Proteomic analysis of microglia-derived exosomes: metabolic role of the aminopeptidase CD13 in neuropeptide catabolism

Vesicle transport is a fundamental mechanism of communication in the CNS. In this study we characterized a novel type of vesicle released by murine brain microglial cells: microglial exosomes. Analysis of their protein content identified several enzymes, chaperones, tetraspanins, and membrane receptors previously reported in B cells and dendritic cell-derived exosomes. Additionally, microglia-derived exosomes expressed the aminopeptidase CD13 and the lactate transporter MCT-1. Exosomal CD13 was metabolically active in cleaving leucine- and methionine-enkephalins peptides by releasing the N-terminal tyrosine. Cleaved neuropeptides were unable to bind to the neuronal opioid receptor as assessed by cAMP response. Microglial exosomal vesicles may represent an important, previously unrecognized, cellular communication system in an organ in which cell motility is highly restricted.

IL-1 Induces Vesicular Secretion of IL-6 without Degranulation from Human Mast Cells

Fc epsilon RI cross-linkage in mast cells results in release of granule-associated mediators, such as histamine and proteases, as well as the production of numerous cytokines, including IL-6. Mast cells have been increasingly implicated in inflammatory processes where explosive degranulation is not commonly observed. Here, we show that IL-1 stimulates secretion of IL-6 without release of the granule-associated protease tryptase in normal human umbilical cord blood-derived mast cells (hCBMCs). IL-6 secretion stimulated by IL-1 in hCBMCs is potentiated by priming with IL-4 and reflects the higher levels of IL-6 secreted from human leukemic mast cell line (HMC-1). Stimulating HMC-1 cells by both IL-1 and TNF-alpha results in synergistic secretion of IL-6. IL-6 is de novo synthesized, as its secretion is blocked by inhibitors of transcription or protein synthesis. IL-1 does not increase intracellular calcium ion levels in either hCBMCs or HMC-1 cells, and IL-6 stimulation proceeds in the absence of extracellular calcium ions. Ultrastructural Immunogold localization shows that IL-6 is excluded from the secretory granules and is compartmentalized in 40- to 80-nm vesicular structures. Selective secretion of IL-6 from mast cells appears distinct from degranulation and may contribute to the development of inflammation, where the importance of IL-6 has been recognized.

Regulation of activity and localization of the WNK1 protein kinase by hyperosmotic stress

Mutations within the WNK1 (with-no-K[Lys] kinase-1) gene cause Gordon's hypertension syndrome. Little is known about how WNK1 is regulated. We demonstrate that WNK1 is rapidly activated and phosphorylated at multiple residues after exposure of cells to hyperosmotic conditions and that activation is mediated by the phosphorylation of its T-loop Ser382 residue, possibly triggered by a transautophosphorylation reaction. Activation of WNK1 coincides with the phosphorylation and activation of two WNK1 substrates, namely, the protein kinases STE20/SPS1-related proline alanine-rich kinase (SPAK) and oxidative stress response kinase-1 (OSR1). Small interfering RNA depletion of WNK1 impairs SPAK/OSR1 activity and phosphorylation of residues targeted by WNK1. Hyperosmotic stress induces rapid redistribution of WNK1 from the cytosol to vesicular structures that may comprise trans-Golgi network (TGN)/recycling endosomes, as they display rapid movement, colocalize with clathrin, adaptor protein complex 1 (AP-1), and TGN46, but not the AP-2 plasma membrane-coated pit marker nor the endosomal markers EEA1, Hrs, and LAMP1. Mutational analysis suggests that the WNK1 C-terminal noncatalytic domain mediates vesicle localization. Our observations shed light on the mechanism by which WNK1 is regulated by hyperosmotic stress.

Leucine-rich repeat kinase 2 associates with lipid rafts

Leucine-Rich Repeat Kinase 2 (LRRK2) is a causative gene for the autosomal dominant form of Parkinson's disease (PD). The gene encodes the approximately 280 kDa LRRK2 protein composed of domains such as leucine-rich repeats, Ras in complex proteins (Roc) followed by C-terminal of Roc (COR), mitogen-activated protein kinase kinase kinase (MAPKKK) and WD40. However, the normal function of the protein as well as its contribution to the pathogenesis of PD remains largely unknown. Here we describe the localization of LRRK2 in Golgi apparatus, plasma membrane and synaptic vesicles in cultured cells including mouse primary neurons. The membrane association of LRRK2 resists solubilization by ice-cold 1% Triton X-100, indicating its association through lipid rafts. To investigate whether mutations found in PD patients affect the localization of LRRK2, we transfected various LRRK2 mutants into cultured cells and performed fractionation experiments. Unexpectedly, the mutants are collected in both membrane and soluble fractions in a manner similar to wild type (WT). I2020T mutant LRRK2 associates with lipid rafts, similar to the WT. The lipid raft association of LRRK2 mutants as well as WT LRRK2 suggests that alteration of LRRK2 function on lipid rafts contributes to the pathogenesis of PD.

