VAMP5 promotes Fcγ receptor-mediated phagocytosis and regulates phagosome maturation in macrophages

Phagosome formation and maturation reportedly occur via sequential membrane fusion events mediated by synaptosomal-associated protein of 23 kDa (SNAP23), a plasma membrane-localized soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family. Vesicle-associated membrane protein 5 (VAMP5), also a plasmalemma SNARE, interacts with SNAP23; however, its precise function in phagocytosis in macrophages remains elusive. To elucidate this aspect, we investigated the characteristics of macrophages in the presence of VAMP5 overexpression or knockdown and found that VAMP5 participates in Fcγ receptor-mediated phagosome formation, although not directly in phagosome maturation. Overexpressed VAMP5 was localized to the early phagosomal membrane but no longer localized to the lysosomal-associated membrane protein 1-positive maturing phagosomal membrane. Analyses using compound-based selective inhibitors demonstrated that VAMP5 dissociation from early phagosomes occurs in a clathrin- and dynamin-dependent manner and is indispensable for SNAP23 function in subsequent membrane fusion during phagosome maturation. Accordingly, to the best of our knowledge, we demonstrate, for the first time, that VAMP5 exerts an immunologically critical function during phagosome formation and maturation via SNARE-based membrane trafficking in macrophages.

Regulatory Mechanism of SNAP23 in Phagosome Formation and Maturation

Synaptosomal associated protein of 23 kDa (SNAP23), a plasma membrane-localized soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE), is a ubiquitously expressed protein that is generally involved in fusion of the plasma membrane and secretory or endosomal recycling vesicles during several types of exocytosis. SNAP23 is expressed in phagocytes, such as neutrophils, macrophages, and dendritic cells, and functions in both exocytosis and phagocytosis. This review focuses on the function of SNAP23 in immunoglobulin G Fc receptor-mediated phagocytosis by macrophages. SNAP23 and its partner SNAREs mediate fusion of the plasma membrane with intracellular organelles or vesicles to form phagosomes as well as the fusion of phagosomes with endosomes or lysosomes to induce phagosome maturation, characterized by reactive oxygen species production and acidification. During these processes, SNAP23 function is regulated by phosphorylation. In addition, microtubule-associated protein 1A/1B light chain 3 (LC3)-associated phagocytosis, which tightly promotes or suppresses phagosome maturation depending on the foreign target, requires SNAP23 function. SNAP23 that is enriched on the phagosome membrane during LC3-associated phagocytosis may be phosphorylated or dephosphorylated, thereby enhancing or inhibiting subsequent phagosome maturation, respectively. These findings have increased our understanding of the SNAP23-associated membrane trafficking mechanism in phagocytes, which has important implications for microbial pathogenesis and innate and adaptive immune responses.

Characterization of MORN2 stability and regulatory function in LC3-associated phagocytosis in macrophages

Microtubule-associated protein A1/B1-light chain 3 (LC3)-associated phagocytosis (LAP) is a type of non-canonical autophagy that regulates phagosome maturation in macrophages. However, the role and regulatory mechanism of LAP remain largely unknown. Recently, the membrane occupation and recognition nexus repeat-containing-2 (MORN2) was identified as a key component of LAP for the efficient formation of LC3-recruiting phagosomes. To characterize MORN2 and elucidate its function in LAP, we established a MORN2-overexpressing macrophage line. At a steady state, MORN2 was partially cleaved by the ubiquitin-proteasome system. MORN2 overexpression promoted not only LC3-II production but also LAP phagosome (LAPosome) acidification during Escherichia coli uptake. Furthermore, the formation of LAPosomes containing the yeast cell wall component zymosan was enhanced in MORN2-overexpressing cells and depended on reactive oxygen species (ROS). Finally, MORN2-mediated LAP was regulated by plasma membrane-localized soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) such as SNAP-23 and syntaxin 11. Taken together, these findings demonstrate that MORN2, whose expression is downregulated via proteasomal digestion, is a limiting factor for LAP, and that membrane trafficking by SNARE proteins is involved in MORN2-mediated LAP.

Increased PD-1-positive macrophages in the tissue of gastric cancer are closely associated with poor prognosis in gastric cancer patients

Background

Programmed cell death 1 (PD-1) is one of the immune checkpoint molecules that negatively regulate the function of T cells. Although recent studies indicate that PD-1 is also expressed on other immune cells besides T cells, its role remains unclear. This study aims to evaluate PD-1 expression on macrophages and examine its effect on anti-tumor immunity in gastric cancer (GC) patients.

Methods

The frequency of PD-1⁺ macrophages obtained from GC tissue was determined by multicolor flow cytometry (n = 15). Double immunohistochemistry staining of PD-1 and CD68 was also performed to evaluate the correlations among the frequency of PD-1⁺ macrophages, clinicopathological characteristics, and prognosis in GC patients (n = 102).

Results

The frequency of PD-1⁺ macrophages was significantly higher in GC tissue than in non-tumor gastric tissue. The phagocytotic activity of PD-1⁺ macrophages was severely impaired compared with that of PD-1⁻ macrophages. The 5-year disease-specific survival rates in patients with PD-1⁺ macrophage^Low (the frequency of PD-1⁺ macrophages; < 0.85%) and those with PD-1⁺ macrophage^High (the frequency of PD-1⁺ macrophages; ≥ 0.85%) were 85.9 and 65.8%, respectively (P = 0.008). Finally, multivariate analysis showed the frequency of PD-1⁺ macrophage to be an independent prognostic factor.

Conclusions

The function of PD-1⁺ macrophage was severely impaired and increased frequency of PD-1⁺ macrophage worsened the prognosis of GC patients. PD-1–PD-L1 therapies may function through a direct effect on macrophages in GC.

Syntaxin 11 regulates the stimulus-dependent transport of Toll-like receptor 4 to the plasma membrane by cooperating with SNAP-23 in macrophages

Syntaxin 11 (stx11) is a soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) that is selectively expressed in immune cells; however, its precise role in macrophages is unclear. We showed that stx11 knockdown reduces the phagocytosis of Escherichia coli in interferon-γ–activated macrophages. stx11 knockdown decreased Toll-like receptor 4 (TLR4) localization on the plasma membrane without affecting total expression. Plasma membrane–localized TLR4 was primarily endocytosed within 1 h by lipopolysaccharide (LPS) stimulation and gradually relocalized 4 h after removal of LPS. This relocalization was significantly impaired by stx11 knockdown. The lack of TLR4 transport to the plasma membrane is presumably related to TLR4 degradation in acidic endosomal organelles. Additionally, an immunoprecipitation experiment suggested that stx11 interacts with SNAP-23, a plasma membrane–localized SNARE protein, whose depletion also inhibits TLR4 replenishment in LPS-stimulated cells. Using an intramolecular Förster resonance energy transfer (FRET) probe for SNAP-23, we showed that the high FRET efficiency caused by LPS stimulation is reduced by stx11 knockdown. These findings suggest that stx11 regulates the stimulus-dependent transport of TLR4 to the plasma membrane by cooperating with SNAP-23 in macrophages. Our results clarify the regulatory mechanisms underlying intracellular transport of TLR4 and have implications for microbial pathogenesis and immune responses.

