intermediate pituitary melanotrope cells are physiologically activated to produce vast amounts of their major secretory cargo, the prohormone proopiomelanocortin (POMC).

Methodology/Principal Findings

Here we find that transgene expression of p24α₃ or p24δ₂ specifically in the Xenopus melanotrope cells in both cases causes an effective displacement of the endogenous p24 proteins, resulting in severely distorted p24 systems and disparate melanotrope cell phenotypes. Transgene expression of p24α₃ greatly reduces POMC transport and leads to accumulation of the prohormone in large, ER-localized electron-dense structures, whereas p24δ₂-transgenesis does not influence the overall ultrastructure of the cells nor POMC transport and cleavage, but affects the Golgi-based processes of POMC glycomaturation and sulfation.

Conclusions/Significance

Transgenic expression of two distinct p24 family members has disparate effects on secretory pathway functioning, illustrating the specificity and non-redundancy of our transgenic approach. We conclude that members of the p24 family furnish subcompartments of the secretory pathway with specific sets of machinery cargo to provide the proper microenvironments for efficient and correct secretory protein transport and processing.

Molecular mechanisms of COPII vesicle formation

The first step in protein secretion from eukaryotic cells is mediated by COPII vesicles, known for the cytoplasmic coat proteins that are the minimal machinery required to generate these small transport carriers. The five COPII coat components coordinate to create a vesicle by locally generating membrane curvature and populating the incipient bud with the appropriate cargo. This review describes the molecular details of how the COPII coat sculpts vesicles from the endoplasmic reticulum and highlights some unresolved questions regarding the regulation of this process in the complex environment of the eukaryotic cell.

Drosophila melanogaster p24 genes have developmental, tissue-specific, and sex-specific expression patterns and functions

Genes encoding members of the p24 family of intracellular trafficking proteins are present throughout animal and plant lineages. However, very little is known about p24 developmental, spatial, or sex-specific expression patterns or how localized expression affects function. We investigated these problems in Drosophila melanogaster, which contains nine genes encoding p24 proteins. One of these genes, logjam (loj), is expressed in the adult female nervous system and ovaries and is essential for oviposition. Nervous system-specific expression of loj, but not ovary-specific expression, rescues the behavioral defect of mutants. The Loj protein localizes to punctate structures in the cellular cytoplasm. These structures colocalize with a marker specific to the intermediate compartment and cis-Golgi, consistent with experimental evidence from other systems suggesting that p24 proteins function in intracellular transport between the endoplasmic reticulum and Golgi. Our findings reveal that Drosophila p24 transcripts are developmentally and tissue-specifically expressed. CG31787 is male-specifically expressed gene that is present during the larval, pupal, and adult stages. Female CG9053 mRNA is limited to the head, whereas males express this gene widely. Together, our studies provide experimental evidence indicating that some p24 genes have sex-specific expression patterns and tissue- and sex-limited functions.

Thematic review series: lipid posttranslational modifications. GPI anchoring of protein in yeast and mammalian cells, or: how we learned to stop worrying and love glycophospholipids

Glycosylphosphatidylinositol (GPI) anchoring of cell surface proteins is the most complex and metabolically expensive of the lipid posttranslational modifications described to date. The GPI anchor is synthesized via a membrane-bound multistep pathway in the endoplasmic reticulum (ER) requiring >20 gene products. The pathway is initiated on the cytoplasmic side of the ER and completed in the ER lumen, necessitating flipping of a glycolipid intermediate across the membrane. The completed GPI anchor is attached to proteins that have been translocated across the ER membrane and that display a GPI signal anchor sequence at the C terminus. GPI proteins transit the secretory pathway to the cell surface; in yeast, many become covalently attached to the cell wall. Genes encoding proteins involved in all but one of the predicted steps in the assembly of the GPI precursor glycolipid and its transfer to protein in mammals and yeast have now been identified. Most of these genes encode polytopic membrane proteins, some of which are organized in complexes. The steps in GPI assembly, and the enzymes that carry them out, are highly conserved. GPI biosynthesis is essential for viability in yeast and for embryonic development in mammals. In this review, we describe the biosynthesis of mammalian and yeast GPIs, their transfer to protein, and their subsequent processing.

Fatty acid remodeling of GPI-anchored proteins is required for their raft association

Whereas most of the cellular phosphatidylinositol (PI) contain unsaturated fatty chains and are excluded from rafts, GPI-anchored proteins (APs) unusually contain two saturated fatty chains in their PI moiety, and they are typically found within lipid rafts. However, the origin of the saturated chains and whether they are essential for raft association are unclear. Here, we report that GPI-APs, with two saturated fatty chains, are generated from those bearing an unsaturated chain by fatty acid remodeling that occurs most likely in the Golgi and requires post-GPI-attachment to proteins (PGAP)2 and PGAP3. The surface GPI-APs isolated from the PGAP2 and -3 double-mutant Chinese hamster ovary (CHO) cells had unsaturated chains, such as oleic, arachidonic, and docosatetraenoic acids in the sn-2 position, whereas those from wild-type CHO cells had exclusively stearic acid, a saturated chain, indicating that the sn-2 chain is exchanged to a saturated chain. We then assessed the association of GPI-APs with lipid rafts. Recovery of unremodeled GPI-APs from the double-mutant cells in the detergent-resistant membrane fraction was very low, indicating that GPI-APs become competent to be incorporated into lipid rafts by PGAP3- and PGAP2-mediated fatty acid remodeling. We also show that the remodeling requires the preceding PGAP1-mediated deacylation from inositol of GPI-APs in the endoplasmic reticulum.

A subset of p23 localized on secretory granules in pancreatic beta-cells

Proteins on the membrane of secretory granules (SGs) involved in their biogenesis and exocytosis are poorly characterized compared with those of synaptic vesicle in neurons. Thus the secretory granule membrane was prepared from a mouse pancreatic beta-cell line MIN6 by subcellular fractionation, and protein constituents were analyzed by microscale two-dimensional liquid chromatography coupled with electrospray ionization tandem mass spectrometry. Using this proteomics approach, one of the p24 family proteins, p23, was unexpectedly found in the granule fraction, although p24 proteins are generally regarded as functioning in the early secretory pathways between the endoplasmic reticulum and the Golgi apparatus. We further showed that p23 is expressed at high levels in endocrine cells. Furthermore, immunocytochemical analyses of pancreatic beta-cells at the light and electron microscopic levels demonstrated that a significant amount of p23 is localized on the insulin granule membrane, although it is most intensely concentrated at the cis-Golgi compartment as previously shown in non-endocrine cells. These findings suggest that a fraction of p23 enters post-Golgi compartments and may function in the biogenesis and/or quality control of SGs.

