Unconventional protein secretion: membrane translocation of FGF-2 does not require protein unfolding

Is there leaderless protein secretion in plants?

The sugar alcohol mannitol and it’s catabolic enzyme mannitol dehydrogenase (MTD), in addition to welldocumented roles in metabolism and osmoprotection, may play roles in hostpathogen interactions. Research suggests that in response to the mannitol that pathogenic fungi secrete to suppress reactive oxygen-mediated host defenses, plants make MTD to catabolize fungal mannitol. Yet previous work suggested that pathogen-secreted mannitol is extracellular, while in healthy plants MTD is cytoplasmic. We have presented results showing that the normally cytoplasmic MTD is exported into the cell wall or extracellular space in response to the endogenous inducer of plant defense responses salicylic acid (SA). This SA-induced secretion is insensitive to brefeldin A, an inhibitor of Golgimediated protein transport. Together with the absence of MTD in Golgi stacks and the lack of a documented extracellular targeting sequence in the MTD protein, this suggests MTD is secreted by a non-Golgi, pathogen-activated secretion mechanism in plants. Here we discuss the potential significance of non-Golgi secretion in response to stress.

Salicylic acid stimulates secretion of the normally symplastic enzyme mannitol dehydrogenase: a possible defense against mannitol-secreting fungal pathogens

The sugar alcohol mannitol is an important carbohydrate with well-documented roles in both metabolism and osmoprotection in many plants and fungi. In addition to these traditionally recognized roles, mannitol is reported to be an antioxidant and as such may play a role in host-pathogen interactions. Current research suggests that pathogenic fungi can secrete mannitol into the apoplast to suppress reactive oxygen-mediated host defenses. Immunoelectron microscopy, immunoblot, and biochemical data reported here show that the normally symplastic plant enzyme, mannitol dehydrogenase (MTD), is secreted into the apoplast after treatment with the endogenous inducer of plant defense responses salicylic acid (SA). In contrast, a cytoplasmic marker protein, hexokinase, remained cytoplasmic after SA-treatment. Secreted MTD retained activity after export to the apoplast. Given that MTD converts mannitol to the sugar mannose, MTD secretion may be an important component of plant defense against mannitol-secreting fungal pathogens such as Alternaria. After SA treatment, MTD was not detected in the Golgi apparatus, and its SA-induced secretion was resistant to brefeldin A, an inhibitor of Golgi-mediated protein transport. Together with the absence of a known extracellular targeting sequence on the MTD protein, these data suggest that a plant's response to pathogen challenge may include secretion of selected defensive proteins by as yet uncharacterized, non-Golgi mechanisms.

An intrinsic quality-control mechanism ensures unconventional secretion of fibroblast growth factor 2 in a folded conformation

Fibroblast growth factor 2 (FGF2) is a proangiogenic mitogen that is secreted by an unconventional mechanism, which does not depend on a functional ER-Golgi system. FGF2 is first recruited to the inner leaflet of plasma membranes, in a process that is mediated by the phosphoinositide PtdIns(4,5)P(2). On the extracellular side, membrane-proximal FGF2-binding sites provided by heparan-sulfate proteoglycans are essential for trapping and accumulating FGF2 in the extracellular space. Here we demonstrate that FGF2 membrane translocation can occur in a folded conformation, i.e. unfolded molecules are not obligatory intermediates in FGF2 secretion. Furthermore, we find that initial sorting into its export pathway requires FGF2 to be folded, because the interaction with PtdIns(4,5)P(2) is lost upon unfolding of FGF2. Our combined findings suggest an intrinsic quality-control mechanism that ensures extracellular accumulation of FGF2 in a biologically active form.