IL-13 attenuates early local CXCL2-dependent neutrophil recruitment for Candida albicans clearance during a severe murine systemic infection

To investigate the role of IL-13 during a severe systemic Candida albicans infection, BALB/c control and IL-13^-/- mice were examined for colony forming units (CFU) in the kidneys and survival days after intravenous infection. Proinflammatory mediators and cell recruitment into the tissue were measured by quantitative real-time PCR, a multiple ELISA system, and morphological cell differentiation. The IL-13^-/- group exhibited a lower CFU number in the kidneys at 4 days and survived longer than the control mice, which was accompanied by significantly higher expression of C-X-C motif ligand 2 (CXCL2), IFN-γ, and polymorphonuclear neutrophils (PMNs) in the infected kidneys. By contrast, the expression of transforming growth factor β (TGF-β) and IL-17 A on day 10 were significantly higher in the control mice than in the IL-13^-/- group. When using an intratracheal infection model, the IL-13^-/- group recruited a greater number of PMNs in 6 h, with rapidly increased CXCL2 in the alveolar space. In vitro testing with cultured bone-marrow-derived cells demonstrated rapid CXCL2 mRNA upregulation at 3 h after contact with C. albicans, which decreased with recombinant IL-13 pretreatment, whereas rIL-13 retained TGF-β upregulation. In a murine model of Candida systemic infection, preexistent IL-13 limits both the rapid CXCL2 elevation and PMN aggregation in the target organ to suppress inflammatory mediators, which also attenuates local pathogen clearance within four days.

Phosphorylation of SNAP-23 at Ser95 causes a structural alteration and negatively regulates Fc receptor-mediated phagosome formation and maturation in macrophages

SNAP-23 is a plasma membrane-localized soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNARE) involved in Fc receptor (FcR)-mediated phagocytosis. However, the regulatory mechanism underlying its function remains elusive. Using phosphorylation-specific antibodies, SNAP-23 was found to be phosphorylated at Ser95 in macrophages. To understand the role of this phosphorylation, we established macrophage lines overexpressing the nonphosphorylatable S95A or the phosphomimicking S95D mutation. The efficiency of phagosome formation and maturation was severely reduced in SNAP-23-S95D–overexpressing cells. To examine whether phosphorylation at Ser95 affected SNAP-23 structure, we constructed intramolecular Förster resonance energy transfer (FRET) probes of SNAP-23 designed to evaluate the approximation of the N termini of the two SNARE motifs. Interestingly, a high FRET efficiency was detected on the membrane when the S95D probe was used, indicating that phosphorylation at Ser95 caused a dynamic structural shift to the closed form. Coexpression of IκB kinase (IKK) 2 enhanced the FRET efficiency of the wild-type probe on the phagosome membrane. Furthermore, the enhanced phagosomal FRET signal in interferon-γ–activated macrophages was largely dependent on IKK2, and this kinase mediated a delay in phagosome-lysosome fusion. These results suggested that SNAP-23 phosphorylation at Ser95 played an important role in the regulation of SNARE-dependent membrane fusion during FcR-mediated phagocytosis.

Quantitative analysis of phagosome formation and maturation using an Escherichia coli probe expressing a tandem fluorescent protein

Phagosome formation and maturation are essential innate immune mechanisms to engulf and digest foreign particles. To analyze these processes quantitatively, we established a specific Escherichia coli probe expressing a tandem fluorescent protein, comprising glutathione S-transferase fused with monomeric Cherry (mCherry) and monomeric Venus (mVenus). We demonstrated that mVenus was more susceptible to bleaching in an acidic environment than mCherry, and that the mVenus:mCherry fluorescence intensity ratio can be used to monitor phagosomal pH changes during maturation. Using this probe, we revealed that synaptosomal-associated protein of 23 kDa, a plasma membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein, actively regulated phagocytosis of E. coli and subsequent phagosome maturation in macrophages. Our results indicated that this probe has the potential to be a powerful tool in understanding the molecular mechanisms of phagosome formation and maturation.

Fate of methylated/unmethylated H19 imprinting control region after paternal and maternal pronuclear injection

The paternal-allele-specific methylation of the Igf2/H19 imprinting control region (ICR) is established during gametogenesis and maintained throughout development. To elucidate the requirement of the germline passage in the maintenance of the imprinting methylation, we established a system introducing a methylated or unmethylated ICR-containing DNA fragment (ICR-F) into the paternal or maternal genome by microinjecting into the paternal or maternal pronucleus of fertilized eggs, and traced the methylation pattern in the ICR-F. When the ICR-F was injected in a methylated form, it was demethylated approximately to half degree at blastocyst stage but was almost completely remethylated at 3 weeks of age. In the case of the unmethylated form, the ICR-F remained unmethylated at the blastocyst stage, but was almost half-methylated at 3 weeks of age. Interestingly, the paternally injected ICR-F was highly methylated compared with maternally injected ICR-F at 3 weeks of age, partially mimicking the endogenous methylation pattern. Moreover, introduction of mutations in the CTCF (CCCTC binding factor) binding sites of the ICR-F, which are known to be important for the maintenance of hypomethylated maternal ICR, induced hypermethylation of the mutated ICR-F in both paternal and maternal pronuclear injected 3-week-old mice. Our results suggest the presence of a protection-against-methylation activity of the CTCF binding site in establishing the preferential paternal methylation during post-fertilization development and the importance of germline passage in the maintenance of the parental specific methylation at H19 ICR.

Histone macroH2A1.2 promotes metabolic health and leanness by inhibiting adipogenesis

Background

Obesity has tremendous impact on the health systems. Its epigenetic bases are unclear. MacroH2A1 is a variant of histone H2A, present in two alternatively exon-spliced isoforms macroH2A1.1 and macroH2A1.2, regulating cell plasticity and proliferation, during pluripotency and tumorigenesis. Their role in adipose tissue plasticity is unknown.

Results

Here, we show evidence that macroH2A1.1 protein levels in the visceral adipose tissue of obese humans positively correlate with BMI, while macroH2A1.2 is nearly absent. We thus introduced a constitutive GFP-tagged transgene for macroH2A1.2 in mice, and we characterized their metabolic health upon being fed a standard chow diet or a high fat diet. Despite unchanged food intake, these mice exhibit lower adipose mass and improved glucose metabolism both under a chow and an obesogenic diet. In the latter regimen, transgenic mice display smaller pancreatic islets and significantly less inflammation. MacroH2A1.2 overexpression in the mouse adipose tissue induced dramatic changes in the transcript levels of key adipogenic genes; genomic analyses comparing pre-adipocytes to mature adipocytes uncovered only minor changes in macroH2A1.2 genomic distribution upon adipogenic differentiation and suggested differential cooperation with transcription factors. MacroH2A1.2 overexpression markedly inhibited adipogenesis, while overexpression of macroH2A1.1 had opposite effects.