Evidence for a role of transmembrane protein p25 in localization of protein tyrosine phosphatase TC48 to the ER

T-cell protein tyrosine phosphatase gives rise to two splice isoforms: TC48, which is localized to the endoplasmic reticulum (ER) and TC45, a nuclear protein. The present study was undertaken to identify proteins that are involved in targeting TC48 to the ER. We identified two TC48-interacting proteins, p25 and p23, from a yeast two-hybrid screen. p23 and p25 are members of a family of putative cargo receptors that are important for vesicular trafficking between Golgi complex and ER. Both p23 and p25 associate with overexpressed TC48 in Cos-1 cells as determined by coimmunoprecipitation. A significant amount of TC48 colocalized initially with ERGIC and Golgi complex markers (in addition to ER and nuclear membrane localization) and was then retrieved to the ER. Coexpression with p25 enhanced ER localization of TC48, whereas coexpression with p23 resulted in its trapping in membranous structures. Coexpression of a p25 mutant lacking the ER-localization signal KKxx resulted in enhanced Golgi localization of TC48. Forty C-terminal amino acid residues of TC48 (position 376-415) were sufficient for interaction with p23 (but not with p25) and targeted green fluorescence protein (GFP) to the Golgi complex. Targeting of GFP to the ER required 66 C-terminal amino acid residues of TC48 (position 350-415), which showed interaction with p25 and p23. We suggest that TC48 translocates to the Golgi complex along the secretory pathway, whereas its ER localization is maintained by selective retrieval enabled by interactions with p25 and p23.

Inositol deacylation by Bst1p is required for the quality control of glycosylphosphatidylinositol-anchored proteins

Misfolded proteins are recognized in the endoplasmic reticulum (ER), transported back to the cytosol, and degraded by the proteasome. A number of proteins are processed and modified by a glycosylphosphatidylinositol (GPI) anchor in the ER, but the quality control mechanisms of GPI-anchored proteins remain unclear. Here, we report on the quality control mechanism of misfolded GPI-anchored proteins. We have constructed a mutant form of the beta-1,3-glucanosyltransferase Gas1p (Gas1*p) as a model misfolded GPI-anchored protein. Gas1*p was modified with a GPI anchor but retained in the ER and was degraded rapidly via the proteasome. Disruption of BST1, which encodes GPI inositol deacylase, caused a delay in the degradation of Gas1*p. This delay was because of an effect on the deacylation activity of Bst1p. Disruption of genes involved in GPI-anchored protein concentration and N-glycan processing caused different effects on the degradation of Gas1*p and a soluble misfolded version of carboxypeptidase Y. Furthermore, Gas1*p associated with both Bst1p and BiP/Kar2p, a molecular chaperone, in vivo. Our data suggest that GPI inositol deacylation plays important roles in the quality control and ER-associated degradation of GPI-anchored proteins.

Acid trehalase in yeasts and filamentous fungi: Localization, regulation and physiological function

Yeasts and filamentous fungi are endowed with two different trehalose-hydrolysing activities, termed acid and neutral trehalases according to their optimal pH for enzymatic activity. A wealth of information already exists on fungal neutral trehalases, while data on localization, regulation and function of fungal acid trehalases have remained elusive. The gene encoding the latter enzyme has now been isolated from two yeast species and two filamentous fungi, and sequences encoding putative acid trehalase can be retrieved from available public sequences. Despite weak similarities between amino acids sequences, this type of trehalase potentially harbours either a transmembrane segment or a signal peptide at the N-terminal sequence, as deduced from domain prediction algorithms. This feature, together with the demonstration that acid trehalase from yeasts and filamentous fungi is localized at the cell surface, is consistent with its main role in the utilisation of exogenous trehalose as a carbon source. The growth on this disaccharide is in fact pretty effective in most fungi except in Saccharomyces cerevisiae. This yeast species actually exhibits a "Kluyver effect" on trehalose. Moreover, an oscillatory behaviour reminiscent of what is observed in aerobic glucose-limited continuous cultures at low dilution rate is also observed in batch growth on trehalose. Finally, the S. cerevisiae acid trehalase may also participate in the catabolism of endogenous trehalose by a mechanism that likely requires the export of the disaccharide, its extracellular hydrolysis, and the subsequent uptake of the glucose released. Based on these recent findings, we suggest to rename "acid" and "neutral" trehalases as "extracellular" and "cytosolic" trehalases, which is more adequate to describe their localization and function in the fungal cell.

Bi-directional protein transport between the ER and Golgi

The endoplasmic reticulum (ER) and the Golgi comprise the first two steps in protein secretion. Vesicular carriers mediate a continuous flux of proteins and lipids between these compartments, reflecting the transport of newly synthesized proteins out of the ER and the retrieval of escaped ER residents and vesicle machinery. Anterograde and retrograde transport is mediated by distinct sets of cytosolic coat proteins, the COPII and COPI coats, respectively, which act on the membrane to capture cargo proteins into nascent vesicles. We review the mechanisms that govern coat recruitment to the membrane, cargo capture into a transport vesicle, and accurate delivery to the target organelle.

Quantitative mass spectrometry reveals a role for the GTPase Rho1p in actin organization on the peroxisome membrane

We have combined classical subcellular fractionation with large-scale quantitative mass spectrometry to identify proteins that enrich specifically with peroxisomes of Saccharomyces cerevisiae. In two complementary experiments, isotope-coded affinity tags and tandem mass spectrometry were used to quantify the relative enrichment of proteins during the purification of peroxisomes. Mathematical modeling of the data from 306 quantified proteins led to a prioritized list of 70 candidates whose enrichment scores indicated a high likelihood of them being peroxisomal. Among these proteins, eight novel peroxisome-associated proteins were identified. The top novel peroxisomal candidate was the small GTPase Rho1p. Although Rho1p has been shown to be tethered to membranes of the secretory pathway, we show that it is specifically recruited to peroxisomes upon their induction in a process dependent on its interaction with the peroxisome membrane protein Pex25p. Rho1p regulates the assembly state of actin on the peroxisome membrane, thereby controlling peroxisome membrane dynamics and biogenesis.

GPI7 Involved in Glycosylphosphatidylinositol Biosynthesis Is Essential for Yeast Cell Separation

GPI7 is involved in adding ethanolaminephosphate to the second mannose in the biosynthesis of glycosylphosphatidylinositol (GPI) in Saccharomyces cerevisiae. We isolated gpi7 mutants, which have defects in cell separation and a daughter cell-specific growth defect at the non-permissive temperature. WSC1, RHO2, ROM2, GFA1, and CDC5 genes were isolated as multicopy suppressors of gpi7-2 mutant. Multicopy suppressors could suppress the growth defect of gpi7 mutants but not the cell separation defect. Loss of function mutations of genes involved in the Cbk1p-Ace2p pathway, which activates the expression of daughter-specific genes for cell separation after cytokinesis, bypassed the temperature-sensitive growth defect of gpi7 mutants. Furthermore, deletion of EGT2, one of the genes controlled by Ace2p and encoding a GPI-anchored protein required for cell separation, ameliorated the temperature sensitivity of the gpi7 mutant. In this mutant, Egt2p was displaced from the septal region to the cell cortex, indicating that GPI7 plays an important role in cell separation via the GPI-based modification of daughter-specific proteins in S. cerevisiae.