Heparan sulfate proteoglycans are receptors for the cell-surface trafficking and biological activity of transglutaminase-2

Transglutaminase type 2 (TG2) is both a protein cross-linking enzyme and a cell adhesion molecule with an elusive unconventional secretion pathway. In normal conditions, TG2-mediated modification of the extracellular matrix modulates cell motility, proliferation and tissue repair, but under continuous cell insult, higher expression and elevated extracellular trafficking of TG2 contribute to the pathogenesis of tissue scarring. In search of TG2 ligands that could contribute to its regulation, we characterized the affinity of TG2 for heparan sulfate (HS) and heparin, an analogue of the chains of HS proteoglycans (HSPGs). By using heparin/HS solid-binding assays and surface plasmon resonance we showed that purified TG2 has high affinity for heparin/HS, comparable to that for fibronectin, and that cell-surface TG2 interacts with heparin/HS. We demonstrated that cell-surface TG2 directly associates with the HS chains of syndecan-4 without the mediation of fibronectin, which has affinity for both syndecan-4 and TG2. Functional inhibition of the cell-surface HS chains of wild-type and syndecan-4-null fibroblasts revealed that the extracellular cross-linking activity of TG2 depends on the HS of HSPG and that syndecan-4 plays a major but not exclusive role. We found that heparin binding did not alter TG2 activity per se. Conversely, fibroblasts deprived of syndecan-4 were unable to effectively externalize TG2, resulting in its cytosolic accumulation. We propose that the membrane trafficking of TG2, and hence its extracellular activity, is linked to TG2 binding to cell-surface HSPG.

Protein folding does not prevent the nonclassical export of FGF1 and S100A13

Newly synthesized proteins are usually exported through the endoplasmic reticulum (ER) and Golgi due to the presence in their primary sequence of a hydrophobic signal peptide that is recognized by the ER translocation system. However, some secreted proteins lack a signal peptide and are exported independently of ER-Golgi. Fibroblast growth factor (FGF)1 is included in this group of polypeptides, as well as S100A13 that is a small calcium-binding protein critical for FGF1 export. Classically secreted proteins are transported into ER in their unfolded states. To determine the role of protein tertiary structure in FGF1 export through the cell membrane, we produced the chimeras of FGF1 and S100A13 with dihydrofolate reductase (DHFR). The specific DHFR inhibitor, aminopterin, prevents its unfolding. We found that aminopterin did not inhibit the release of FGF1:DHFR and S100A13:DHFR. Thus, FGF1 and S100A13 can be exported in folded conformation.

Secretion without Golgi

A growing number of proteins devoid of signal peptides have been demonstrated to be released through the non-classical pathways independent of endoplasmic reticulum and Golgi. Among them are two potent proangiogenic cytokines FGF1 and IL1alpha. Stress-induced transmembrane translocation of these proteins requires the assembly of copper-dependent multiprotein release complexes. It involves the interaction of exported proteins with the acidic phospholipids of the inner leaflet of the cell membrane and membrane destabilization. Not only stress, but also thrombin treatment and inhibition of Notch signaling stimulate the export of FGF1. Non-classical release of FGF1 and IL1alpha presents a promising target for treatment of cardiovascular, oncologic, and inflammatory disorders.

Rerouting of fibroblast growth factor 2 to the classical secretory pathway results in post-translational modifications that block binding to heparan sulfate proteoglycans

FGF-2 is a proangiogenic growth factor secreted by unconventional means. It is unknown why FGF-2 takes an ER/Golgi-independent secretory route. We find that secretion of FGF-2 via the ER/Golgi system causes post-translational modifications that prevent binding to heparan sulfate proteoglycans (HSPGs), an interaction that is critically important for both FGF-2 storage and signal transduction. This loss of function is due to artificial O-glycosylation mainly resulting in the addition of glycosaminoglycan chains of the chrondroitin sulfate type. Our findings suggest that the unconventional mechanism of FGF-2 export is an ancient pathway of protein secretion that, in the course of evolution, has been kept due to the inability of the classical secretory pathway to export FGF-2 in a functional form.