Conclusions

MacroH2A1.2 is an unprecedented chromatin component powerfully promoting metabolic health by modulating anti-adipogenic transcriptional networks in the differentiating adipose tissue. Strategies aiming at enhancing macroH2A1.2 expression might counteract excessive adiposity in humans.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-016-0098-9) contains supplementary material, which is available to authorized users.

SNAP23/25 and VAMP2 mediate exocytic event of transferrin receptor-containing recycling vesicles

We recently showed that Rab11 is involved not only in formation of recycling vesicles containing the transferrin (Tfn)–transferrin receptor (TfnR) complex at perinuclear recycling endosomes but also in tethering of recycling vesicles to the plasma membrane (PM) in concert with the exocyst tethering complex. We here aimed at identifying SNARE proteins responsible for fusion of Tfn–TfnR-containing recycling vesicles with the PM, downstream of the exocyst. We showed that exocyst subunits, Sec6 and Sec8, can interact with SNAP23 and SNAP25, both of which are PM-localizing Q_bc-SNAREs, and that depletion of SNAP23 and/or SNAP25 in HeLa cells suppresses fusion of Tfn–TfnR-containing vesicles with the PM, leading to accumulation of the vesicles at the cell periphery. We also found that VAMP2, an R-SNARE, is colocalized with endocytosed Tfn on punctate endosomal structures, and that its depletion in HeLa cells suppresses recycling vesicle exocytosis. These observations indicate that fusion of recycling vesicles with the PM downstream of the exocyst is mediated by SNAP23/25 and VAMP2, and provide novel insight into non-neuronal roles of VAMP2 and SNAP25.

SNAP-23 regulates phagosome formation and maturation in macrophages

Using macrophages overexpressing or reducing SNAP-23, this study shows that SNAP-23 is implicated in phagosome formation and maturation, presumably by mediating SNARE-based membrane traffic. Indeed, a conformational change in SNAP-23 structure based on FRET signal is observed on the phagosome membrane of cells overexpressing the lysosomal SNARE VAMP7.

Synaptosomal associated protein of 23 kDa (SNAP-23), a plasma membrane–localized soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE), has been implicated in phagocytosis by macrophages. For elucidation of its precise role in this process, a macrophage line overexpressing monomeric Venus–tagged SNAP-23 was established. These cells showed enhanced Fc receptor–mediated phagocytosis. Detailed analyses of each process of phagocytosis revealed a marked increase in the production of reactive oxygen species within phagosomes. Also, enhanced accumulation of a lysotropic dye, as well as augmented quenching of a pH-sensitive fluorophore were observed. Analyses of isolated phagosomes indicated the critical role of SNAP-23 in the functional recruitment of the NADPH oxidase complex and vacuolar-type H⁺-ATPase to phagosomes. The data from the overexpression experiments were confirmed by SNAP-23 knockdown, which demonstrated a significant delay in phagosome maturation and a reduction in uptake activity. Finally, for analyzing whether phagosomal SNAP-23 entails a structural change in the protein, an intramolecular Förster resonance energy transfer (FRET) probe was constructed, in which the distance within a TagGFP2-TagRFP was altered upon close approximation of the N-termini of its two SNARE motifs. FRET efficiency on phagosomes was markedly enhanced only when VAMP7, a lysosomal SNARE, was coexpressed. Taken together, our results strongly suggest the involvement of SNAP-23 in both phagosome formation and maturation in macrophages, presumably by mediating SNARE-based membrane traffic.

Disruption of the fusion of Leishmania parasitophorous vacuoles with ER vesicles results in the control of the infection

Parasitophorous vacuoles (PV) that harbour Leishmania parasites acquire some characteristics from fusion with host cell vesicles. Recent studies have shown that PVs acquire and display resident endoplasmic reticulum (ER) molecules. We investigated the importance of ER molecules to PV biology by assessing the consequence of blocking the fusion of PVs with vesicles that originate from the early secretory pathway. This was achieved by targeting the N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) that mediate the fusion of early secretory vesicles. In the presence of dominant negative variants of sec22b or some of its known cognate partners, D12 and syntaxin 18, PVs failed to distend and harboured fewer parasites. These observations were confirmed in studies in which each of the SNAREs listed above including the intermediate compartment ER/Golgi SNARE, syntaxin 5, was knocked down. The knock-down of these SNARES had little or no measurable effect on the morphology of the ER or on activated secretion even though they resulted in a more significant reduction of PV size. Moreover, the knock-down of the ER/Golgi SNAREs resulted in significant reduction in parasite replication. Taken together, these studies provide further evidence that PVs acquire ER components by fusing with vesicles derived from the early secretory pathway; disruption of this interaction results in inhibition of the development of PVs as well as the limitation of parasite replication within infected cells.

Leishmania parasitophorous vacuoles interact continuously with the host cell's endoplasmic reticulum; parasitophorous vacuoles are hybrid compartments

Macrophages that express representative endoplasmic reticulum (ER) molecules tagged with green fluorescence protein were generated to assess the recruitment of ER molecules to Leishmania parasitophorous vacuoles (PVs). More than 90% of PVs harbouring Leishmania pifanoi or Leishmania donovani parasites recruited calnexin, to their PV membrane. An equivalent proportion of PVs also recruited the membrane-associated soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), Sec22b. Both ER molecules appeared to be recruited very early in the formation of nascent PVs. Electron microscopy analysis of infected Sec22b/YFP expressing cells confirmed that Sec22b was recruited to Leishmania PVs. In contrast to PVs, it was found that no more than 20% of phagosomes that harboured Zymosan particles recruited calnexin or Sec22b to their limiting phagosomal membrane. The retrograde pathway that ricin employs to access the cell cytosol was exploited to gain further insight into ER-PV interactions. Ricin was delivered to PVs in infected cells incubated with ricin. Incubation of cells with brefeldin A blocked the transfer of ricin to PVs. This implied that molecules that traffic to the ER are transferred to PVs. Moreover the results show that PVs are hybrid compartments that are composed of both host ER and endocytic pathway components.

Sec22b is a negative regulator of phagocytosis in macrophages

The endoplasmic reticulum (ER) is proposed to be a membrane donor for phagosome formation. In support of this, we have previously shown that the expression level of syntaxin 18, an ER-localized SNARE protein, correlates with phagocytosis activity. To obtain further insights into the involvement of the ER in phagocytosis we focused on Sec22b, another ER-localized SNARE protein that is also found on phagosomal membranes. In marked contrast to the effects of syntaxin 18, we report here that phagocytosis was nearly abolished in J774 macrophages stably expressing mVenus-tagged Sec22b, without affecting the cell surface expression of the Fc receptor or other membrane proteins related to phagocytosis. Conversely, the capacity of the parental J774 cells for phagocytosis was increased when endogenous Sec22b expression was suppressed. Domain analyses of Sec22b revealed that the R-SNARE motif, a selective domain for forming a SNARE complex with syntaxin18 and/or D12, was responsible for the inhibition of phagocytosis. These results strongly support the ER-mediated phagocytosis model and indicate that Sec22b is a negative regulator of phagocytosis in macrophages, most likely by regulating the level of free syntaxin 18 and/or D12 at the site of phagocytosis.