Sorting signals can direct receptor-mediated export of soluble proteins into COPII vesicles

Soluble secretory proteins are first translocated across endoplasmic reticulum (ER) membranes and folded in a specialized ER luminal environment. Fully folded and assembled secretory cargo are then segregated from ER-resident proteins into COPII-derived vesicles or tubular elements for anterograde transport. Mechanisms of bulk-flow, ER-retention and receptor-mediated export have been suggested to operate during this transport step, although these mechanisms are poorly understood. In yeast, there is evidence to suggest that Erv29p functions as a transmembrane receptor for the export of certain soluble cargo proteins including glycopro-alpha-factor (gpalphaf), the precursor of alpha-factor mating pheromone. Here we identify a hydrophobic signal within the pro-region of gpalphaf that is necessary for efficient packaging into COPII vesicles and for binding to Erv29p. When fused to Kar2p, an ER-resident protein, the pro-region sorting signal was sufficient to direct Erv29p-dependent export of the fusion protein into COPII vesicles. These findings indicate that specific motifs within soluble secretory proteins function in receptor-mediated export from the ER. Moreover, positive sorting signals seem to predominate over potential ER-retention mechanisms that may operate in localizing ER-resident proteins such as Kar2p.

Inositol Deacylation of Glycosylphosphatidylinositol-anchored Proteins Is Mediated by Mammalian PGAP1 and Yeast Bst1p

The inositol moiety of mammalian glycosylphosphatidylinositol (GPI) is acylated at an early step in GPI biosynthesis. The inositol acylation is essential for the generation of mature GPI capable of attachment to proteins. However, the acyl group is usually absent from GPI-anchored proteins (GPI-APs) on the cell surface due to inositol deacylation that occurs in the endoplasmic reticulum (ER) soon after GPI-anchor attachment. Mammalian GPI inositol-deacylase has not been cloned, and the biological significance of the deacylation has been unclear. Here we report a GPI inositol-deacylase-deficient Chinese hamster ovary cell line established by taking advantage of resistance to phosphatidylinositol-specific phospholipase C and the gene responsible, which was termed PGAP1 for Post GPI Attachment to Proteins 1. PGAP1 encoded an ER-associated, 922-amino acid membrane protein bearing a lipase consensus motif. Substitution of a conserved putative catalytic serine with alanine resulted in a complete loss of function, indicating that PGAP1 is the GPI inositol-deacylase. The mutant cells showed a clear delay in the maturation of GPI-APs in the Golgi and accumulation of GPI-APs in the ER. Thus, the GPI inositol deacylation is important for efficient transport of GPI-APs from the ER to the Golgi.

Active and specific recruitment of a soluble cargo protein for endoplasmic reticulum exit in the absence of functional COPII component Sec24p

Exit of proteins from the yeast endoplasmic reticulum (ER) is thought to occur in vesicles coated by four proteins, Sec13p, Sec31p, Sec23p and Sec24p, which assemble at ER exit sites to form the COPII coat. Sec13p may serve a structural function, whereas Sec24p has been suggested to operate in selection of cargo proteins into COPII vesicles. We showed recently that the soluble glycoprotein Hsp150 exited the ER in the absence of Sec13p function. Here we show that its ER exit did not require functional Sec24p. Hsp150 was secreted to the medium in a sec24-1 mutant at restrictive temperature 37 degrees C, while cell wall invertase and vacuolar carboxypeptidase Y remained in the ER. The determinant guiding Hsp150 to this transport route was mapped to the C-terminal domain of 114 amino acids by deletion analysis, and by an HRP fusion protein-based EM technology adapted here for yeast. This domain actively mediated ER exit of Sec24p-dependent invertase in the absence of Sec24p function. However, the domain was entirely dispensable for ER exit when Sec24p was functional. The Sec24p homolog Sfb2p was shown not to compensate for nonfunctional Sec24p in ER exit of Hsp150. Our data show that a soluble cargo protein, Hsp150, is selected actively and specifically to budding sites lacking normal Sec24p by a signature residing in its C-terminal domain.

Microarray analysis reveals that TP53- and ATM-mutant B-CLLs share a defect in activating proapoptotic responses after DNA damage but are distinguished by major differences in activating prosurvival responses

The ATM/p53-dependent DNA damage response pathway plays an important role in the progression of lymphoid tumors. Inactivation of the ATM or TP53 gene is frequent in B-cell lymphocytic leukemia (B-CLL) and leads to aggressive disease. Although the ATM and p53 pathways overlap, they are not congruent, and it is unclear how the mechanism of tumor progression differs between ATM- and p53-deficient tumors. Using microarray analysis of ATM-mutant, TP53-mutant, and ATM/TP53 wild-type B-CLLs, we show that after exposure to DNA damage transcriptional responses are entirely dependent on ATM function. The p53 proapoptotic responses comprise only a part of ATM-regulated transcription; additionally, ATM regulates prosurvival responses independently of p53. Consequently, the greater severity of the TP53-mutant B-CLLs compared with ATM-mutant B-CLLs is consistent with the additive effect of defective apoptotic and elevated survival responses after DNA damage in these tumors. We also show that transcription expression profiles of ATM-deficient, TP53-deficient, and wild-type B-CLLs are indistinguishable before irradiation. Therefore, damage-induced transcriptional fingerprinting can be used to stratify tumors according to their biologic differences and simultaneously identify potential targets for treating refractory tumors.

A cell-specific transgenic approach in Xenopus reveals the importance of a functional p24 system for a secretory cell

The p24alpha, -beta, -gamma, and -delta proteins are major multimeric constituents of cycling endoplasmic reticulum-Golgi transport vesicles and are thought to be involved in protein transport through the early secretory pathway. In this study, we targeted transgene overexpression of p24delta2 specifically to the Xenopus intermediate pituitary melanotrope cell that is involved in background adaptation of the animal and produces high levels of its major secretory cargo proopiomelanocortin (POMC). The transgene product effectively displaced the endogenous p24 proteins, resulting in a melanotrope cell p24 system that consisted predominantly of the transgene p24delta2 protein. Despite the severely distorted p24 machinery, the subcellular structures as well as the level of POMC synthesis were normal in these cells. However, the number and pigment content of skin melanophores were reduced, impairing the ability of the transgenic animal to fully adapt to a black background. This physiological effect was likely caused by the affected profile of POMC-derived peptides observed in the transgenic melanotrope cells. Together, our results suggest that in the early secretory pathway an intact p24 system is essential for efficient secretory cargo transport or for supplying cargo carriers with the correct protein machinery to allow proper secretory protein processing.