Microtubule-dependent matrix metalloproteinase-2/matrix metalloproteinase-9 exocytosis: prerequisite in human melanoma cell invasion

Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases that cleave and degrade a wide spectrum of extracellular matrix components. By enhancing turnover of extracellular matrix, MMP activity is also known to play a key role in tumor cell invasion. Because extracellular protease activity requires efficient release of these proteases to the cellular surface, we investigated storage, transport, and exocytosis of MMP-2 and MMP-9 in human melanoma cells using immunofluorescence, electrical, and biochemical techniques. Immunolabeling of melanoma cells with antibodies specific for MMP-2 and MMP-9 led to the identification of two distinct populations of small cytoplasmatic vesicles containing MMP-2 or MMP-9, respectively. In combination with alpha-tubulin-specific antibodies, both vesicle populations were found to be aligned along the microtubular network. Moreover, the molecular motor protein kinesin is shown to be localized on most of these vesicles, providing evidence that the identified vesicles are actively propelled along microtubules toward the plasma membrane. The functional relevance of these findings is demonstrated using low dosage (5.9 nmol/L) of paclitaxel to affect the microtubular function of melanoma cells. Although cell proliferation is not altered, paclitaxel treatment impairs secretion of MMP-2/MMP-9 and significantly reduces invasive activity in our new cell invasion assay. In conclusion, we demonstrate in melanoma cells that microtubule-dependent traffic of MMP-containing vesicles and exocytosis are critical steps for invasive behavior and therefore are potential targets for specific antitumor drugs.

The molecular basis of differential subcellular localization of C2 domains of protein kinase C-alpha and group IVa cytosolic phospholipase A2

The C2 domain is a Ca(2+)-dependent membrane-targeting module found in many cellular proteins involved in signal transduction or membrane trafficking. C2 domains are unique among membrane targeting domains in that they show a wide range of lipid selectivity for the major components of cell membranes, including phosphatidylserine and phosphatidylcholine. To understand how C2 domains show diverse lipid selectivity and how this functional diversity affects their subcellular targeting behaviors, we measured the binding of the C2 domains of group IVa cytosolic phospholipase A(2) (cPLA(2)) and protein kinase C-alpha (PKC-alpha) to vesicles that model cell membranes they are targeted to, and we monitored their subcellular targeting in living cells. The surface plasmon resonance analysis indicates that the PKC-alpha C2 domain strongly prefers the cytoplasmic plasma membrane mimic to the nuclear membrane mimic due to high phosphatidylserine content in the former and that Asn(189) plays a key role in this specificity. In contrast, the cPLA(2) C2 domain has specificity for the nuclear membrane mimic over the cytoplasmic plasma membrane mimic due to high phosphatidylcholine content in the former and aromatic and hydrophobic residues in the calcium binding loops of the cPLA(2) C2 domain are important for its lipid specificity. The subcellular localization of enhanced green fluorescent protein-tagged C2 domains and mutants transfected into HEK293 cells showed that the subcellular localization of the C2 domains is consistent with their lipid specificity and could be tailored by altering their in vitro lipid specificity. The relative cell membrane translocation rate of selected C2 domains was also consistent with their relative affinity for model membranes. Together, these results suggest that biophysical principles that govern the in vitro membrane binding of C2 domains can account for most of their subcellular targeting properties.

The first stage of transforming growth factor beta1 activation is release of the large latent complex from the extracellular matrix of growth plate chondrocytes by matrix vesicle stromelysin-1 (MMP-3)