Regulated motion of glycoproteins revealed by direct visualization of a single cargo in the endoplasmic reticulum

The quality of cargo proteins in the endoplasmic reticulum (ER) is affected by their motion during folding. To understand how the diffusion of secretory cargo proteins is regulated in the ER, we directly analyze the motion of a single cargo molecule using fluorescence imaging/fluctuation analyses. We find that the addition of two N-glycans onto the cargo dramatically alters their diffusion by transient binding to membrane components that are confined by hyperosmolarity. Via simultaneous observation of a single cargo and ER exit sites (ERESs), we could exclude ERESs as the binding sites. Remarkably, actin cytoskeleton was required for the transient binding. These results provide a molecular basis for hypertonicity-induced immobilization of cargo, which is dependent on glycosylation at multiple sites but not the completion of proper folding. We propose that diffusion of secretory glycoproteins in the ER lumen is controlled from the cytoplasm to reduce the chances of aggregation.

Meltrin beta (ADAM19) mediates ectodomain shedding of Neuregulin beta1 in the Golgi apparatus: fluorescence correlation spectroscopic observation of the dynamics of ectodomain shedding in living cells

Membrane-anchored Neuregulin beta1 sheds its ectodomain as soluble factors. Two proteases that belong to a disintegrin and metalloprotease (ADAM) family are known to cleave Neuregulin beta1. One is tumor necrosis factor-alpha converting enzyme (TACE/ADAM17). The other is Meltrin beta (ADAM19). Against our expectation that shedding by ADAM proteases occurs at the cell surface, here we found that Meltrin beta mediates the ectodomain shedding of Neuregulin beta1 in the Golgi apparatus. Meltrin beta was localized in and around the Golgi apparatus in developing sensory neurons. Subcellular fractionation revealed that Meltrin beta generated soluble Neuregulin beta1 in Golgi-enriched fractions while TACE-cleaved Neuregulin beta1 was recovered in lighter fractions. To examine whether Meltrin beta-mediated ectodomain shedding occurs in the Golgi apparatus in living cells, we took advantage of different diffusion properties of cleavage products from those of membrane-anchored precursor proteins. Fluorescence correlation spectroscopy (FCS) is the most sensitive method to determine milli approximately submillisecond diffusion in vivo. Protease-active Meltrin beta caused a shift in autocorrelation function in FCS of green fluorescent protein (GFP)-tagged Neuregulin beta1 in the Golgi apparatus, suggesting a conversion of Neuregulin beta1 molecules from membrane-anchored to soluble forms in that organelle. The Golgi apparatus is a site of processing Neuregulin beta1 by Meltrin beta.

Involvement of syntaxin 18, an endoplasmic reticulum (ER)-localized SNARE protein, in ER-mediated phagocytosis

The endoplasmic reticulum (ER) is thought to play an important structural and functional role in phagocytosis. According to this model, direct membrane fusion between the ER and the plasma or phagosomal membrane must precede further invagination, but the exact mechanisms remain elusive. Here, we investigated whether various ER-localized SNARE proteins are involved in this fusion process. When phagosomes were isolated from murine J774 macrophages, we found that ER-localized SNARE proteins (syntaxin 18, D12, and Sec22b) were significantly enriched in the phagosomes. Fluorescence and immuno-EM analyses confirmed the localization of syntaxin 18 in the phagosomal membranes of J774 cells stably expressing this protein tagged to a GFP variant. To examine whether these SNARE proteins are required for phagocytosis, we generated 293T cells stably expressing the Fc gamma receptor, in which phagocytosis occurs in an IgG-mediated manner. Expression in these cells of dominant-negative mutants of syntaxin 18 or D12 lacking the transmembrane domain, but not a Sec22b mutant, impaired phagocytosis. Syntaxin 18 small interfering RNA (siRNA) selectively decreased the efficiency of phagocytosis, and the rate of phagocytosis was markedly enhanced by stable overexpression of syntaxin 18 in J774 cells. Therefore, we conclude that syntaxin 18 is involved in ER-mediated phagocytosis, presumably by regulating the specific and direct fusion of the ER and plasma or phagosomal membranes.

Involvement of a Novel Q-SNARE, D12, in Quality Control of the Endomembrane System

The cellular endomembrane system requires the proper kinetic balance of synthesis and degradation of its individual components, which is maintained in part by a specific membrane fusion apparatus. In this study, we describe the molecular properties of D12, which was identified from a mouse expression library. This C-terminal anchored membrane protein has sequence similarity to both a yeast soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE), Use1p/Slt1p, and a recently identified human syntaxin 18-binding protein, p31. D12 formed a tight complex with syntaxin 18 as well as Sec22b and bound to alpha-SNAP, indicating that D12 is a SNARE protein. Although the majority of D12 is located in the endoplasmic reticulum and endoplasmic reticulum-Golgi intermediate compartments at steady state, overexpression or knockdown of D12 had no obvious effects on membrane trafficking in the early secretory pathway. However, suppression of D12 expression caused rapid appearance of lipofuscin granules, accompanied by apoptotic cell death without the apparent activation of the unfolded protein response. The typical cause of lipofuscin formation is the impaired degradation of mitochondria by lysosomal degradative enzymes, and, consistent with this, we found that proper post-Golgi maturation of cathepsin D was impaired in D12-deficient cells. This unexpected observation was supported by evidence that D12 associates with VAMP7, a SNARE in the endosomal-lysosomal pathway. Hence, we suggest that D12 participates in the degradative function of lysosomes.

Reticulon 3 is involved in membrane trafficking between the endoplasmic reticulum and Golgi

Reticulons (RTNs) constitute a family of endoplasmic reticulum (ER)-associated proteins with a reticular distribution. Despite the implication of their neuronal isoforms in axonal regeneration, the function of their widely expressed isoforms is largely unknown. In this study, we examined the role of the ubiquitously expressed RTN3 in membrane trafficking. Ectopically expressed RTN3 exhibited heterogeneous patterns; filamentous, reticular, and granular distributions. The ER morphology changed accordingly. In cells where RTN3 displayed a filamentous/reticular distribution, protein transport between the ER and Golgi was blocked, and Golgi proteins were dispersed. In contrast, ERGIC-53, a marker for the ER-Golgi intermediate compartment, accumulated at the perinuclear region, and remained there even after cells were treated with agents that induce redistribution of Golgi proteins to the ER, indicating an inhibition of Golgi-to-ER transport of ERGIC-53. These results suggest that RTN3 plays a role in membrane trafficking in the early secretory pathway.