The trans-membrane protein p25 forms highly specialized domains that regulate membrane composition and dynamics

Trans-membrane proteins of the p24 family are abundant, oligomeric proteins predominantly found in cis-Golgi membranes. They are not easily studied in vivo and their functions are controversial. We found that p25 can be targeted to the plasma membrane after inactivation of its canonical KKXX motif (KK to SS, p25SS), and that p25SS causes the co-transport of other p24 proteins beyond the Golgi complex, indicating that wild-type p25 plays a crucial role in retaining p24 proteins in cis-Golgi membranes. We then made use of these observations to study the intrinsic properties of these proteins, when present in a different membrane context. At the cell surface, the p25SS mutant segregates away from both the transferrin receptor and markers of lipid rafts, which are enriched in cholesterol and glycosphingolipids. This suggests that p25SS localizes to, or contributes to form, specialized membrane domains, presumably corresponding to oligomers of p25SS and other p24 proteins. Once at the cell surface, p25SS is endocytosed, together with other p24 proteins, and eventually accumulates in late endosomes, where it remains confined to well-defined membrane regions visible by electron microscopy. We find that this p25SS accumulation causes a concomitant accumulation of cholesterol in late endosomes, and an inhibition of their motility – two processes that are functionally linked. Yet, the p25SS-rich regions themselves seem to exclude not only Lamp1 but also accumulated cholesterol. One may envision that p25SS accumulation, by excluding cholesterol from oligomers, eventually overloads neighboring late endosomal membranes with cholesterol beyond their capacity (see Discussion). In any case, our data show that p25 and presumably other p24 proteins are endowed with the intrinsic capacity to form highly specialized domains that control membrane composition and dynamics. We propose that p25 and other p24 proteins control the fidelity of membrane transport by maintaining cholesterol-poor membranes in the Golgi complex.

The early secretory pathway contributes to autophagy in yeast

Autophagy is a starvation response in eukaryotes by which the cell delivers cytoplasmic components to the vacuole for degradation, and is mediated by a double membrane structure called the autophagosome. We have previously proposed that the specific combination of COPII like components, including Sec24p, is required for autophagy (Ishihara, N. et al. (2001) Mol. Biol. Cell, 12: 3690-3702). The autophagic defect in sec24 deleted mutant cells was, however, suppressed upon the recovery of its secretory flow by the overexpression of its homologue, Sfb2p. We have also reported that the autophagic defect is not observed in sec13 and sec31 mutants, a phenomenon that can be explained by the fact that starvation stress suppresses the secretory defect of these mutants. These observations indicate that the active flow in the early secretory pathway plays an important role in autophagy; that is, autophagy proceeds in the presence, but not in the absence of the early secretory flow. Both autophagy and its closely related cytoplasm to vacuole-targeting (Cvt) pathway occur through a pre-autophagosomal structure (PAS), and since the PAS and the functional Cvt pathway exist in all sec mutants, the early secretory pathway must be involved specifically in autophagy, subsequent to PAS formation.

Lipid-dependent subcellular relocalization of the acyl chain desaturase in yeast

The degree of acyl chain desaturation of membrane lipids is a critical determinant of membrane fluidity. Temperature-sensitive mutants of the single essential acyl chain desaturase, Ole1p, of yeast have previously been isolated in screens for mitochondrial inheritance mutants (Stewart, L.C. and Yaffe, M.P. (1991). J. Cell Biol. 115, 1249-1257). We now report that the mutant desaturase relocalizes from its uniform ER distribution to a more punctuate localization at the cell periphery upon inactivation of the enzyme. This relocalization takes place within minutes at nonpermissive conditions, a time scale at which mitochondrial morphology and inheritance is not yet affected. Relocalization of the desaturase is fully reversible and does not affect the steady state localization of other ER resident proteins or the kinetic and fidelity of the secretory pathway, indicating a high degree of selectivity for the desaturase. Relocalization of the desaturase is energy independent but is lipid dependent because it is rescued by supplementation with unsaturated fatty acids. Relocalization of the desaturase is also observed in cells treated with inhibitors of the enzyme, indicating that it is independent of temperature-induced alterations of the enzyme. In the absence of desaturase function, lipid synthesis continues, resulting in the generation of lipids with saturated acyl chains. A model is discussed in which the accumulation of saturated lipids in a microdomain around the desaturase could induce the observed segregation and relocalization of the enzyme.

Selective protein exit from yeast endoplasmic reticulum in absence of functional COPII coat component Sec13p

Sec13p has been thought to be an essential component of the COPII coat, required for exit of proteins from the yeast endoplasmic reticulum (ER). We show herein that normal function of Sec13p was not required for ER exit of the Hsp150 glycoprotein. Hsp150 was secreted to the medium under restrictive conditions in a sec13-1 mutant. The COPII components Sec23p and Sec31p and the GTP/GDP exchange factor Sec12p were required in functional form for secretion of Hsp150. Hsp150 leaves the ER in the absence of retrograde COPI traffic, and the responsible determinant is a peptide repeated 11 times in the middle of the Hsp150 sequence. Herein, we localized the sorting determinant for Sec13p-independent ER exit to the C-terminal domain. Sec13p-dependent invertase left the ER in the absence of normal Sec13p function, when fused to the C-terminal domain of Hsp150, demonstrating that this domain contained an active mediator of Sec13p-independent secretion. Thus, Hsp150 harbors two different signatures that regulate its ER exit. Our data show that transport vesicles lacking functional Sec13p can carry out ER-to-Golgi transport, but select only specific cargo protein(s) for ER exit.

Oligomeric state and stoichiometry of p24 proteins in the early secretory pathway

The p24 proteins belong to a highly conserved family of membrane proteins that cycle in the early secretory pathway. They bind to the coat proteins of COPI and COPII vesicles, and are proposed to be involved in vesicle biogenesis, cargo uptake, and quality control, but their precise function is still under debate. Most p24 proteins form hetero-oligomers, essential for their correct localization and stability. Functional insights regarding the mechanisms of their steady state localization and the role of interaction with coat proteins has been hampered by a lack of data on their concentration and state of oligomerization within the endoplasmic reticulum, the intermediate compartment, and Golgi complex. We have determined for all mammalian p24 family members the size of the oligomers formed and their stoichiometric relation in each of these individual organelles. In contrast to earlier reports, we show that individual members exist as dimers and monomers and that the ratio between these two forms depends on both the organelle investigated and the p24 protein. We find unequal quantities, with p23 and p27 building up concentration gradients, ruling out a simple 1:1 stoichiometry. In addition, we show differential cycling of individual p24 members. These data point to a complex and dynamic system of altering dimerizations of the family members.

Cargo selection in vesicular transport: the making and breaking of a coat

Intracellular traffic is mediated by vesicular/tubular carriers. The carriers are formed by the activity of cytosolic coat proteins that are recruited to their target membranes and deform these membranes into buds and vesicles. Specific interactions between recruited coat subunits and short peptide sequences (transport motifs) on cargo proteins direct the incorporation of cargo into budded vesicles. Here, we focus on cargo selection reactions mediated by COPII and AP-2/clathrin vesicle coat complexes to explore common mechanisms by which coat assembly support localized and selective cargo sorting. Recent findings suggest that multiple, low-affinity interactions are employed in a cooperative manner to support coat assembly and enable cargo recognition. Thus low-binding affinities between coat subunits and transport motifs are transiently transformed into high-avidity, multivalent and selective interactions at vesicle bud sites. The temporal and regulated nature of the interactions provide the key to cargo selection.