A direct role for phosphatidylinositol-4,5-bisphosphate in unconventional secretion of fibroblast growth factor 2

Fibroblast growth factor 2 (FGF-2) is a mitogen that is exported from cells by an endoplasmic reticulum/Golgi-independent secretory pathway. Recent findings have shown that FGF-2 export occurs by direct translocation from the cytoplasm across the plasma membrane into the extracellular space. Here, we report that FGF-2 contains a binding site for phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)], the principal phosphoinositide species associated with plasma membranes. Intriguingly, in the context of a lipid bilayer, the interaction between FGF-2 and PI(4,5)P(2) is shown to depend on a lipid background that resembles plasma membranes. We show that the interaction with PI(4,5)P(2) is critically important for FGF-2 secretion as experimental conditions reducing cellular levels of PI(4,5)P(2) resulted in a substantial drop in FGF-2 export efficiency. Likewise, we have identified FGF-2 variant forms deficient for binding to PI(4,5)P(2) that were found to be severely impaired with regard to export efficiency. These data show that a transient interaction with PI(4,5)P(2) associated with the inner leaflet of plasma membranes represents the initial step of the unconventional secretory pathway of FGF-2.

Unconventional secretion of fibroblast growth factor 2 and galectin-1 does not require shedding of plasma membrane-derived vesicles

Various molecular mechanisms of unconventional secretion of fibroblast growth factor 2 and galectin-1 have been proposed. A non-vesicular pathway that is based on direct translocation across the plasma membrane has been described. In other studies, however, release into the extracellular space of cell-derived vesicles was implicated in both FGF-2 and Gal-1 secretion. Such vesicles were proposed to originate either from plasma membrane shedding or by the release of exosomes. Employing an inhibitor of plasma membrane blebbing and based on a quantitative biochemical analysis of cell culture supernatants for vesicles potentially carrying FGF-2 or Gal-1, we demonstrate that both FGF-2 and Gal-1 are not exported by shedding of plasma membrane-derived vesicles.

Extracellular annexin A5: functions of phosphatidylserine-binding and two-dimensional crystallization

In normal healthy cells phosphatidylserine is located in the inner leaflet of the plasma membrane. However, on activated platelets, dying cells and under specific circumstances also on various types of viable leukocytes phosphatidylserine is actively externalized to the outer leaflet of the plasma membrane. Annexin A5 has the ability to bind in a calcium-dependent manner to phosphatidylserine and to form a membrane-bound two-dimensional crystal lattice. Based on these abilities various functions for extracellular annexin A5 on the phosphatidylserine-expressing plasma membrane have been proposed. In this review we describe possible mechanisms for externalization of annexin A5 and various processes in which extracellular annexin A5 may play a role such as blood coagulation, apoptosis, phagocytosis and formation of plasma membrane-derived microparticles. We further highlight the recent discovery of internalization of extracellular annexin A5 by phosphatidylserine-expressing cells.

Evidence for a major role of endogenous fibroblast growth factor-2 in apoptotic cortex-induced subventricular zone cell proliferation

In the adult mammalian brain, neural stem cells persist in the subventricular zone (SVZ) of lateral ventricles. It is well established that cortical damage leads to SVZ cell proliferation and neuronal differentiation. We have previously demonstrated in rat that, when treated with the apoptosis-inducing agent staurosporine, cortex explants release heat-labile factors that promote SVZ cell culture proliferation. In the present report, we investigated in vitro mechanisms involved in cortex injury-triggered neurogenesis in the rat. We demonstrated, using immunoblotting analysis and fibroblast growth factor (FGF)-2 enzyme-linked sandwich immunosorbent assay, that treatment of cortex explants with apoptosis-inducing agents increases the release of FGF-2. We next determined the effects of apoptotic cortex-released factors in regulating SVZ cell proliferation and neuronal differentiation by using bromodeoxyuridine incorporation and microtubule-associated protein 2 immunostaining assays, respectively. We found that conditioned media derived from staurosporine-treated cortex explants enhanced SVZ cell culture proliferation and differentiation by over 50 and 80%, respectively. Finally, we showed that immunodepletion of FGF-2 or pharmacological blockade of FGF-2 receptor by SU5402 completely abolished staurosporine-treated cortex mitogenic activity on SVZ cultures but did not alter its activity on neuronal cell differentiation. Altogether, the present report establishes that the release of endogenous FGF-2 by apoptotic cortex explants plays a major role in the induction of SVZ cell proliferation but not neuronal differentiation, which probably depends on the release of other as yet unidentified cortical factors.

