Isolation, characterization and localization of annexin V from chicken liver

Mule duck "foie gras" shows different metabolic states according to its quality phenotype by using a proteomic approach

This study aimed at identifying the mechanisms implicated in "foie gras" quality variability through the study of the relationships between liver protein compositions and four liver quality phenotypes: liver weight, melting rate, and protein contents on crude or dry matter. Spots of soluble proteins were separated by bidimensional electrophoresis, and the relative abundance of proteins according to quality traits values was investigated. Twenty-three protein spots (19 unique identified proteins) showed different levels of abundance according to one or more of the traits' values. These abundance differences highlighted two groups of livers with opposite trends of abundance levels. Proteins of the first group, associated with low liver weight and melting rate, are involved in synthesis and anabolism processes, whereas proteins of the second group, associated with high liver weight and melting rate, are proteins involved in stress response. Altogether, these results highlight the variations in metabolic states underlying foie gras quality traits.

Characterization of platelet aminophospholipid externalization reveals fatty acids as molecular determinants that regulate coagulation

Aminophospholipid (APL) trafficking across the plasma membrane is a key event in cell activation, apoptosis, and aging and is required for clearance of dying cells and coagulation. Currently the phospholipid molecular species externalized are unknown. Using a lipidomic method, we show that thrombin, collagen, or ionophore-activated human platelets externalize two phosphatidylserines (PSs) and five phosphatidylethanolamines (PEs). Four percent of the total cellular PE/PS pool (∼300 ng/2 × 10(8) cells, thrombin), is externalized via calcium mobilization and protease-activated receptors-1 and -4, and 48% is contained in microparticles. Apoptosis and energy depletion (aging) externalized the same APLs in a calcium-dependent manner, and all stimuli externalized oxidized phospholipids, termed hydroxyeicosatetraenoic acid-PEs. Transmembrane protein-16F (TMEM-16F), the protein mutated in Scott syndrome, was required for PE/PS externalization during thrombin activation and energy depletion, but not apoptosis. Platelet-specific APLs optimally supported tissue factor-dependent coagulation in human plasma, vs. APL with longer or shorter fatty acyl chains. This finding demonstrates fatty acids as molecular determinants of APL that regulate hemostasis. Thus, the molecular species of externalized APL during platelet activation, apoptosis, and energy depletion were characterized, and their ability to support coagulation revealed. The findings have therapeutic implications for bleeding disorders and transfusion therapy. The assay could be applied to other cell events characterized by APL externalization, including cell division and vesiculation.

A biphasic endothelial stress-survival mechanism regulates the cellular response to vascular endothelial growth factor A

Vascular endothelial growth factor A (VEGF-A) is an essential cytokine that regulates endothelial function and angiogenesis. VEGF-A binding to endothelial receptor tyrosine kinases such as VEGFR1 and VEGFR2 triggers cellular responses including survival, proliferation and new blood vessel sprouting. Increased levels of a soluble VEGFR1 splice variant (sFlt-1) correlate with endothelial dysfunction in pathologies such as pre-eclampsia; however the cellular mechanism(s) underlying the regulation and function of sFlt-1 are unclear. Here, we demonstrate the existence of a biphasic stress response in endothelial cells, using serum deprivation as a model of endothelial dysfunction. The early phase is characterized by a high VEGFR2:sFlt-1 ratio, which is reversed in the late phase. A functional consequence is a short-term increase in VEGF-A-stimulated intracellular signaling. In the late phase, sFlt-1 is secreted and deposited at the extracellular matrix. We hypothesized that under stress, increased endothelial sFlt-1 levels reduce VEGF-A bioavailability: VEGF-A treatment induces sFlt-1 expression at the cell surface and VEGF-A silencing inhibits sFlt-1 anchorage to the extracellular matrix. Treatment with recombinant sFlt-1 inhibits VEGF-A-stimulated in vitro angiogenesis and sFlt-1 silencing enhances this process. In this response, increased VEGFR2 levels are regulated by the phosphatidylinositol-3-kinase and PKB/Akt signaling pathways and increased sFlt-1 levels by the ERK1/2 signaling pathway. We conclude that during serum withdrawal, cellular sensing of environmental stress modulates sFlt-1 and VEGFR2 levels, regulating VEGF-A bioavailability and ensuring cell survival takes precedence over cell proliferation and migration. These findings may underpin an important mechanism contributing to endothelial dysfunction in pathological states.