Erv14p directs a transmembrane secretory protein into COPII-coated transport vesicles

Erv14p is a conserved integral membrane protein that traffics in COPII-coated vesicles and localizes to the early secretory pathway in yeast. Deletion of ERV14 causes a defect in polarized growth because Axl2p, a transmembrane secretory protein, accumulates in the endoplasmic reticulum and is not delivered to its site of function on the cell surface. Herein, we show that Erv14p is required for selection of Axl2p into COPII vesicles and for efficient formation of these vesicles. Erv14p binds to subunits of the COPII coat and binding depends on conserved residues in a cytoplasmically exposed loop domain of Erv14p. When mutations are introduced into this loop, an Erv14p-Axl2p complex accumulates in the endoplasmic reticulum, suggesting that Erv14p links Axl2p to the COPII coat. Based on these results and further genetic experiments, we propose Erv14p coordinates COPII vesicle formation with incorporation of specific secretory cargo.

Cell-type-specific and selectively induced expression of members of the p24 family of putative cargo receptors

Members of the p24 family of type I transmembrane proteins are highly abundant in transport vesicles and are thought to be involved in selective protein transport between the endoplasmic reticulum and the Golgi complex. The p24 proteins have been grouped into four subfamilies (α, β,γ, and δ) and appear to assemble into tetrameric complexes that contain only one representative from each subfamily. Here we molecularly dissected the p24 family in a single cell type, namely in the intermediate pituitary melanotrope cells of the amphibian Xenopus laevis. The biosynthetic activity of these cells for production of their major cargo protein proopiomelanocortin (POMC) can be physiologically manipulated via the process of background adaptation (∼30-fold induction, with highly active cells in black toads and virtually inactive cells in white animals). Extensive cDNA library screening revealed the identity of six p24 proteins expressed in the Xenopus melanotrope cells, namely one member of the p24α(α3), one of the p24β (β1), two of the p24γ (γ2, γ3) and two of the p24δ (δ1, δ2) subfamily. Two other Xenopus p24 proteins, Xp24α2 and-γ1, were not expressed in the melanotrope cells, pointing to cell-type specific p24 expression. Of the six melanotrope p24 proteins, the expression of four (Xp24α3, -β1,-γ3 and -δ2) was 20- to 30-fold induced in active versus inactive melanotropes, whereas that of the other two members(Xp24γ2 and -δ1) had not or only slightly increased. The four proteins were induced only in the intermediate melanotrope cells and not in the anterior pituitary cells, and displayed similar overall tissue distributions that differed from those of Xp24γ1,-γ2 and -δ1. Together, our results reveal that p24 expression can be cell-type specific and selectively induced, and suggest that in Xenopus melanotrope cells anα 3/β1/γ3/δ2p24 complex is involved in POMC transport through the early stages of the secretory pathway.

Localization of p24 putative cargo receptors in the early secretory pathway depends on the biosynthetic activity of the cell

Members of the p24 family of putative cargo receptors (subdivided into p24-alpha, -beta, -gamma and -delta) are localized in the intermediate-and cis-Golgi compartments of the early secretory pathway, and are thought to play an important role in protein transport. In the present study, we wondered what effect increased biosynthetic cell activity with resulting high levels of protein transport would have on the subcellular localization of p24. We examined p24 localization in Xenopus intermediate pituitary melanotrope cells, which in black- and white-adapted animals are biosynthetically highly active and virtually inactive respectively. In addition, p24 localization was studied in Xenopus anterior pituitary cells whose activity is not changed during background adaptation. Using organelle fractionation, we found that in the inactive melanotropes and moderately active anterior pituitary cells of white-adapted animals, the p24-alpha, -beta, -gamma and -delta proteins are all located in the Golgi compartment. In the highly active melanotropes, but not in the anterior cells of black-adapted animals, the steady-state distribution of all four p24 members changed towards the intermediate compartment and subdomains of the endoplasmic reticulum (ER), most probably the ER exit sites. In the active melanotropes, the major cargo protein pro-opiomelanocortin was mostly localized to ER subdomains and partially co-localized with the p24 proteins. Furthermore, in the active cells, in vitro blocking of protein biosynthesis by cycloheximide or dispersion of the Golgi complex by brefeldin A led to a redistribution of the p24 proteins, indicating their involvement in ER-to-Golgi protein transport and extensive cycling in the early secretory pathway. We conclude that the subcellular localization of p24 proteins is dynamic and depends on the biosynthetic activity of the cell.

Distinct roles for the cytoplasmic tail sequences of Emp24p and Erv25p in transport between the endoplasmic reticulum and Golgi complex

Heteromeric complexes of p24 proteins cycle between early compartments of the secretory pathway and are required for efficient protein sorting. Here we investigated the role of cytoplasmically exposed tail sequences on two p24 proteins, Emp24p and Erv25p, in directing their movement and subcellular location in yeast. Studies on a series of deletion and chimeric Emp24p-Erv25p proteins indicated that the tail sequences impart distinct functional properties that were partially redundant but not entirely interchangeable. Export of an Emp24p-Erv25p complex from the endoplasmic reticulum (ER) did not depend on two other associated p24 proteins, Erp1 and Erp2p. To examine interactions between the Emp24p and Erv25p tail sequences with the COPI and COPII coat proteins, binding experiments with immobilized tail peptides and coat proteins were performed. The Emp24p and Erv25p tail sequences bound the Sec13p/Sec31p subunit of the COPII coat (K(d) approximately 100 microm), and binding depended on a pair of aromatic residues found in both tail sequences. COPI subunits also bound to these Emp24p and Erv25p peptides; however, the Erv25p tail sequence, which contains a dilysine motif, bound COPI more efficiently. These results suggest that both the Emp24p and Erv25p cytoplasmic sequences contain a di-aromatic motif that binds subunits of the COPII coat and promotes export from the ER. The Erv25p tail sequence binds COPI and is responsible for returning this complex to the ER.

Distinct retrieval and retention mechanisms are required for the quality control of endoplasmic reticulum protein folding

Proteins destined for the secretory pathway must first fold and assemble in the lumen of endoplasmic reticulum (ER). The pathway maintains a quality control mechanism to assure that aberrantly processed proteins are not delivered to their sites of function. As part of this mechanism, misfolded proteins are returned to the cytosol via the ER protein translocation pore where they are ubiquitinated and degraded by the 26S proteasome. Previously, little was known regarding the recognition and targeting of proteins before degradation. By tracking the fate of several mutant proteins subject to quality control, we demonstrate the existence of two distinct sorting mechanisms. In the ER, substrates are either sorted for retention in the ER or are transported to the Golgi apparatus via COPII-coated vesicles. Proteins transported to the Golgi are retrieved to the ER via the retrograde transport system. Ultimately, both retained and retrieved proteins converge at a common machinery at the ER for degradation. Furthermore, we report the identification of a gene playing a novel role specific to the retrieval pathway. The gene, BST1, is required for the transport of misfolded proteins to the Golgi, although dispensable for the transport of many normal cargo proteins.