Unconventional secretion: an extracellular trap for export of fibroblast growth factor 2

Several secretory proteins are released from cells by mechanisms that are distinct from the classical endoplasmic reticulum (ER)/Golgi-mediated secretory pathway. Recent studies unexpectedly revealed that the interaction between one such protein, fibroblast growth factor 2 (FGF-2), and cell surface heparan sulfate proteoglycans (HSPGs) is essential for secretion. FGF-2 mutants that cannot bind to heparan sulfates are not secreted, and cells that do not express functional HSPGs cannot secrete wild-type FGF-2. FGF-2 appears to be secreted by direct translocation across the plasma membrane in an ATP- and membrane-potential-independent manner. I propose that its translocation across the membrane is a diffusion-controlled process in which cell surface HSPGs function as an extracellular molecular trap that drives directional transport of FGF-2.

Relevance of partially structured states in the non-classical secretion of acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) is a signal peptide-less protein that is secreted into the extracellular compartment as part of a multiprotein release complex, consisting of aFGF, S100A13 (a calcium binding protein), and a 40 kDa (p40) form of synaptotagmin (Syt1), a protein that participates in the docking of a variety of secretory vesicles. p40 Syt1, and specifically its C2A domain, is believed to play a major role in the non-classical secretion of the aFGF release complex mediated by the interaction of aFGF and p40 Syt1with the phospholipids of the cell membrane inner leaflet. In the present study, we investigate the structural characteristics of aFGF and the C2A domain of p40 Syt1 under acidic conditions, using a variety of biophysical techniques including multidimensional NMR spectroscopy. Urea-induced equilibrium unfolding (at pH 3.4) of both aFGF and the C2A domain are non-cooperative and proceed with the accumulation of stable intermediate states. 1-Anilino-8-napthalene sulfonate (ANS) binding and size-exclusion chromatography results suggest that both aFGF and the C2A domain exist as partially structured states under acidic conditions (pH 3.4). Limited trypsin digestion analysis and 1H-15N chemical shift perturbation data reveal that the flexibility of certain portions of the protein backbone is increased in the partially structured state(s) of aFGF. The residues that are perturbed in the partially structured state(s) in aFGF are mostly located at the N- and C-terminal ends of the protein. In marked contrast, most of the interactions stabilizing the native secondary structure are preserved in the partially structured state of the C2A domain. Isothermal titration calorimetry data indicate that the binding affinity between aFGF and the C2A domain is significantly enhanced at pH 3.4. In addition, both aFGF and the C2A domain exhibit much higher lipid binding affinity in their partially structured states. The translocation of the multiprotein FGF release complex across the membrane appears to be facilitated by the formation of partially structured states of aFGF and the C2A domain of p40 Syt1.

Fibroblast growth factor-2 mediates transforming growth factor-beta action in prostate cancer reactive stroma

Transforming growth factor-beta (TGF-beta) is overexpressed at sites of wound repair and in most adenocarcinomas including prostate cancer. In stromal tissues, TGF-beta regulates cell proliferation, phenotype and matrix synthesis. To address mechanisms of TGF-beta action in cancer-associated reactive stroma, we developed prostate stromal cells null for TGF-beta receptor II (TbetaRII) or engineered to express a dominant-negative Smad3 to attenuate TGF-beta signaling. The differential reactive stroma (DRS) xenograft model was used to evaluate altered stromal TGF-beta signaling on LNCaP tumor progression. LNCaP xenograft tumors constructed with TbetaRII null or dominant-negative Smad3 stromal cells exhibited a significant reduction in mass and microvessel density relative to controls. Additionally, decreased cellular fibroblast growth factor-2 (FGF-2) immunostaining was associated with attenuated TGF-beta signaling in stroma. In vitro, TGF-beta stimulated stromal FGF-2 expression and release. However, stromal cells with attenuated TGF-beta signaling were refractory to TGF-beta-stimulated FGF-2 expression and release. Re-expression of FGF-2 in these stromal cells in DRS xenografts resulted in restored tumor mass and microvessel density. In summary, these data show that TGF-beta signaling in reactive stroma is angiogenic and tumor promoting and that this effect is mediated in part through a TbetaRII/Smad3-dependent upregulation of FGF-2 expression and release.