Transforming growth factor beta-1 (TGF-beta1) is secreted in a biologically inactive form and stored in the extracellular matrix as a 290 kDa complex consisting of the mature TGF-beta1 homodimer (Mr 25 kDa), the latency-associated peptide (LAP; Mr 75 kDa), and the latent TGF-beta1 binding protein-1 (LTBP1; Mr 190 kDa). Latent TGF-beta1, composed of these three components, is known as the "large latent TGF-beta1 complex." In contrast, latent TGF-beta1 without LTBP1 is known as "small latent TGF-beta1." For all latent forms, dissociation of the TGF-beta1 homodimer from LAP is necessary for growth factor activation and acquisition of biological activity. Matrix vesicles produced by growth plate chondrocytes contain matrix metalloproteinases that can activate small latent TGF-beta1. The enzyme responsible for this is matrix metalloproteinase-3 (MMP-3), although matrix vesicles also contain MMP-2 and plasminogen activator. The present study tested the hypothesis that matrix vesicle enzymes are also involved in the release of the large latent TGF-beta1 complex stored in the extracellular matrix. Matrix vesicles were isolated from cultures of resting zone and growth zone chondrocytes and metalloproteinases present in the matrix vesicles extracted with guanidine-HCl. Chondrocyte extracellular matrices were prepared by lysing confluent cultures and removing the lysed cells. The matrices were incubated with matrix vesicle extracts and the release of total and active TGF-beta1 was determined. To determine if MMP-2 or MMP-3 was involved in the release, matrix vesicle extracts were preincubated with anti-MMP-2 antibody or anti-MMP-3 antibody to selectively deplete the enzyme activity. Matrices were also treated with rhMMP-2 or rhMMP-3. To determine the identity of the released protein(s), digests were separated on SDS-polyacrylamide gels and Western blotting analysis was performed using a specific antibody to LTBP1. Matrix vesicle extracts released both active and total (=latent + active) TGF-beta1 in a time-dependent manner, with peak release after 1 hour of incubation. The amount of total TGF-beta1 released was 10 times higher than the release of active TGF-beta1. The effect of the matrix vesicle extracts was dose-dependent; in addition, the amount and ratio of active to total TGF-b1 released was very similar, irrespective of the source of matrix or matrix vesicle extracts. Pre-incubation of matrix vesicle extracts with anti-MMP-3 antibody blocked the release of active and total TGF-beta1, whereas pre-incubation with pre-immune IgG or anti-MMP-2 antibody had no effect. The addition of rhMMP-3, but not rhMMP-2, caused a dose-dependent increase in the release of total, but not active, TGF-beta1. Western analysis confirmed that both matrix vesicle extracts and rhMMP-3 released the large latent TGF-beta1 complex from the matrix. In addition to the expected 290, 230, and 190 kDa bands, samples run without reduction also contained proteins of molecular weights 110 and 50 kDa that reacted with the anti-LTBP1 antibody. When these same samples were electrophoresed after reduction, the high molecular weight immunoreactive bands disappeared and three bands of molecular weight 75, 32, and 25 kDa were observed. These results indicate that matrix vesicles contain enzymes, especially MMP-3, which are responsible for the release of TGF-beta1 from the matrix, most of which is in latent form. Further, the data suggest that release of the large complex occurs via cleavage at several novel sites in the 130 kDa LTBP1 molecule. Since matrix vesicle MMP-3 is also able to activate small latent TGF-beta1, these results suggest that the large latent TGF-beta1 complex protects against activation of the small latent TGF-beta1. Thus, the data suggest that release of the large latent TGF-bl complex from the matrix and activation of the latent growth factor are only two steps of what must be at least a three-step process.

Roles of autophagy in chloroplast recycling

Chloroplasts are the primary energy suppliers for plants, and much of the total leaf nitrogen is distributed to these organelles. During growth and reproduction, chloroplasts in turn represent a major source of nitrogen to be recovered from senescing leaves and used in newly-forming and storage organs. Chloroplast proteins also can be an alternative substrate for respiration under suboptimal conditions. Autophagy is a process of bulk degradation and nutrient sequestration that is conserved in all eukaryotes. Autophagy can selectively target chloroplasts as whole organelles and or as Rubisco-containing bodies that are enclosed by the envelope and specifically contain the stromal portion of the chloroplast. Although information is still limited, recent work indicates that chloroplast recycling via autophagy plays important roles not only in developmental processes but also in organelle quality control and adaptation to changing environments. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

Urease produced by Coccidioides posadasii contributes to the virulence of this respiratory pathogen

Urease activity during in vitro growth in the saprobic and parasitic phases of Coccidioides spp. is partly responsible for production of intracellular ammonia released into the culture media and contributes to alkalinity of the external microenvironment. Although the amino acid sequence of the urease of Coccidioides posadasii lacks a predicted signal peptide, the protein is transported from the cytosol into vesicles and the central vacuole of parasitic cells (spherules). Enzymatically active urease is released from the contents of mature spherules during the parasitic cycle endosporulation stage. The endospores, together with the urease and additional material which escape from the ruptured parasitic cells, elicit an intense host inflammatory response. Ammonia production by the spherules of C. posadasii is markedly increased by the availability of exogenous urea found in relatively high concentrations at sites of coccidioidal infection in the lungs of mice. Direct measurement of the pH at these infection sites revealed an alkaline microenvironment. Disruption of the urease gene of C. posadasii resulted in a marked reduction in the amount of ammonia secreted in vitro by the fungal cells. BALB/c mice challenged intranasally with the mutant strain showed increased survival, a well-organized granulomatous response to infection, and better clearance of the pathogen than animals challenged with either the parental or the reconstituted (revertant) strain. We conclude that ammonia and enzymatically active urease released from spherules during the parasitic cycle of C. posadasii contribute to host tissue damage, which exacerbates the severity of coccidioidal infection and enhances the virulence of this human respiratory pathogen.