NVL2 is a nucleolar AAA-ATPase that interacts with ribosomal protein L5 through its nucleolar localization sequence

NVL (nuclear VCP-like protein), a member of the AAA-ATPase family, is known to exist in two forms with N-terminal extensions of different lengths in mammalian cells. Here, we show that they are localized differently in the nucleus; NVL2, the major species, is mainly present in the nucleolus, whereas NVL1 is nucleoplasmic. Mutational analysis demonstrated the presence of two nuclear localization signals in NVL2, one of which is shared with NVL1. In addition, a nucleolar localization signal was found to exist in the N-terminal extra region of NVL2. The nucleolar localization signal is critical for interaction with ribosomal protein L5, which was identified as a specific interaction partner of NVL2 on yeast two-hybrid screening. The interaction of NVL2 with L5 is ATP-dependent and likely contributes to the nucleolar translocation of NVL2. The physiological implication of this interaction was suggested by the finding that a dominant negative NVL2 mutant inhibits ribosome biosynthesis, which is known to take place in the nucleolus.

Implication of ZW10 in membrane trafficking between the endoplasmic reticulum and Golgi

ZW10, a dynamitin-interacting protein associated with kinetochores, is known to participate directly in turning off of the spindle checkpoint. In the present study, we show that ZW10 is located in the endoplasmic reticulum as well as in the cytosol during interphase, and forms a subcomplex with RINT-1 (Rad50-interacting protein) and p31 in a large complex comprising syntaxin 18, an endoplasmic reticulum-localized t-SNARE implicated in membrane trafficking. Like conventional syntaxin-binding proteins, ZW10, RINT-1 and p31 dissociated from syntaxin 18 upon Mg(2+)-ATP treatment in the presence of NSF and alpha-SNAP, whereas the subcomplex was not disassembled. Overexpression, microinjection and knockdown experiments revealed that ZW10 is involved in membrane trafficking between the endoplasmic reticulum and Golgi. The present results disclose an unexpected role for a spindle checkpoint protein, ZW10, during interphase.

Determinants of liposome fusion mediated by synaptic SNARE proteins

Synaptic exocytosis requires the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins syntaxin 1, SNAP-25, and synaptobrevin (VAMP). Assembly of the SNAREs into a stable core complex is supposed to catalyze membrane fusion, and proteoliposomes reconstituted with synaptic SNARE proteins spontaneously fuse with each other. We now show that liposome fusion mediated by synaptic SNAREs is inhibited by botulinum neurotoxin E (BoNT/E) but can be rescued by supplementing the C-terminal portion of SNAP-25. Furthermore, fusion is prevented by a SNAP-25-specific antibody known to block exocytosis in chromaffin cells, and it is competed for by soluble fragments of the R-SNAREs synaptobrevin 2, endobrevin/VAMP-8, and tomosyn. No accumulation of clustered vesicles is observed during the reaction. Rapid artificial clustering of SNARE-containing proteoliposomes enhances the fusion rate at low but not at saturating liposome concentrations. We conclude that the rate of liposome fusion is dominated by the intrinsic properties of the SNAREs rather than by the preceding docking step.

The R-SNARE motif of tomosyn forms SNARE core complexes with syntaxin 1 and SNAP-25 and down-regulates exocytosis

Tomosyn is a 130-kDa syntaxin-binding protein that contains a large N-terminal domain with WD40 repeats and a C-terminal domain homologous to R-SNAREs. Here we show that tomosyn forms genuine SNARE core complexes with the SNAREs syntaxin 1 and SNAP-25. In vitro studies with recombinant proteins revealed that complex formation proceeds from unstructured monomers to a stable four-helical bundle. The assembled complex displayed features typical for SNARE core complexes, including a profound hysteresis upon unfolding-refolding transitions. No stable complexes were formed between the SNARE motif of tomosyn and either syntaxin or SNAP-25 alone. Furthermore, both native tomosyn and its isolated C-terminal domain competed with synaptobrevin for binding to endogenous syntaxin and SNAP-25 on inside-out sheets of plasma membranes. Tomosyn-SNARE complexes were effectively disassembled by the ATPase N-ethylmaleimide-sensitive factor together with its cofactor alpha-SNAP. Moreover, the C-terminal domain of tomosyn was as effective as the cytoplasmic portion of synaptobrevin in inhibiting evoked exocytosis in a cell-free preparation derived from PC12 cells. Similarly, overexpression of tomosyn in PC12 cells resulted in a massive reduction of exocytosis, but the release parameters of individual exocytotic events remained unchanged. We conclude that tomosyn is a soluble SNARE that directly competes with synaptobrevin in the formation of SNARE complexes and thus may function in down-regulating exocytosis.

Gaf-1b is an alternative splice variant of Gaf-1/Rip11

Gaf-1/Rip11 encoded by the clone KIAA0857 participates in endosomal recycling through the interaction with both gamma-SNAP, a member of the soluble NSF attachment protein family, and a small GTPase, Rab11. Gaf-1/Rip11 and other Rab11-interacting proteins constitute a novel protein family that is involved in the endocytic pathways. Here we report the presence of an alternative splice variant of Gaf-1/Rip11 named Gaf-1b. Gaf-1b also interacts with gamma-SNAP and is expressed ubiquitously in tissues except for liver. Subcellular fractionation analysis revealed that Gaf-1b, as well as Gaf-1/Rip11, is mainly present in the microsomal fraction. Overexpression of Gaf-1b, like that of Gaf-1/Rip11, affected the morphology of recycling endosomes. These results suggest that Gaf-1b has a similar function to Gaf-1/Rip11.

Mapping of functional domains of gamma-SNAP

gamma-Soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (gamma-SNAP) is capable of stabilizing a 20 S complex consisting of NSF, alpha-SNAP, and SNAP receptors (SNAREs), but its function in vesicular transport is not fully understood. Our two-hybrid analysis revealed that gamma-SNAP, unlike alpha-SNAP, interacts directly with NSF, as well as Gaf-1/Rip11, but not with SNAREs. Gaf-1/Rip11 is a gamma-SNAP-associated factor that belongs to the Rab11-interacting protein family. To gain insight into the molecular basis for the interactions of gamma-SNAP with NSF and Gaf-1/Rip11, we determined the regions of the three proteins involved in protein-protein interactions. gamma-SNAP bound to NSF via its extreme C-terminal region, and the full-length NSF was needed to interact with gamma-SNAP. Both the N-terminal and C-terminal regions of gamma-SNAP were required for the binding to Gaf-1/Rip11. Gaf-1/Rip11 bound to gamma-SNAP via its C-terminal domain comprising a putative coiled-coil region. Although the C-terminal domain of Gaf-1/Rip11 also interacts with Rab11, the binding of gamma-SNAP and Rab11 to Gaf-1/Rip11 was not mutually exclusive. Rather, Gaf-1/Rip11 was capable of serving a link between gamma-SNAP and Rab11. A complex comprising gamma-SNAP and Gaf-1/Rip11 was disassembled in a process coupled to NSF-mediated ATP hydrolysis, suggesting that the interaction between gamma-SNAP and Gaf-1/Rip11 is of functional significance.