Dual localization: proteins in extracellular and intracellular compartments

The goal of this article is to provide a comprehensive catalog of those proteins documented to exhibit dual localization, being found in both the extracellular compartment (cell surface and extracellular medium) as well as the intracellular compartment (cytosol and nucleus). A large subset of these proteins that show dual localization is found both in the nucleus and outside of cells. Proteins destined to be secreted out of the cell or to be expressed at the cell surface usually enter the endomembrane pathway on the basis of a signal sequence that targets them into the endoplasmic reticulum. Proteins destined for import into the nucleus, on the other hand, usually carry a nuclear localization signal. We have organized our catalog in terms of the presence and absence of these trafficking signals: (a) proteins that contain a signal sequence but no nuclear localization signal; (b) proteins that contain both a signal sequence as well as a nuclear localization signal; (c) proteins that contain a nuclear localization signal but lack a signal sequence; and (d) proteins containing neither a signal sequence nor a nuclear localization signal. Novel insights regarding the activities of several classes of proteins exhibiting dual localization can be derived when one targeting signal is experimentally abrogated. For example, the mitogenic activity of both fibroblasts growth factor-1 and schwannoma-derived growth factor clearly requires nuclear localization, independent of the activation of the receptor tyrosine kinase signaling pathway. In addition, there is a growing list of integral membrane receptors that undergo translocation to the nucleus, with bona fide nuclear localization signals and transcription activation activity. The information provided in this descriptive catalog will, hopefully, stimulate investigations into the pathways and mechanisms of transport between these compartments and the physiological significance of dual localization.

Cell-surface heparan sulfate proteoglycans are essential components of the unconventional export machinery of FGF-2

FGF-2 is an unconventionally secreted lectin that transmits proangiogenic signals through a ternary complex with high-affinity FGF receptors and heparan sulfate proteoglycans (HSPGs). Although FGF-2 signal transduction is understood in great detail, its mechanism of release from cells, which is independent of the classical secretory pathway, remains elusive. To test the hypothesis that FGF-2 secretion is linked to its cell-surface ligands, we studied FGF-2 release using mutants defective for HSPG binding and cells with impaired HSPG biosynthesis. Here, we report that a functional interaction between FGF-2 and HSPGs is required for net export of FGF-2 from mammalian cells. FGF-2 release requires extracellular, membrane-proximal HSPGs. We propose that extracellular HSPGs form a molecular trap that drives FGF-2 translocation across the plasma membrane.

Pattern of FGF-2 isoform expression correlated with its biological action in experimental prolactinomas

Fibroblast growth factor-2 (FGF-2) synthesized in the pituitary is involved in the formation and progression of pituitary tumors. The aim of this study was to analyze the pattern expression of two FGF-2 isoforms at different subcellular levels and to determine its correlation with prolactinoma development. Estrogen administration to male rats for 7, 20, and 60 days generated pituitary tumors, with lactotrophs being the prevalent cell type. Ultrastructural immunolabeling showed FGF-2 in the cytosolic and nuclear compartments of somatotrophs, lactotrophs and gonadotrophs, as well as in folliculo-stellate cells of normal rats. Estrogen stimulation increased FGF-2 immunoreactivity in various tumors and enhanced the expression of two FGF-2 isoforms, 18 and 22 kDa, as quantified by western blot. The 18 kDa isoform observed in cytosol extracts reached the highest levels after 60 days of hormonal stimulation and this was related to lactotroph proliferation. However, the 22 kDa FGF-2 isoform was only detected in the nuclear compartment and achieved the maximum expression at 7 days of estrogen treatment, without any correlation with lactotroph proliferation. These results suggest that the 18 kDa FGF-2 may play a role in the modulation of lactotroph proliferation in prolactinomas induced by estrogen. The overproduction of both FGF-2 isoforms appears to be implicated in autocrine-paracrine-intracrine mitogenic loops; this FGF-2 activity could lead to uncontrolled cell growth, angiogenesis, and tumor formation.