The ricinosomes of senescing plant tissue bud from the endoplasmic reticulum

The ricinosome (synonym, precursor protease vesicle) is a novel organelle, found so far exclusively in plant cells. Electron microscopic studies suggest that it buds off from the endoplasmic reticulum in senescing tissues. Biochemical support for this unusual origin now comes from the composition of the purified organelle, which contains large amounts of a 45-kDa cysteine endoprotease precursor with a C-terminal KDEL motif and the endoplasmic reticulum lumen residents BiP (binding protein) and protein disulfide isomerase. Western blot analysis, peptide sequencing, and mass spectrometry demonstrate retention of KDEL in the protease proform. Acidification of isolated ricinosomes causes castor bean cysteine endopeptidase activation, with cleavage of the N-terminal propeptide and the C-terminal KDEL motif. We propose that ricinosomes accumulate during senescence by programmed cell death and are activated by release of protons from acidic vacuoles.

Activation of latent transforming growth factor beta1 by stromelysin 1 in extracts of growth plate chondrocyte-derived matrix vesicles

Previous studies have shown that matrix vesicles isolated from cultures of costochondral growth zone chondrocytes and treated with 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] can activate recombinant human latent transforming growth factor beta1 (rhTGF-beta1). It is unknown what enzyme or other factor in the extracellular organelles is responsible for the activation. This study tested the hypothesis that enzymes present in matrix vesicles can activate latent TGF-beta1 and that this is regulated by 1alpha,25(OH)2D3. To do this, we examined the ability of matrix vesicle extracts to activate small latent rhTGF-beta1. In addition, enzymes previously determined to be present in matrix vesicles were screened for their ability to activate small latent rhTGF-beta1. Recombinant human matrix metalloproteinase 2 (rhMMP-2; 72 kDa gelatinase), rhMMP-3 (stromelysin 1), purified human plasminogen, and purified urokinase (plasminogen activator) were each tested at varying concentrations. To assess the role of cell maturation, we used a cell culture model in which chondrocytes are derived from two distinct zones of rat costochondral cartilage, the resting zone and the growth zone. Matrix vesicles were isolated from these cultures and then tested. The results showed that extracts of matrix vesicles produced by both growth zone and resting zone chondrocytes were able to activate small latent rhTGF-beta1. The effects were dose and time dependent, with greater activity being found in extracts of matrix vesicles from the growth zone chondrocyte cultures. Only rhMMP-3 was able to activate small latent rhTGF-beta1, indicating that stromelysin-1, but not MMP-2, plasminogen, or urokinase, was involved. As observed in the extracts, the effect of rhMMP-3 was time and dose dependent. When anti-MMP-3 antibody was added to matrix vesicle extracts from both cell types, activation of small latent rhTGF-beta1 was dose-dependently blocked. Neither 1alpha,25(OH)2D3 nor 24R,25(OH)2D3 had a direct effect on activation of small latent rhTGF-beta1 by the extracts. However, when intact matrix vesicles were treated with 1alpha,25(OH)2D3, their ability to activate small latent rhTGF-beta1 was increased. Inhibition of phospholipase A2 with quinacrine blocked the 1alpha,25(OH)2D3-dependent effect. These results suggest that the ability of 1alpha,25(OH)2D3-treated matrix vesicles to activate small latent TGF-beta1 is via action of the secosteroid on the matrix vesicle membrane, not on the enzymes responsible for activating latent TGF-beta1. Because matrix vesicles isolated from growth zone chondrocytes have been shown to contain increased phospholipase A2 activity after treatment with 1alpha,25(OH)2D3, it is likely that this secosteroid promotes loss of membrane integrity through phospholipase A2-dependent formation of lysophospholipids, resulting in the release of MMP-3 into the matrix, where latent TGF-beta1 is stored. Taken together, the results of the current study show that matrix vesicles produced by growth plate chondrocytes contain MMP-3, that this enzyme is at least partially responsible for activation of small latent TGF-beta1 in the matrix, and that 1alpha,25(OH)2D3 regulates MMP release from matrix vesicles.