SVIP is a novel VCP/p97-interacting protein whose expression causes cell vacuolation

VCP/p97 is involved in a variety of cellular processes, including membrane fusion and ubiquitin-dependent protein degradation. It has been suggested that adaptor proteins such as p47 and Ufd1p confer functional versatility to VCP/p97. To identify novel adaptors, we searched for proteins that interact specifically with VCP/p97 by using the yeast two-hybrid system, and discovered a novel VCP/p97-interacting protein named small VCP/p97-interacting protein (SVIP). Rat SVIP is a 76-amino acid protein that contains two putative coiled-coil regions, and potential myristoylation and palmitoylation sites at the N terminus. Binding experiments revealed that the N-terminal coiled-coil region of SVIP, and the N-terminal and subsequent ATP-binding regions (ND1 domain) of VCP/p97, interact with each other. SVIP and previously identified adaptors p47 and ufd1p interact with VCP/p97 in a mutually exclusive manner. Overexpression of full-length SVIP or a truncated mutant did not markedly affect the structure of the Golgi apparatus, but caused extensive cell vacuolation reminiscent of that seen upon the expression of VCP/p97 mutants or polyglutamine proteins in neuronal cells. The vacuoles seemed to be derived from endoplasmic reticulum membranes. These results together suggest that SVIP is a novel VCP/p97 adaptor whose function is related to the integrity of the endoplasmic reticulum.

Determination of functional regions of p125, a novel mammalian Sec23p-interacting protein

The Sec23p-Sec24p complex is a component of coat protein II-coated vesicles involved in protein export from the endoplasmic reticulum. We previously identified a novel Sec23p-interacting protein, p125, which consists of 1000 amino acids and comprises a proline-rich region and a phospholipase A(1) homology region. p125, when ectopically expressed in cultured cells, localizes to endoplasmic reticulum-Golgi intermediate regions. In the present study we showed that expressed p125 principally colocalizes with p115 and GM130, both of which are involved in vesicle tethering to Golgi membranes. Next, we determined the functional regions of p125 by expressing a p125 series with deletions. The results showed that the proline-rich region (residues 135-259) is responsible for the binding to Sec23p. For the correct localization of p125, a region (residues 135-1000) comprising both the proline-rich and phospholipase A(1) homology regions was required.

Implication of sphingolipid metabolism in the stability of the Golgi apparatus

We examined the effects of short chain and long chain ceramides on the stability of the Golgi apparatus. Short chain ceramides, C(2)- and C(6)-ceramides, blocked brefeldin A-induced Golgi disassembly without affecting the rapid release of Golgi coat proteins, whereas they did not inhibit brefeldin A-induced tubulation of endosomes. Both short chain ceramides also retarded Golgi disassembly induced by nordihydroguaiaretic acid and nocodazole, suggesting that they stabilize the Golgi apparatus. In contrast to short chain ceramides, natural long chain ceramides, when incorporated into cells or formed within cells upon treatment with sphingomyelinase or metabolic inhibitors, enhanced brefeldin A-induced Golgi disassembly. These results suggest that sphingolipid metabolism is implicated in the stability of the Golgi apparatus.

N-ethylmaleimide-sensitive factor is associated with the nuclear envelope

N-Ethylmaleimide-sensitive factor (NSF) is an ATPase involved in many membrane fusion events within the exocytic and endocytotic pathways. In the present study we showed that NSF is associated with the nuclear envelope. Golgi-associated NSF was released from membranes upon incubation with Mg(2+)-ATP, reflecting the disassembly of a complex consisting of NSF, soluble NSF attachment proteins (SNAPs), and SNAP receptors (SNAREs). In contrast nuclear envelope-associated NSF in interphase cells was not released by the same treatment. During mitosis, however, it was released from nuclear membranes by Mg(2+)-ATP. These results suggest that the binding mode of nuclear membrane-associated NSF changes during the cell cycle.

Syntaxin 18, a SNAP receptor that functions in the endoplasmic reticulum, intermediate compartment, and cis-Golgi vesicle trafficking

Members of the syntaxin family are target-soluble N-ethylmaleimide-sensitive factor-attachment protein receptors involved in vesicle docking and/or fusion within the exocytic and endocytotic pathways. By using the yeast two-hybrid system, we have identified a novel member of the syntaxin family, syntaxin 18, that binds to alpha-soluble N-ethylmaleimide-sensitive factor-attachment protein. Subcellular fractionation and immunocytochemical analysis revealed that syntaxin 18 is principally located in the endoplasmic reticulum. We examined the effect of overexpression of FLAG-tagged syntaxin 18 and a mutant lacking the N-terminal 81 amino acid residues on protein transport and organelles in the early secretory pathway. Both expressed proteins localized to the endoplasmic reticulum, and the expressed FLAG-syntaxin 18 caused remarkable aggregation of endoplasmic reticulum membranes. Although expression of the FLAG-syntaxin 18 lacking the N-terminal region produced less effect on the morphology of the endoplasmic reticulum, dispersion of the endoplasmic reticulum-Golgi intermediate compartment and cis-Golgi was elicited. Moreover, overexpression of the FLAG-syntaxin 18 mutant inhibited protein export from the endoplasmic reticulum. These results taken together suggest that syntaxin 18 functions in transport between the endoplasmic reticulum and Golgi.

Regulation of the golgi structure by the alpha subunits of heterotrimeric G proteins

Disassembly of the Golgi apparatus is elicited by the action of nordihydroguaiaretic acid (NDGA) and this disassembly is prevented by the activation of heterotrimeric G proteins. In the present study we showed that overexpression of Galpha(z) or Galpha(i2) significantly suppresses the disassembly of the Golgi apparatus induced by NDGA. Overexpression of Gbeta(1)gamma(2), on the other hand, had no effect on NDGA-induced Golgi disassembly. Galpha(z) neither blocked Golgi disassembly induced by brefeldin A or nocodazole, nor interfered with protein transport, suggesting its specificity on the action of NDGA. Our results suggest that the alpha subunits of heterotrimeric G proteins are responsible for the maintenance of the Golgi structure.

A hydrophobic region locating at the center of fibroblast growth factor-9 is crucial for its secretion

Fibroblast growth factor (FGF)-9 is a glycosylated neurotrophic polypeptide highly expressed in brain. The mechanism for its secretion from expressing cells is unclear, because its primary structure lacks a cleavable signal sequence. We, therefore, investigated the mechanism and structural requirements for secretion of FGF-9. As with other secreted proteins, in vitro translation of FGF-9 was inhibited by signal recognition particle, which binds to the signal sequence. When translated in vitro, full-length FGF-9 was translocated into microsomes, glycosylated, and protected from trypsin digestion. By using various FGF-9 deletion mutants, we found that two hydrophobic domains, located at the N terminus and at the center of the FGF-9 primary structure, were crucial for translocation. Examination of various point mutants revealed that local hydrophobicity of the central hydrophobic domain, but not the N terminus, was crucial for translocation. Analogous results were obtained with respect to FGF-9 secretion from transfectant cells. Upon deletion of the complete sequence preceding it, the previously uncleavable hydrophobic domain appeared to serve as a cleavable signal sequence. Our results suggest that nascent FGF-9 polypeptides translocate into endoplasmic reticulum without peptide cleavage via a co-translational pathway in which both the N terminus and the central hydrophobic domain are important; thereafter, FGF-9 is glycosylated and secreted.