A flavoprotein oxidase defines a new endoplasmic reticulum pathway for biosynthetic disulphide bond formation

Ero1 and Pdi1 are essential elements of the pathway for the formation of disulphide bonds within the endoplasmic reticulum (ER). By screening for alternative oxidation pathways in Saccharomyces cerevisiae, we identified ERV2 as a gene that when overexpressed can restore viability and disulphide bond formation to an ero1-1 mutant strain. ERV2 encodes a luminal ER protein of relative molecular mass 22,000. Purified recombinant Erv2p is a flavoenzyme that can catalyse O2-dependent formation of disulphide bonds. Erv2p transfers oxidizing equivalents to Pdi1p by a dithiol-disulphide exchange reaction, indicating that the Erv2p-dependent pathway for disulphide bond formation closely parallels that of the previously identified Ero1p-dependent pathway.

Structure of the Sec23p/24p and Sec13p/31p complexes of COPII

COPII-coated vesicles carry proteins from the endoplasmic reticulum to the Golgi complex. This vesicular transport can be reconstituted by using three cytosolic components containing five proteins: the small GTPase Sar1p, the Sec23p/24p complex, and the Sec13p/Sec31p complex. We have used a combination of biochemistry and electron microscopy to investigate the molecular organization and structure of Sec23p/24p and Sec13p/31p complexes. The three-dimensional reconstruction of Sec23p/24p reveals that it has a bone-shaped structure, (17 nm in length), composed of two similar globular domains, one corresponding to Sec23p and the other to Sec24p. Sec13p/31p is a heterotetramer composed of two copies of Sec13p and two copies of Sec31p. It has an elongated shape, is 28-30 nm in length, and contains five consecutive globular domains linked by relatively flexible joints. Putting together the architecture of these Sec complexes with the interactions between their subunits and the appearance of the coat in COPII-coated vesicles, we present a model for COPII coat organization.

A genomic study of the bipolar bud site selection pattern in Saccharomyces cerevisiae

A genome-wide screen of 4168 homozygous diploid yeast deletion strains has been performed to identify nonessential genes that participate in the bipolar budding pattern. By examining bud scar patterns representing the sites of previous cell divisions, 127 mutants representing three different phenotypes were found: unipolar, axial-like, and random. From this screen, 11 functional classes of known genes were identified, including those involved in actin-cytoskeleton organization, general bud site selection, cell polarity, vesicular transport, cell wall synthesis, protein modification, transcription, nuclear function, translation, and other functions. Four characterized genes that were not known previously to participate in bud site selection were also found to be important for the haploid axial budding pattern. In addition to known genes, we found 22 novel genes (20 are designated BUD13-BUD32) important for bud site selection. Deletion of one resulted in unipolar budding exclusively from the proximal pole, suggesting that this gene plays an important role in diploid distal budding. Mutations in 20 other novel BUD genes produced a random budding phenotype and one produced an axial-like budding defect. Several of the novel Bud proteins were fused to green fluorescence protein; two proteins were found to localize to sites of polarized cell growth (i.e., the bud tip in small budded cells and the neck in cells undergoing cytokinesis), similar to that postulated for the bipolar signals and proteins that target cell division site tags to their proper location in the cell. Four others localized to the nucleus, suggesting that they play a role in gene expression. The bipolar distal marker Bud8 was localized in a number of mutants; many showed an altered Bud8-green fluorescence protein localization pattern. Through the genome-wide identification and analysis of different mutants involved in bipolar bud site selection, an integrated pathway for this process is presented in which proximal and distal bud site selection tags are synthesized and localized at their appropriate poles, thereby directing growth at those sites. Genome-wide screens of defined collections of mutants hold significant promise for dissecting many biological processes in yeast.

Deletion of yeast p24 genes activates the unfolded protein response

Yeast cells lacking a functional p24 complex accumulate a subset of secretory proteins in the endoplasmic reticulum (ER) and increase the extracellular secretion of HDEL-containing ER residents such as Kar2p/BiP. We report that a loss of p24 function causes activation of the unfolded protein response (UPR) and leads to increased KAR2 expression. The HDEL receptor (Erd2p) is functional and traffics in p24 deletion strains as in wild-type strains, however the capacity of the retrieval pathway is exceeded. Other conditions that activate the UPR and elevate KAR2 expression also lead to extracellular secretion of Kar2p. Using an in vitro assay that reconstitutes budding from the ER, we detect elevated levels of Kar2p in ER-derived vesicles from p24 deletion strains and from wild-type strains with an activated UPR. Silencing the UPR by IRE1 deletion diminished Kar2p secretion under these conditions. We suggest that activation of the UPR plays a major role in extracellular secretion of Kar2p.

Dsl1p, an essential protein required for membrane traffic at the endoplasmic reticulum/Golgi interface in yeast

To identify novel factors required for ER to Golgi transport in yeast we performed a screen for genes that, when mutated, confer a dependence on a dominant mutant form of the ER to Golgi vesicle docking factor Sly1p, termed Sly1-20p. DSL1, a novel gene isolated in the screen, encodes an essential protein with a predicted molecular mass of 88 kDa. DSL1 is required for transport between the ER and the Golgi because strains bearing mutant alleles of this gene accumulate the pre-Golgi form of transported proteins at the restrictive temperature. Two strains bearing temperature-sensitive alleles of DSL1 display distinct phenotypes as observed by electron microscopy at the restrictive temperature; although both strains accumulate ER membrane, one also accumulates vesicles. Interestingly, the inviability of strains bearing several mutant alleles of DSL1 can be suppressed by expression of either Erv14p (a protein required for the movement of a specific protein from the ER to the Golgi), Sec21p (the gamma-subunit of the COPI coat protein complex coatomer), or Sly1-20p. Because the strongest suppressor is SEC21, we proposed that Dsl1p functions primarily in retrograde Golgi to ER traffic, although it is possible that Dsl1p functions in anterograde traffic as well, perhaps at the docking stage, as suggested by the suppression by SLY1-20.