PAK5 kinase is an inhibitor of MARK/Par-1, which leads to stable microtubules and dynamic actin

MARK/Par-1 is a kinase involved in development of embryonic polarity. In neurons, MARK phosphorylates tau protein and causes its detachment from microtubules, the tracks of axonal transport. Because the target sites of MARK on tau occur at an early stage of Alzheimer neurodegeneration, we searched for interaction partners of MARK. Here we report that MARK2 is negatively regulated by PAK5, a neuronal member of the p21-activated kinase family. PAK5 suppresses the activity of MARK2 toward its target, tau protein. The inhibition requires the binding between the PAK5 and MARK2 catalytic domains, but does not require phosphorylation. In transfected Chinese hamster ovary (CHO) cells both kinases show a vesicular distribution with partial colocalization on endosomes containing AP-1/2. Although MARK2 transfected alone destabilizes microtubules and stabilizes actin stress fibers, PAK5 keeps microtubules stable through the down-regulation of MARK2 but destabilizes the F-actin network so that stress fibers and focal adhesions disappear and cells develop filopodia. The results point to an inverse relationship between actin- and microtubule-related signaling by the PAK5 and MARK2 pathways that affect both cytoskeletal networks.

Mutations in the SPG3A gene encoding the GTPase atlastin interfere with vesicle trafficking in the ER/Golgi interface and Golgi morphogenesis

Mutations in SPG3A causing autosomal dominant pure spastic paraplegia led to identification of atlastin, a new dynamin-like large GTPase. Atlastin is localized in the endoplasmic reticulum, the Golgi, neurites and growth cones and has been implicated in neurite outgrowth. To investigate whether it exerts its activity in the early secretory system, we expressed normal and mutant atlastin in cell culture. Pathogenic mutations in the GTPase domain interfered with the maturation of Golgi complexes by preventing the budding of vesicles from the endoplasmic reticulum, whereas mutations in other regions of the protein disrupted fission of endoplasmic reticulum-derived vesicles or their migration to their Golgi target. Atlastin, therefore, plays a role in vesicle trafficking in the ER/Golgi interface. Furthermore, atlastin partially co-localized with proteins of the p24/emp/gp25L family that regulate vesicle budding and trafficking in the early secretory pathway, and co-immunoprecipitated p24, suggesting a functional relationship that should be further explored.

Leishmania amazonensis: early proteinase activities during promastigote-amastigote differentiation in vitro

Leishmania proteinase activity is known as parasite differentiation marker, and has been considered relevant for leishmanial survival and virulence. These properties suggest that Leishmania proteinases can be promising targets for development of anti-leishmania drugs. Here, we analyze the activities of four proteinases during the early phase of the Leishmania amazonensis promastigotes differentiation into amastigotes induced by heat shock. We have examined activities of cysteine-, metallo-, serine-, and aspartic-proteinase by hydrolysis of specific chromogenic substrates at pH 5.0 and at the optimal pH for each enzyme. Our results show that metallo-, serine-, and aspartic-proteinases activities were down-regulated during the shock-induced transformation of promastigotes into amastigotes. In contrast, cysteine-proteinase activity increased concomitantly with the promastigote differentiation. Immunocytochemical localization using two anti-cysteine-proteinase monospecific rabbit antibodies detected the enzyme in several cell compartments of both parasite stages. Our results show different proteinase activity modulation and expression during the early phases of the shock-induced parasite transformation.

Golgi linked protein glycosylation and associated diseases

One of the Golgi's main functions is the glycosylation of secreted proteins. A large variety of glycan chains can be synthesized in the Golgi, and it is increasingly clear that these are critical in basic cellular functions as well as the development of multicellular organisms. The structurally best-documented glycans are N-glycans, yet these are also the most enigmatic in their function. In contrast, O-glycan function is far better understood, but here the structures and biosynthetic pathways are very incomplete. The critical importance of glycans is highlighted by the broad spectrum of diseases they are associated with, such as a number of inherited diseases, but also cancers or diabetes. The molecular clues to these, however, are only just being elucidated. Although some glycan structures are known to be involved in signaling or adhesion to the extracellular matrix, for most the functions are not yet known. This review aims at summarizing current knowledge as much as to point out critical areas key for future progress.