Functional diversity of FGF-2 isoforms by intracellular sorting

Regulation of the subcellular localization of certain proteins is a mechanism for the regulation of their biological activities. FGF-2 can be produced as distinct isoforms by alternative initiation of translation on a single mRNA and the isoforms are differently sorted in cells. High molecular weight FGF-2 isoforms are not secreted from the cell, but are transported to the nucleus where they regulate cell growth or behavior in an intracrine fashion. 18 kDa FGF-2 can be secreted to the extracellular medium where it acts as a conventional growth factor by binding to and activation of cell-surface receptors. Furthermore, following receptor-mediated endocytosis, the exogenous FGF-2 can be transported to the nuclei of target cells, and this is of importance for the transmittance of a mitogenic signal. The growth factor is able to interact with several intracellular proteins. Here, the mode of action and biological role of intracellular FGF-2 are discussed.

The long and winding road: Protein trafficking mechanisms in the Plasmodium falciparum infected erythrocyte

Mature human erythrocytes infected with the human malarial parasite Plasmodium falciparum are extensively modified to provide a more comfortable "home" for their intracellular guests. This process is mediated by parasite-encoded factors that are exported into, and through the host erythrocyte. This intra- yet simultaneously extra-cellular protein trafficking and sorting system has, in the past decades received much attention, also due to its unusual nature. Recent reports have highlighted the importance of a short peptide sequence, referred to individually as Plasmodium export element (PEXEL), vacuolar translocation signal (VTS) or generally as host cell targeting signal (HCT) in the export of both soluble and membrane bound proteins, allowing the partial definition of the parasite's "exportome". Mechanistically however, the discovery of this sequence raises as many questions as it answers. In this article, we comment on current models of protein transport to the host cell, discuss the mechanistic problems highlighted by these signals, and suggest what might be the next important steps in studying the protein export mechanisms of an obligate intracellular parasite that chooses to inhabit a de-nucleated host cell.

Cell surface counter receptors are essential components of the unconventional export machinery of galectin-1

Galectin-1 is a component of the extracellular matrix as well as a ligand of cell surface counter receptors such as β-galactoside–containing glycolipids, however, the molecular mechanism of galectin-1 secretion has remained elusive. Based on a nonbiased screen for galectin-1 export mutants we have identified 26 single amino acid changes that cause a defect of both export and binding to counter receptors. When wild-type galectin-1 was analyzed in CHO clone 13 cells, a mutant cell line incapable of expressing functional galectin-1 counter receptors, secretion was blocked. Intriguingly, we also find that a distant relative of galectin-1, the fungal lectin CGL-2, is a substrate for nonclassical export from Chinese hamster ovary (CHO) cells. Alike mammalian galectin-1, a CGL-2 mutant defective in β-galactoside binding, does not get exported from CHO cells. We conclude that the β-galactoside binding site represents the primary targeting motif of galectins defining a galectin export machinery that makes use of β-galactoside–containing surface molecules as export receptors for intracellular galectin-1.