p125 is a novel mammalian Sec23p-interacting protein with structural similarity to phospholipid-modifying proteins

COPII-coated vesicles are involved in protein transport from the endoplasmic reticulum to the Golgi apparatus. COPII consists of three parts: Sar1p and the two protein complexes, Sec23p-Sec24p and Sec13p-Sec31p. Using a glutathione S-transferase fusion protein with mouse Sec23p, we identified a novel mammalian Sec23p-interacting protein, p125, which is clearly distinct from Sec24p. The N-terminal region of p125 is rich in proline residues, and the central and C-terminal regions exhibit significant homology to phospholipid-modifying proteins, especially phosphatidic acid preferring-phospholipase A1. We transiently expressed p125 and mouse Sec23p in mammalian cells and examined their interaction. The results showed that the N-terminal region of p125 is important for the interaction with Sec23p. We confirmed the interaction between the two proteins by a yeast two-hybrid assay. Overexpression of p125, like that of mammalian Sec23p, caused disorganization of the endoplasmic reticulum-Golgi intermediate compartment and Golgi apparatus, suggesting its role in the early secretory pathway.

Hypothetical protein KIAA0079 is a mammalian homologue of yeast Sec24p

The Sec23p-Sec24p complex is a component of COPII-coated vesicles that mediate protein transport from the endoplasmic reticulum in yeast. The mammalian hypothetical protein KIAA0079 (KIAA0079p) exhibits sequence similarity to yeast Sec24p. KIAA0079p was co-eluted with mammalian Sec23p on gel filtration. In vitro binding experiments revealed that the C-terminal region of KIAA0079p binds to the N-terminal region of mammalian Sec23p. Overexpression of KIAA0079p caused a defect in protein export from the endoplasmic reticulum. These results support the idea that KIAA0079p is a functional homologue of yeast Sec24p.

ADP-ribosylation factor-1 is sensitive to N-ethylmaleimide

The treatment of normal rat kidney cells with N-ethylmaleimide caused the release of beta-COP, a component of coatomer, from the Golgi apparatus without causing disassembly of the organelle. The release of beta-COP, which was not due to depolymerization of microtubules, was markedly blocked by the activation of GTP-binding proteins by aluminum fluoride or a nonhydrolyzable analogue of GTP. To determine which component is N-ethylmaleimide-sensitive, we reconstituted the recruitment of coatomer from the bovine brain cytosol onto the Golgi apparatus in digitonin-permeabilized cells. In cells treated with N-ethylmaleimide before permeabilization, beta-COP was still recruited onto the Golgi apparatus. In contrast, beta-COP was not recruited when N-ethylmaleimide-treated bovine brain cytosol was used. These results suggest that the N-ethylmaleimide-sensitive factor(s) are present in the cytosol. It is known that coatomer and ADP-ribosylation factor-1 (ARF1) are the only cytoplasmic proteins needed for the assembly of Golgi-derived coated vesicles. N-Ethylmaleimide treatment of a coatomer-rich fraction did not affect the binding of beta-COP to the Golgi apparatus, whereas the same treatment of an ARF-rich fraction abolished beta-COP binding. Similar results were obtained using purified recombinant ARF1. Concomitant with inactivation, 0.85 mol of N-ethylmaleimide was incorporated into 1 mol of ARF1. ARF1 contains only one cysteine residue (Cys-159), which is located near the base moiety of the bound guanine nucleotide.

NSF is required for the brefeldin A-promoted disassembly of the Golgi apparatus

N-Ethylmaleimide-sensitive factor (NSF) is required for multiple pathways of vesicle-mediated protein transport. Microinjection of a monoclonal anti-NSF antibody almost completely blocked brefeldin A-promoted Golgi disassembly without affecting the rapid release of beta-COP, a subunit of the Golgi coat proteins (COPI), from the Golgi apparatus. Similar results were obtained using a dominant-negative NSF which is known to compete with endogenous NSF. The present results suggest that an NSF-mediated step is present in the brefeldin A-promoted disassembly of the Golgi apparatus.

Possible involvement of heterotrimeric G proteins in the organization of the Golgi apparatus

Nordihydroguaiaretic acid (NDGA) caused disassembly of the Golgi apparatus of NRK cells in a dose-, time-, and energy-dependent manner but not in a microtubule-dependent manner. In contrast to brefeldin A, NDGA did not cause release of beta-COP, a component of Golgi-derived vesicles. However, NDGA-induced disassembly was blocked by AlF4-, an activator of the heterotrimeric but not the small GTP-binding proteins. In digitonin-permeabilized cells, guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) as well as AlF4- blocked the NDGA-promoted disassembly of the Golgi apparatus, and Gbetagamma (betagamma subunits of heterotrimeric G proteins) reversed this effect. Our present results suggest the possible involvement of heterotrimeric G proteins in the organization of the Golgi apparatus.

The hydrophobic region of signal peptides is a determinant for SRP recognition and protein translocation across the ER membrane

Newly recognized mammalian secretory proteins such as preprolactin are translocated across the endoplasmic reticulum (ER) in a signal recognition particle (SRP)-dependent manner. Recent studies revealed that there are two recognition steps for signal peptides during this translocation. The first step is recognition by SRP, which results in elongation arrest, and the second step is interaction between signal peptides and the translocation channel embedded in the ER membrane. To determine the roles of the hydrophobic region of signal peptides in the recognition by SRP and the membrane-embedded translocation machinery, we constructed chimeric proteins consisting of the mature region of preprolactin and signal peptides containing different numbers of leucine residues. The translocation of these chimeric proteins was completely dependent on SRP, and the efficiency increased as the number of leucine residues increased up to 10 and then decreased. Although the efficiency of elongation arrest also increased as the number of leucine residues increased up to 10, it only slightly decreased as the number increased up to 20. Similar results were obtained when the hydrophobic region was replaced by alternate leucine and alanine residues, except that the most efficient translocation occurred when the number was 14. Taken together, the present results suggests that the total hydrophobicity of the hydrophobic region of signal peptides is a determinant for recognition by both SRP and the membrane-embedded translocation machinery, although the specificities of the two signal recognition steps are slightly different from each other.

Sequence requirements for endoproteolytic processing of precursor proteins by furin: transfection and in vitro experiments

We have recently presented some sequence rules that govern the endoproteolytic processing of precursor proteins within the constitutive secretory pathway [Watanabe, T. et al. (1992) J. Biol. Chem. 267, 8270-8274; Watanabe, T. et al. (1993) FEBS Lett. 320, 215-218]. This cleavage seems to be catalyzed by furin, a mammalian homologue of the yeast processing endoprotease Kex2. In this study, not only by coexpression of furin and substrate precursors in a furin-deficient cell line but also by in vitro experiments using purified recombinant furin and substrate precursors, we determined the substrate specificity of furin. The sequence requirements of furin determined by the coexpression and in vitro experiments were essentially the same, and were consistent with the sequence rules for precursor cleavage within the constitutive secretory pathway.