Targeted induction of apoptosis by chimeric granzyme B fusion proteins carrying antibody and growth factor domains for cell recognition

The serine protease granzyme B (GrB) of cytotoxic lymphocytes efficiently induces apoptosis by direct activation of caspases and cleavage of central caspase substrates. We employed human GrB as an effector function in chimeric fusion proteins that also contain the EGFR ligand TGFalpha or an ErbB2-specific single-chain antibody fragment (scFv) for selective targeting to tumor cells. GrB-TGFalpha (GrB-T) and GrB-scFv(FRP5) (GrB-5) molecules expressed in the yeast Pichia pastoris were bifunctional, cleaving synthetic and natural GrB substrates, and binding specifically to cells expressing EGFR or ErbB2 target receptors. Upon cell binding the chimeric molecules were internalized into intracellular vesicles, but could be released into the cytosol by the endosomolytic reagent chloroquine. Treatment with picomolar to nanomolar concentrations of GrB-5 and GrB-T resulted in selective and rapid tumor cell killing, accompanied by clear signs of apoptosis such as chromatin condensation, membrane blebbing, formation of apoptotic bodies and activation of endogenous initiator and effector caspases.

The role of the epsilon subunit in the Escherichia coli ATP synthase. The C-terminal domain is required for efficient energy coupling

The role of the C-domain of the epsilon subunit of ATP synthase was investigated by fusing either the 20-kDa flavodoxin (Fd) or the 5-kDa chitin binding domain (CBD) to the N termini of both full-length epsilon and a truncation mutant epsilon(88-stop). All mutant epsilon proteins were stable in cells and supported F1F0 assembly. Cells expressing the Fd-epsilon or Fd-epsilon(88-stop) mutants were unable to grow on acetate minimal medium, indicating their inability to carry out oxidative phosphorylation because of steric blockage of rotation. The other forms of epsilon supported growth on acetate. Membrane vesicles containing Fd-epsilon showed 23% of the wild type ATPase activity but no proton pumping, suggesting that the ATP synthase is intrinsically partially uncoupled. Vesicles containing CBD-epsilon were indistinguishable from the wild type in ATPase activity and proton pumping, indicating that the N-terminal fusions alone do not promote uncoupling. Fd-epsilon(88-stop) caused higher rates of uncoupled ATP hydrolysis than Fd-epsilon, and epsilon(88-stop) showed an increased rate of membrane-bound ATP hydrolysis but decreased proton pumping relative to the wild type. Both results demonstrate the role of the C-domain in coupling. Analysis of the wild type and epsilon(88-stop) mutant membrane ATPase activities at concentrations of ATP from 50 mum to 8 mm showed no significant dependence of the ratio of bound/released ATPase activity on ATP concentration. These results support the hypothesis that the main function of the C-domain in the Escherichia coli epsilon subunit is to reduce uncoupled ATPase activity, rather than to regulate coupled activity.

Vesicular localization and characterization of a novel post-proline-cleaving aminodipeptidase, quiescent cell proline dipeptidase

A large number of chemokines, cytokines, and signal peptides share a highly conserved X-Pro motif on the N-terminus. The cleavage of this N-terminal X-Pro dipeptide results in functional alterations of chemokines such as RANTES, stroma-derived factor-1, and macrophage-derived chemokine. Until recently, CD26/DPPIV was the only known protease with the ability to cleave N-terminal X-Pro motifs at neutral pH. We have isolated and cloned a novel serine protease, quiescent cell proline dipeptidase (QPP), with substrate specificity similar to that of CD26/DPPIV. In this paper we show that QPP, like CD26/DPPIV, is synthesized with a propeptide and undergoes N:-glycosylation. Interestingly, this glycosylation is required for QPP enzymatic activity, but not for its localization. Unlike the cell surface molecule, CD26/DPPIV, QPP is targeted to intracellular vesicles that are distinct from lysosomes. Proteinase K treatment of intact vesicles indicates that QPP is located within the vesicles. These vesicles appear to have a secretory component, as QPP is secreted in a functionally active form in response to calcium release. The presence of QPP in the vesicular compartment suggests that molecules bearing the N-terminal X-Pro motif can be cleaved at multiple sites within and outside the cell. These results expand the potential site(s) and scope of a process that appears to be an important mechanism of post-translational regulation.

N(6)-Adenine DNA-methyltransferase in wheat seedlings

The N(6)-adenine DNA-methyltransferase was isolated from the vacuolar vesicle fraction of wheat coleoptiles. In the presence of S-adenosyl-L-methionine the enzyme de novo methylates the first adenine residue in the TGATCA sequence in the single- or double-stranded DNA substrates but it prefers single-stranded structures. Wheat adenine DNA-methyltransferase (wadmtase) is a Mg(2+)- or Ca(2+)-dependent enzyme with a maximum activity at pH 7.5-8.0. Wadmtase seems to be responsible for mitochondrial DNA modification that might be involved in the regulation of replication of mitochondria in plants.