Dual acylation accounts for the localization of {alpha}19-giardin in the ventral flagellum pair of Giardia lamblia

A Giardia-specific protein family denominated as alpha-giardins, represents the major protein component, besides tubulin, of the cytoskeleton of the human pathogenic parasite Giardia lamblia. One of its members, alpha19-giardin, carries an N-terminal sequence extension of MGCXXS, which in many proteins serves as a target for dual lipid conjugation: myristoylation at the glycine residue after removal of the methionine and palmitoylation at the cysteine residue. As the first experimental evidence of a lipid modification, we found alpha19-giardin to be associated with the membrane fraction of disrupted trophozoites. After heterologous coexpression of alpha19-giardin with giardial N-myristoyltransferase (NMT) in Escherichia coli, we found the protein in a myristoylated form. Additionally, after heterologous expression together with the palmitoyl transferase Pfa3 in Saccharomyces cerevisiae, alpha19-giardin associates with the membrane of the main vacuole. Immunocytochemical colocalization studies on wild-type Giardia trophozoites with tubulin provide evidence that alpha19-giardin exclusively localizes to the ventral pair of the giardial flagella. A mutant in which the putatively myristoylated N-terminal glycine residue was replaced by alanine lost this specific localization. Our findings suggest that the dual lipidation of alpha19-giardin is responsible for its specific flagellar localization.

Key role of the N-terminus of chicken annexin A5 in vesicle aggregation

Annexins are calcium-dependent phospholipid-binding proteins involved in calcium signaling and intracellular membrane trafficking among other functions. Vesicle aggregation is a crucial event to make possible the membrane remodeling but this process is energetically unfavorable, and phospholipid membranes do not aggregate and fuse spontaneously. This issue can be circumvented by the presence of different agents such as divalent cations and/or proteins, among them some annexins. Although human annexin A5 lacks the ability to aggregate vesicles, here we demonstrate that its highly similar chicken ortholog induces aggregation of vesicles containing acidic phospholipids even at low protein and/or calcium concentration by establishment of protein dimers. Our experiments show that the ability to aggregate vesicles mainly resides in the N-terminus as truncation of the N-terminus of chicken annexin A5 significantly decreases this process and replacement of the N-terminus of human annexin A5 by that of chicken switches on aggregation; in both cases, there are no changes in the overall protein structure and only minor changes in phospholipid binding. Electrostatic repulsions between negatively charged residues in the concave face of the molecule, mainly in the N-terminus, seem to be responsible for the impairment of dimer formation in human annexin A5. Taking into account that chicken annexin A5 presents a high sequence and structural similarity with mammalian annexins absent in birds, as annexins A3 and A4, some of the physiological functions exerted by these proteins may be carried out by chicken annexin A5, even those that could require calcium-dependent membrane aggregation.

pDMAEMA is internalised by endocytosis but does not physically disrupt endosomes

Earlier workers proposed that poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) facilitates cell transfection by being endocytosed, complexed with DNA, and subsequently acting as a "proton sponge" to burst endosomes/lysosomes and release DNA to the cytosol. It also seemed feasible that the cytotoxicity of pDMAEMA might result from lysosomal bursting, which can induce cell death. Experiments were performed to determine the extent of cytotoxicity of uncomplexed pDMAEMA, the mode of cell death it induces (i.e. apoptosis or necrosis), its mechanism of entry into cells, and its ability to disrupt endosomes/lysosomes and release molecules into the cell cytosol. The results indicate that (i). pDMAEMA is highly cytotoxic and induces rapid, primarily necrotic cell death, (ii). it is internalised into cells via fluid-phase endocytosis, and (iii). although pDMAEMA affected the morphology of late endosomes/lysosomes, it did not physically disrupt them to release their contents to the cytosol. The lack of endosomal disruptive activity suggests that this is not involved in the cytotoxicity of pDMAEMA or in its ability to transfect cells. Further work will be required to establish the molecular mechanism(s) by which pDMAEMA facilitates transfection.