Evidence for involvement of furin in cleavage and activation of diphtheria toxin

Proteolytic cleavage (nicking) of diphtheria toxin (DT) in the 14-amino acid loop subtended by the disulfide bond between Cys186 and Cys201 is required for the cytotoxic action of DT. The loop includes the consensus motif for cleavage by a membrane-anchored protease, furin. We found that a soluble form of furin cleaves intact DT between Arg103 and Ser194 in vitro. LoVo cells, a human colon carcinoma cell line, do not produce functional furin. We show here that intact DT is not cleaved by LoVo cells. The cells are resistant to intact DT, although they are sensitive to DT nicked by furin before it is added to the medium. When intact DT is added to LoVo/Fur1 cells, a stable transfectant of LoVo cells expressing mouse furin, nicked DT associated with the cells is observed. LoVo/Fur1 cells are sensitive to both intact and nicked DT. These results indicate that furin is involved in the toxicity of intact DT. Bafilomycin A1, an inhibitor of intracellular vesicle acidification, did not inhibit cleavage of intact DT by LoVo/Fur1 or Vero cells, indicating that cleavage can proceed in a neutral environment. Inhibitors of endocytosis decreased DT cleavage but did not eliminate it. We also found a small amount of nicked DT in the culture medium. These results may indicate that intact DT is cleaved age by cell-associated furin on the cell surface as well as in endocytotic vesicles.

A mutation of furin causes the lack of precursor-processing activity in human colon carcinoma LoVo cells

Furin has been proposed to be the endoprotease responsible for precursor cleavage at Arg-X-Lys/Arg-Arg (RXK/RR) sites within the constitutive secretory pathway. However, there was a possibility that other protease(s) is involved in this cleavage. We here characterized furin in human colon carcinoma LoVo cells, since these cells lacked the endogenous processing activity toward RXK/RR sites and recovered the activity by transfection of furin cDNA. Furin cDNA cloned from LoVo cells had one nucleotide deletion in the region covering the homo B domain which is essential for the endoproteolytic activity. LoVo cells transfected with a furin construct with the mutation showed no activity. Based on these data, we conclude that furin is the endoprotease that is involved in the precursor cleavage at RXK/RR sites within the constitutive secretory pathway.

Proteolytic processing of the hepatocyte growth factor/scatter factor receptor by furin

The hepatocyte growth factor/scatter factor (HGF/SF) receptor consists of an alpha- and a beta-subunit, which are derived from a single-chain precursor by endoproteolytic processing. The precursor is not proteolytically processed in LoVo colon carcinoma cells. The uncleaved receptor immunopurified from the cells was cleaved in vitro by furin. Furthermore, the HGF/SF receptor was proteolytically processed in LoVo cells transfected with furin cDNA. These results indicate that furin is a processing endoprotease for the HGF/SF receptor. Tyrosine autophosphorylation of the uncleaved receptor was induced by HGF/SF, and the growth of the cells expressing the uncleaved receptor was stimulated by HGF/SF, indicating that the proteolytic processing of the receptor is not essential for the signal transduction of HGF/SF.

Proteolytic cleavages of proalbumin and complement Pro-C3 in vitro by a truncated soluble form of furin, a mammalian homologue of the yeast Kex2 protease

We have recently purified and characterized a truncated soluble form of furin from which the predicted transmembrane domain and cytoplasmic tail were deleted (Hatsuzawa, K., Nagahama, M., Takahashi, S., Takada, K., Murakami, K., and Nakayama, K. (1992) J. Biol. Chem. 267, 16094-16099). Our results showed that furin resembles the yeast Kex2 protease with respect to both its enzymic properties and substrate specificity. Here we demonstrate that the soluble form of furin is capable of converting the precursors of albumin and the third component of complement (proalbumin and pro-C3, respectively) in vitro to mature proteins. Thus furin mimics the Ca(2+)-dependent proalbumin and pro-C3 convertases found in the Golgi membranes (Brennan, S. O., and Peach, R. J. (1988) FEBS Lett. 229, 167-170; Oda, K. (1992) J. Biol. Chem. 267, 17465-17471). Furthermore we show that the variant alpha 1-antitrypsin Pittsburgh, which is a specific inhibitor of the Golgi proalbumin convertase, inhibits not only the Golgi pro-C3 convertase, but also the soluble furin. These results suggest a role for furin in the cleavage of proproteins transported via the constitutive pathway.

Purification and characterization of furin, a Kex2-like processing endoprotease, produced in Chinese hamster ovary cells

Furin, a mammalian homolog of the yeast Kex2 protease, is associated with Golgi membranes and is involved in cleavage of precursor proteins at sites marked by the Arg-X-Lys/Arg-Arg (RXK/RR) motif. We have recently shown that a furin mutant lacking the transmembrane domain can be secreted from cDNA-transfected cells with proteolytic activity for the fluorogenic peptide t-butoxycarbonyl-Arg-Val-Arg-Arg-4-methylcoumarin-7- amide. In this study, we purified and characterized the recombinant furin from the conditioned medium of these cells. Furin was purified as a mixture of 83- and 81-kDa forms and a 96-kDa form. The differences in molecular mass were not due to differences in molecular mass were not due to differences in glycosylation. Moreover, all forms had the same NH2-terminal sequence beginning at the residue after the Arg-Ala-Lys-Arg sequence. These data suggest that the three different forms may be produced by differential COOH-terminal processing of a furin molecule and that mature furin may be autocatalytically produced. Both enzyme preparations showed a pH optimum at 7.0, required Ca2+ for the activity, and showed essentially the same inhibitor profile. These properties resembled those of the Kex2 protease. Both preparations efficiently cleaved fluorogenic peptides with an RXK/RR sequence and moderately cleaved a peptide with an RXXR sequence, but did not cleave dibasic peptides. The sequence requirements determined in vitro were compatible with those determined by expression studies in cultured cells. These data unequivocally demonstrate that furin is an endogenous cellular protease responsible for cleavage of precursor proteins mainly at RXK/RR sites.

Consensus sequence for precursor processing at mono-arginyl sites. Evidence for the involvement of a Kex2-like endoprotease in precursor cleavages at both dibasic and mono-arginyl sites

Many peptide hormones and neuropeptides are produced from larger, inactive precursors through endoproteolysis at sites usually marked by paired basic residues (primarily Lys-Arg and Arg-Arg), or occasionally by a monobasic residue (primarily Arg). Based upon data concerning processing of prorenin and its mutants around the native Lys-Arg cleavage site expressed in mouse pituitary AtT-20 cells, we present the following sequence rules that govern mono-arginyl cleavages: (a) a basic residue at the fourth (position -4) or the sixth (position -6) residue upstream of the cleavage site is required, (b) at position -4, Arg is more favorable than Lys, and (c) at position 1, a hydrophobic aliphatic residue is not suitable. These rules are compatible with those proposed by comparison of precursor sequences around mono-arginyl cleavage sites. We also provide evidence that precursor cleavages at mono-arginyl and dibasic sites can be catalyzed by the same Kex2-like processing endoprotease, PC1/PC3.