Unconventional secretory routes: direct protein export across the plasma membrane of mammalian cells

The vast majority of extracellular proteins are exported from mammalian cells by the endoplasmic reticulum/Golgi-dependent secretory pathway. For poorly understood reasons, however, a heterogenous group of extracellular proteins has been discovered that does not make use of signal peptide-dependent secretory transport. Both the release mechanisms and the molecular identity of the secretory machines involved have remained elusive. Recent studies now have established a subgroup of unconventional secretory proteins capable of translocating from the cytoplasm directly across the plasma membrane to get access to the exterior of eukaryotic cells. This review aims to focus on a detailed comparison of the subcellular site of membrane translocation of various unconventional secretory proteins such as the proangiogenic molecule fibroblast growth factor-2 (FGF-2) and Leishmania hydrophilic acylated surface protein B (HASP B). A potential link between membrane translocation and quality control as an integral part of unconventional secretory processes is discussed.

Metallothionein mediates leukocyte chemotaxis

BACKGROUND

Metallothionein (MT) is a cysteine-rich, metal-binding protein that can be induced by a variety of agents. Modulation of MT levels has also been shown to alter specific immune functions. We have noticed that the MT genes map close to the chemokines Ccl17 and Cx3cl1. Cysteine motifs that characterize these chemokines are also found in the MT sequence suggesting that MT might also act as a chemotactic factor.

RESULTS

In the experiments reported here, we show that immune cells migrate chemotactically in the presence of a gradient of MT. This response can be specifically blocked by two different monoclonal anti-MT antibodies. Exposure of cells to MT also leads to a rapid increase in F-actin content. Incubation of Jurkat T cells with cholera toxin or pertussis toxin completely abrogates the chemotactic response to MT. Thus MT may act via G-protein coupled receptors and through the cyclic AMP signaling pathway to initiate chemotaxis.

CONCLUSION

These results suggest that, under inflammatory conditions, metallothionein in the extracellular environment may support the beneficial movement of leukocytes to the site of inflammation. MT may therefore represent a "danger signal"; modifying the character of the immune response when cells sense cellular stress. Elevated metallothionein produced in the context of exposure to environmental toxicants, or as a result of chronic inflammatory disease, may alter the normal chemotactic responses that regulate leukocyte trafficking. Thus, MT synthesis may represent an important factor in immunomodulation that is associated with autoimmune disease and toxicant exposure.

Direct transport across the plasma membrane of mammalian cells of Leishmania HASPB as revealed by a CHO export mutant

Leishmania HASPB is a lipoprotein that is exported to the extracellular space from both Leishmania parasites and mammalian cells via an unconventional secretory pathway. Exported HASPB remains anchored in the outer leaflet of the plasma membrane mediated by myristate and palmitate residues covalently attached to the N-terminal SH4 domain of HASPB. HASPB targeting to the plasma membrane depends on SH4 acylation that occurs at intracellular membranes. How acylated HASPB is targeted to the plasma membrane and, in particular, the subcellular site of HASPB membrane translocation is unknown. In order to address this issue, we screened for clonal CHO mutants that are incapable of exporting HASPB. A detailed characterization of such a CHO mutant cell line revealed that the expression level of the HASPB reporter molecule is unchanged compared to CHO wild-type cells; that it is both myristoylated and palmitoylated; and that it is mainly localized to the plasma membrane as judged by confocal microscopy and subcellular fractionation. However, based on a quantitative flow cytometry assay and a biochemical biotinylation assay of surface proteins, HASPB transport to the outer leaflet of the plasma membrane is largely reduced in this mutant. From these data, we conclude that the subcellular site of HASPB membrane translocation is the plasma membrane as the reporter molecule accumulates in this location when export is blocked. Thus, these results allow us to define a two-step process of HASPB cell surface biogenesis in which SH4 acylation of HASPB firstly mediates intracellular targeting to the plasma membrane. In a second step, the plasma membrane-resident machinery, which is apparently disrupted in the CHO mutant cell line, mediates membrane translocation of HASPB. Intriguingly, the angiogenic growth factor FGF-2, another protein secreted by unconventional means, is shown to be secreted normally from the HASPB export mutant cell line. These observations demonstrate that the export machinery component defective in the export mutant cell line functions specifically in the HASPB export pathway.