p62 (SQSTM1) and cyclic AMP phosphodiesterase-4A4 (PDE4A4) locate to a novel, reversible protein aggregate with links to autophagy and proteasome degradation pathways

Chronic challenge of cyclic AMP phosphodiesterase-4A4 (PDE4A4) with certain PDE4 selective inhibitors causes it to reversibly form intracellular aggregates that are not membrane-encapsulated. These aggregates are neither stress granules (SGs) nor processing bodies (PBs) as they contain neither PABP-1 nor Dcp1a, respectively. However, the PDE4 inhibitor rolipram decreases arsenite-induced SGs and increases the amount of PBs, while arsenite challenge ablates rolipram-induced PDE4A4 aggregates. PDE4A4 aggregates are neither autophagic vesicles (autophagosomes) nor aggresomes, although microtubule disruptors ablate PDE4A4 aggregate formation. PDE4A4 constitutively co-immunoprecipitates with p62 protein (sequestosome1, SQSTM1), which locates to both PDE4A4 aggregates and autophagosomes in cells constitutively challenged with rolipram. The mTor inhibitor, rapamycin, activates autophagy, prevents PDE4A4 from forming intracellular aggregates and triggers the loss of bound p62 from PDE4A4. siRNA-mediated knockdown of p62 attenuates PDE4A4 aggregate formation. The p62-binding protein, light chain 3 (LC3), is not found in PDE4A4 aggregates. Blockade of proteasome activity and activation of autophagy with MG132 both increases the level of ubiquitinated proteins found associated with PDE4A4 and inhibits PDE4A4 aggregate formation. Activation of autophagy with either thapsigargin or ionomycin inhibits PDE4A4 aggregate formation. Inhibition of autophagy with either wortmannin or LY294002 activates PDE4A4 aggregate formation. The protein kinase C inhibitors, RO 320432 and GO 6983, and the ERK inhibitors UO 126 and PD 98059 all activated PDE4A4 aggregate formation, whilst roscovitine, thalidomide and the tyrosine kinase inhibitors, genistein and AG17, all inhibited this process. We suggest that the fate of p62-containing protein aggregates need not necessarily be terminal, through delivery to autophagic vesicles and aggresomes. Instead, we propose a novel regulatory mechanism where a sub-population of p62-containing protein aggregates would form in a rapid, reversible manner so as to sequester specific cargo away from their normal, functionally important site(s) within the cell. Thus an appropriate conformational change in the target protein would confer reversible recruitment into a sub-population of p62-containing protein aggregates and so provide a regulatory function by removing these cargo proteins from their functionally important site(s) in a cell.

A phosphoglucose isomerase mutant in Aspergillus nidulans is defective in hyphal polarity and conidiation

Upon germination Aspergillus nidulans swoM1 exhibits abnormal development by extending a primary germ tube that quickly reverts to isotropic growth and results in an enlarged, swollen apex with pronounced wall thickenings. Apical lysis occurs in 38% of the germlings. A point mutation in the AN6037.3 gene encoding the only phosphoglucose isomerase in A. nidulans is responsible for the defect. Loss of polarity is bypassed when glucose is replaced with alternate carbon sources but in all cases the mutant is unable to conidiate due to a block in conidiophore development at vesicle formation. In conidiophores SwoM::GFP localizes to multiple punctate, foci within each actively growing cell type, and to multiple foci in mature dormant conidia. In hyphae SwoM::GFP localized to two rings spanning the center of mature septa. In hyphae localization is concentrated at actively growing hyphal tips.

Requirement of phospholipase D for ilimaquinone-induced Golgi membrane fragmentation

Although organelles such as the endoplasmic reticulum and Golgi apparatus are highly compartmentalized, these organelles are interconnected through a network of vesicular trafficking. The marine sponge metabolite ilimaquinone (IQ) is known to induce Golgi membrane fragmentation and is widely used to study the mechanism of vesicular trafficking. Although IQ treatment causes protein kinase D (PKD) activation, the detailed mechanism of IQ-induced Golgi membrane fragmentation remains unclear. In this work, we found that IQ treatment of cells caused a robust activation of phospholipase D (PLD). In the presence of 1-butanol but not 2-butanol, IQ-induced Golgi membrane fragmentation was completely blocked. In addition, IQ failed to induce Golgi membrane fragmentation in PLD knock-out DT40 cells. Furthermore, IQ-induced PKD activation was completely blocked by treatment with either 1-butanol or propranolol. Notably, IQ-induced Golgi membrane fragmentation was also blocked by propranolol treatment. These results indicate that PLD-catalyzed formation of phosphatidic acid is a prerequisite for IQ-induced Golgi membrane fragmentation and that enzymatic conversion of phosphatidic acid to diacylglycerol is necessary for subsequent activation of PKD and IQ-induced Golgi membrane fragmentation.