Erv41p and Erv46p: new components of COPII vesicles involved in transport between the ER and Golgi complex

Proteins contained on purified COPII vesicles were analyzed by matrix-assisted laser desorption ionization mass spectrometry combined with database searching. We identified four known vesicle proteins (Erv14p, Bet1p, Emp24p, and Erv25p) and an additional nine species (Yip3p, Rer1p, Erp1p, Erp2p, Erv29p, Yif1p, Erv41p, Erv46p, and Emp47p) that had not been localized to ER vesicles. Using antibodies, we demonstrate that these proteins are selectively and efficiently packaged into COPII vesicles. Three of the newly identified vesicle proteins (Erv29p, Erv41p, and Erv46p) represent uncharacterized integral membrane proteins that are conserved across species. Erv41p and Erv46p were further characterized. These proteins colocalized to ER and Golgi membranes and exist in a detergent-soluble complex that was isolated by immunoprecipitation. Yeast strains lacking Erv41p and/or Erv46p are viable but display cold sensitivity. The expression levels of Erv41p and Erv46p are interdependent such that Erv46p was reduced in an erv41Delta strain, and Erv41p was not detected in an erv46Delta strain. When the erv41Delta or ev46Delta alleles were combined with other mutations in the early secretory pathway, altered growth phenotypes were observed in some of the double mutant strains. A cell-free assay that reproduces transport between the ER and Golgi indicates that deletion of the Erv41p-Erv46p complex influences the membrane fusion stage of transport.

Coupled transport of p24 family members

Recent studies show that small trans-membrane proteins of approximately 22–24 kDa (the p24 family), which are grouped into 4 sub-families by sequence homology (p23, p24, p25 and p26), are involved in the early secretory pathway. In this study, we have investigated the mutual requirements of ectopically expressed members of the p24 family for targeting to their proper cellular destination. We find that coexpression of p23 and p24 is both necessary and sufficient for each protein to be transported to the cis-Golgi network/Golgi complex. Proteins from other subfamilies did not substitute for either p23 or p24, even after multiple coexpression. However, trafficking of the p23/p24 couple was facilitated by coexpression of proteins from other sub-families. In addition, we find that the sequence resembling an endoplasmic reticulum retrieval signal present in the cytoplasmic domain of p23 (but not p24) is dispensable. In contrast, the conserved coiled-coil region in the lumenal domain is absolutely required in both p23 and p24 for proper targeting of the p23/p24 couple. These data demonstrate that p23 and p24 must interact with each other to reach their destination, but that this strict requirement is combined with a mutual dependence amongst p24 proteins. We speculate that p24 proteins can form different oligomeric complexes, which contribute to confer specialized sorting/trafficking properties to membranes of the early secretory pathway, perhaps serving as membrane organizers.

Sfb2p, a yeast protein related to Sec24p, can function as a constituent of COPII coats required for vesicle budding from the endoplasmic reticulum

The COPII coat is required for vesicle budding from the endoplasmic reticulum (ER), and consists of two heterodimeric subcomplexes, Sec23p/Sec24p, Sec13p/Sec31p, and a small GTPase, Sar1p. We characterized a yeast mutant, anu1 (abnormal nuclear morphology) exhibiting proliferated ER as well as abnormal nuclear morphology at the restrictive temperature. Based on the finding that ANU1 is identical to SEC24, we confirmed a temperature-sensitive protein transport from the ER to the Golgi in anu1-1/sec24-20 cells. Overexpression of SFB2, a SEC24 homologue with 56% identity, partially suppressed not only the mutant phenotype of sec24-20 cells but also rescued the SEC24-disrupted cells. Moreover, the yeast two-hybrid assay revealed that Sfb2p, similarly to Sec24p, interacted with Sec23p. In SEC24-disrupted cells rescued by overexpression of SFB2, some cargo proteins were still retained in the ER, while most of the protein transport was restored. Together, these findings strongly suggest that Sfb2p functions as the component of COPII coats in place of Sec24p, and raise the possibility that each member of the SEC24 family of proteins participates directly and/or indirectly in cargo-recognition events with its own cargo specificity at forming ER-derived vesicles.

The p24 proteins are not essential for vesicular transport in Saccharomyces cerevisiae

To investigate the factors involved in the sorting of cargo proteins into COPII endoplasmic reticulum (ER) to Golgi apparatus transport vesicles, we have created a strain of S. cerevisiae (p24Delta8) that lacks all eight members of the p24 family of transmembrane proteins (Emp24p, Erv25p, and Erp1p to Erp6p). The p24 proteins have been implicated in COPI and COPII vesicle formation, cargo protein sorting, and regulation of vesicular transport in eukaryotic cells. We find that p24Delta8 cells grow identically to wild type and show delays of invertase and Gas1p ER-to-Golgi transport identical to those seen in a single Deltaemp24 deletion strain. Thus, p24 proteins do not have an essential function in the secretory pathway. Instead, they may serve as quality control factors to restrict the entry of proteins into COPII vesicles.

Prohormone transport through the secretory pathway of neuroendocrine cells

En route through the secretory pathway of neuroendocrine cells, prohormones pass a series of membrane-bounded compartments. During this transport, the prohormones are sorted to secretory granules and proteolytically cleaved to bioactive peptides. Recently, progress has been made in a number of aspects concerning secretory protein transport and sorting, particularly with respect to transport events in the early regions of the secretory pathway. In this review we will deal with some of these aspects, including: i) selective exit from the endoplasmic reticulum via COPII-coated vesicles and the potential role of p24 putative cargo receptors in this process, ii) cisternal maturation as an alternative model for protein transport through the Golgi complex, and iii) the mechanisms that may be involved in the sorting of regulated secretory proteins to secretory granules. Although much remains to be learned, interesting new insights into the functioning of the secretory pathway have been obtained.Key words: regulated secretory pathway, p24 family, vesicular transport, POMC, protein sorting, secretory granule, Xenopus laevis.

Applications of protein mass spectrometry in cell biology

Advances in mass spectrometry combined with accelerated progress in genome sequencing projects have facilitated the rapid identification of proteins by enzymatic digestion, mass analysis, and sequence database searching. Applications for this technology range from the surveillance of protein expression in cells, tissues, and whole organisms, to the identification of proteins and posttranslational modifications. Here we consider practical aspects of the application of mass spectrometry in cell biology and illustrate these with examples from our own laboratories.

Identification of a lumenal sequence specifying the assembly of Emp24p into p24 complexes in the yeast secretory pathway

The p24 proteins are transmembrane proteins of the endomembrane system that play a poorly defined role in vesicle traffic between the endoplasmic reticulum and the Golgi apparatus. Various lines of evidence indicate that p24 proteins fall into four subfamilies (alpha, beta, gamma, and delta) and that tetramers are assembled containing one representative from each subfamily; however, the nature of the protein-protein interactions within these hetero-oligomers is unknown. We have identified a lumenal segment of yeast p24beta (Emp24p) that is necessary for its assembly into p24 complexes. Replacement of 52 C-terminal residues of Emp24p with the corresponding sequence from Erv25p (p24delta) generates a chimeric protein able to replace Emp24p in p24 complexes that retain partial function in vivo, ruling out a role for the transmembrane and cytosolic domains in specifying p24 interactions. Substitution of a further 50 residues, encompassing a heptad repeat region, abolishes the ability of the chimera to replace Emp24p but instead creates a protein that resembles its Erv25p parent in its requirement for stabilization by Emp24p. These data point to a role for coiled-coil interactions in directing subfamily-specific assembly of p24 oligomers that project into the lumen of transport vesicles, where they may act to exclude secretory cargo from coat protein complex type I-coated retrograde transport vesicles.