Dynamics of polymorphism of acidocalcisomes in Leishmania parasites

Growth of Leishmania mexicana amazonensis promastigotes in different culture media resulted in structurally and chemically different acidocalcisomes. When grown in SDM-79 medium, the promastigotes showed large spherical acidocalcisomes of up to 1.2 microm diameter distributed throughout the cell. X-ray microanalysis and elemental mapping of the organelles showed large amounts of oxygen, phosphorus, sodium, potassium, magnesium, calcium, and zinc. Immunofluorescence microscopy using antisera raised against a peptide sequence of the vacuolar-type proton pyrophosphatase of Arabidopsis thaliana that is conserved in the Leishmania enzyme, indicated localization in acidocalcisomes. When cells were transferred to Warren's medium, the acidocalcisomes transformed from spherical into branched tubular organelles. The labeling pattern of the vacuolar proton-pyrophosphatase, considered as a marker for the organelle, changed accompanying the structural changes of the acidocalcisomes, and the enzyme showed an apparently lower proton-transporting activity when measured in digitonin-permeabilized promastigotes. X-ray microanalysis and elemental mapping of these structures revealed the additional presence of iron. Together, the results reveal that the morphology and composition of acidocalcisomes are greatly influenced by the culture conditions.

Protein kinase D 3 is localized in vesicular structures and interacts with vesicle-associated membrane protein 2

Protein kinase D localizes in the Golgi and regulates protein transport from the Golgi to the plasma membrane. In the present study, we found that PKD3, a novel member of the PKD family, and its fluorescent protein fusions localized in the Golgi and in the vesicular structures that are in part marked by endosome markers. Fluorescent recovery after photobleaching (FRAP) showed that the PKD3-associated vesicular structures were constantly forming and dissolving, reflecting active subcellular structures. FRAP on plasma membrane-located PKD3 indicated a slower recovery of PKD3 fluorescent signal compared to those of PKC isoforms, implying a different targeting mechanism at the plasma membrane. VAMP2, the vesicle-localized v-SNARE, was later identified as a novel binding partner of PKD3 through yeast two-hybrid screening. PKD3 directly interacted with VAMP2 in vitro and in vivo, and colocalized in part with VAMP2 vesicles in cells. PKD3 did not phosphorylate VAMP-GFP and the purified GST-VAMP2 protein in in vitro phosphorylation assays. Rather, PKD3 was found to promote the recruitment of VAMP2 vesicles to the plasma membrane in response to PMA, while the kinase dead PKD3 abolished this effect. Thus, the kinase activity of PKD3 was required for PMA-induced plasma membrane trafficking of VAMP2. In summary, our findings suggest that PKD3 localizes to vesicular structures that are part of the endocytic compartment. The vesicular distribution may be attributed in part to the direct interaction between PKD3 and vesicle-associated membrane protein VAMP2, through which PKD3 may regulate VAMP2 vesicle trafficking by facilitating its recruitment to the target membrane.

Receptor activation regulates cortical, but not vesicular localization of NDP kinase

We used immunofluorescence techniques to determine the localization of nucleoside diphosphate (NDP) kinase in NIH-3T3 fibroblasts. We found that cytoplasmic NDP kinase can be separated into two populations according to subcellular localization and response to extracellular stimuli. Specifically, within minutes of stimulation of resting fibroblasts with serum, growth factors or bombesin, a portion of NDP kinase becomes associated with membrane ruffles and lamellipodia. Another pool of NDP kinase accumulates independently of stimulation around intracellular vesicles. Transfection of cells with activated Rac mimics, whereas expression of dominant negative Rac inhibits, the effects of extracellular stimulation on the translocation of NDP kinase to the cell cortex. Neither Rac mutant affects the vesicle-associated pool. Association of NDP kinase with vesicles depends on microtubule integrity and is disrupted by nocodazole. In cell-free assays NDP kinase binds tightly to membrane vesicles associated with taxol-stabilized microtubules. Binding of NDP kinase to this fraction is reduced by ATP and abolished by GTP, as well as guanine nucleotides that are NDP kinase substrates. Thus, the localization of the two NDP kinase pools identified here is regulated independently by distinct cellular components: the appearance of cortical NDP kinase is a consequence of Rac activation, whereas vesicular NDP kinase is responsive to microtubule dynamics and nucleotides, in particular GTP. These results suggest that in fibroblasts NDP kinase participates in Rac-related cortical events and in GTP-dependent processes linked to intracellular vesicle trafficking.