Decoding of sorting signals by coatomer through a GTPase switch in the COPI coat complex

Sorting signals on cargo proteins are recognized by coatomer for selective uptake into COPI (coatomer)-coated vesicles. This study shows that coatomer couples sorting signal recognition to the GTP hydrolysis reaction on ARF1. Coatomer responds differently to different signals. The cytoplasmic signal sequence of hp24a inhibits coatomer-dependent GTP hydrolysis. By contrast, the dilysine retrieval signal, which competes for the same binding site on coatomer, has no effect on GTPase activity. It is inferred that, in vivo, sorting signal selection is under kinetic control, with coatomer governing a GTPase discard pathway that excludes dilysine-tagged proteins from one class of COPI-coated vesicles. The concept of competing sets of sorting signals that act positively and negatively during vesicle budding through a GTPase switch in the COPI coat complex suggests mechanisms for cargo segregation in which specificity is conferred by GTP hydrolysis.

The transmembrane protein p23 contributes to the organization of the Golgi apparatus

In previous studies we have shown that p23, a member of the p24-family of small transmembrane proteins, is highly abundant in membranes of the cis-Golgi network (CGN), and is involved in sorting/trafficking in the early secretory pathway. In the present study, we have further investigated the role of p23 after ectopic expression. We found that ectopically expressed p23 folded and oligomerized properly, even after overexpression. However, in contrast to endogenous p23, exogenous p23 molecules did not localize to the CGN, but induced a significant expansion of characteristic smooth ER membranes, where they accumulated in high amounts. This ER-derived, p23-rich subdomain displayed a highly regular morphology, consisting of tubules and/or cisternae of constant diameter, which were reminiscent of the CGN membranes containing p23 in control cells. The expression of exogenous p23 also led to the specific relocalization of endogenous p23, but not of other proteins, to these specialized ER-derived membranes. Relocalization of p23 modified the ultrastructure of the CGN and Golgi membranes, but did not affect anterograde and retrograde transport reactions to any significant extent. We conclude (i) that p23 has a morphogenic activity that contributes to the morphology of CGN-membranes; and (ii) that the presence of p23 in the CGN is necessary for the proper organization of the Golgi apparatus.

The Emp24 complex recruits a specific cargo molecule into endoplasmic reticulum-derived vesicles

Members of the yeast p24 family, including Emp24p and Erv25p, form a heteromeric complex required for the efficient transport of selected proteins from the endoplasmic reticulum (ER) to the Golgi apparatus. The specific functions and sites of action of this complex are unknown. We show that Emp24p is directly required for efficient packaging of a lumenal cargo protein, Gas1p, into ER-derived vesicles. Emp24p and Erv25p can be directly cross-linked to Gas1p in ER-derived vesicles. Gap1p, which was not affected by emp24 mutation, was not cross-linked. These results suggest that the Emp24 complex acts as a cargo receptor in vesicle biogenesis from the ER.

Differential induction of two p24delta putative cargo receptors upon activation of a prohormone-producing cell

The p24 family consists of type I transmembrane proteins that are present abundantly in transport vesicles, may play a role in endoplasmic reticulum-to-Golgi cargo transport, and have been classified into subfamilies named p24alpha, -beta, -gamma, and -delta. We previously identified a member of the p24delta subfamily that is coordinately expressed with the prohormone proopiomelanocortin (POMC) in the melanotrope cells of the intermediate pituitary during black background adaptation of the amphibian Xenopus laevis ( approximately 30-fold increase in POMC mRNA). In this study, we report on the characterization of this p24delta member (Xp24delta(2)) and on the identification and characterization of a second member (Xp24delta(1)) that is also expressed in the melanotrope cells and that has 66% amino acid sequence identity to Xp24delta(2). The two p24delta members are ubiquitously expressed, but Xp24delta(2) is neuroendocrine enriched. During black background adaptation, the amount of the Xp24delta(2) protein in the intermediate pituitary was increased approximately 25 times, whereas Xp24delta(1) protein expression was increased only 2.5 times. Furthermore, the level of Xp24delta(2) mRNA was approximately 5-fold higher in the melanotrope cells of black-adapted animals than in those of white-adapted animals, whereas Xp24delta(1) mRNA expression was not induced. Therefore, the expression of Xp24delta(2) specifically correlates with the expression of POMC. Together, our findings suggest that p24delta proteins have a role in selective protein transport in the secretory pathway.

Coat proteins regulating membrane traffic

This review focuses on the roles of coat proteins in regulating the membrane traffic of eukaryotic cells. Coat proteins are recruited to the donor organelle membrane from a cytosolic pool by specific small GTP-binding proteins and are required for the budding of coated vesicles. This review first describes the four types of coat complexes that have been characterized so far: clathrin and its adaptors, the adaptor-related AP-3 complex, COPI, and COPII. It then discusses the ascribed functions of coat proteins in vesicular transport, including the physical deformation of the membrane into a bud, the selection of cargo, and the targeting of the budded vesicle. It also mentions how the coat proteins may function in an alternative model for transport, namely via tubular connections, and how traffic is regulated. Finally, this review outlines the evidence that related coat proteins may regulate other steps of membrane traffic.

Shr3p mediates specific COPII coatomer-cargo interactions required for the packaging of amino acid permeases into ER-derived transport vesicles

The SHR3 gene of Saccharomyces cerevisiae encodes an integral membrane component of the endoplasmic reticulum (ER) with four membrane-spanning segments and a hydrophilic, cytoplasmically oriented carboxyl-terminal domain. Mutations in SHR3 specifically impede the transport of all 18 members of the amino acid permease (aap) gene family away from the ER. Shr3p does not itself exit the ER. Aaps fully integrate into the ER membrane and fold properly independently of Shr3p. Shr3p physically associates with the general aap Gap1p but not Sec61p, Gal2p, or Pma1p in a complex that can be purified from N-dodecylmaltoside-solubilized membranes. Pulse-chase experiments indicate that the Shr3p-Gap1p association is transient, a reflection of the exit of Gap1p from the ER. The ER-derived vesicle COPII coatomer components Sec13p, Sec23p, Sec24p, and Sec31p but not Sar1p bind Shr3p via interactions with its carboxyl-terminal domain. The mutant shr3-23p, a nonfunctional membrane-associated protein, is unable to associate with aaps but retains the capacity to bind COPII components. The overexpression of either Shr3p or shr3-23p partially suppresses the temperature-sensitive sec12-1 allele. These results are consistent with a model in which Shr3p acts as a packaging chaperone that initiates ER-derived transport vesicle formation in the proximity of aaps by facilitating the membrane association and assembly of COPII coatomer components.