Annexin IV (Xanx-4) has a functional role in the formation of pronephric tubules

Vertebrate kidney organogenesis is characterised by the successive formation of the pronephros, the mesonephros and the metanephros. The pronephros is the first to form and is the functional embryonic kidney of lower vertebrates; although it is vestigial in higher vertebrates, it is a necessary precursor for the other kidney types. The Xenopus pronephros is a simple paired organ; each nephron consists of a single large glomus, one set of tubules and a single duct. The simple organisation of the pronephros and the amenability of Xenopus laevis embryos to manipulation make the Xenopus pronephros an attractive system in which to study organogenesis. It has been shown that pronephric tubules can be induced to form in presumptive ectodermal tissue by treatment with RA and activin. We have used this system in a subtractive hybridisation screen that resulted in the cloning of Xenopus laevis annexin IV (Xanx-4). Xanx-4 transcripts are specifically located to the developing pronephric tubules, and the protein to the luminal surface of these tubules. Temporal expression shows zygotic transcription is upregulated at the time of pronephric tubule specification and persists throughout pronephric development. The temporal and spatial expression pattern of Xanx-4 suggests it may have a role in pronephric tubule development. Overexpression of Xanx-4 yields no apparent phenotype, but Xanx-4 depletion, using morpholinos, produces a shortened, enlarged tubule phenotype. The phenotype observed can be rescued by co-injection of Xanx-4 mRNA. Although the function of annexins is not yet clear, studies have suggested a role for annexins in a number of cellular processes. Annexin IV has been shown to have an inhibitory role in the regulation of epithelial calcium-activated chloride ion conductance. The enlarged pronephric tubule phenotype observed may be attributed to incorrect modulation of exocytosis, membrane plasticity or ion channels and/or water homeostasis. In this study, we demonstrate an in vivo role for annexin IV in the development of the pronephric tubules in Xenopus laevis.

Identification of a novel Ras-regulated proapoptotic pathway

BACKGROUND

The Ras-GTPase controls cell fate decisions through the binding of an array of effector molecules, such as Raf and PI 3-kinase, in a GTP-dependent manner. NORE1, a noncatalytic polypeptide, binds specifically to Ras-GTP and to several other Ras-like GTPases. NORE is homologous to the putative tumor suppressor RASSF1 and to the Caenorhabditis elegans polypeptide T24F1.3.

RESULTS

We find that all three NORE-related polypeptides bind selectively to the proapoptotic protein kinase MST1, a member of the Group II GC kinases. Endogenous NORE and MST1 occur in a constitutive complex in vivo that associates with endogenous Ras after serum stimulation. Targeting recombinant MST1 to the membrane, either through NORE or myristoylation, augments the apoptotic efficacy of MST1. Overexpression of constitutively active Ki-RasG12V promotes apoptosis in a variety of cell lines; Ha-RasG12V is a much less potent proapoptotic agent; however, a Ha-RasG12V effector loop mutant (E37G) that binds NORE, but not Raf or PI 3-kinase, exhibits proapoptotic efficacy approaching that of Ki-RasG12V. The apoptotic action of both Ki-RasG12V and Ha-RasG12V, E37G is suppressed by overexpression of the MST1 carboxy-terminal noncatalytic segment or by the NORE segment that binds MST1.

CONCLUSIONS

MST1 is a phylogenetically conserved partner of the NORE/RASSF polypeptide family, and the NORE-MST1 complex is a novel Ras effector unit that mediates the apoptotic effect of Ki-RasG12V.

Annexin expressions are temporally and spatially regulated during rat hepatocyte differentiation

Annexin (Anx) 1, 2, 5, and 6 expressions were determined at the transcriptional and translational levels in the rat hepatocytes from gestational day 15 to postnatal day 17. Dramatic shifts were observed in Anx 1 and 2 levels, which peaked at day 1 and gestational day 20, respectively, and reached low levels thereafter. However, Anx 5 and 6 rates were more constant. Prenatal administration of dexamethasone (dex) resulted in a decrease of Anx 1 mRNA levels, and a strong increase in Anx 2 mRNA contents. In adult hepatocytes cultured in the presence of EGF or HGF, Anx 1 and 2 expressions resumed. By immunohistochemistry, Anx 1 was detected only in the cytoplasm of hepatocytes of 1- to 3-day-old rats, Anx 2 and 6 both exhibited a redistribution from the cytoplasm toward the plasma membrane, and Anx 5 was present in the nucleus, cytoplasm, and plasma membrane. Thus, Anx 1, 2, 5, and 6 have individual modes of expression and localization in the differentiating hepatocytes, where they might play unique roles at well defined phases of liver ontogeny.

Phosphatidylserine-dependent adhesion of T cells to endothelial cells

Phosphatidylserine (PS) was exposed at the surface of human umbilical vein endothelial cells (HUVECs) and cultured cell lines by agonists that increase cytosolic Ca(2+), and factors governing the adhesion of T cells to the treated cells were investigated. Thrombin, ionophore A23187 and the Ca(2+)-ATPase inhibitor 2, 5-di-tert-butyl-1,4-benzohydroquinone each induced a PS-dependent adhesion of Jurkat T cells. A23187, which was the most effective agonist in releasing PS-bearing microvesicles, was the least effective in inducing the PS-dependent adhesion of Jurkat cells. Treatment of ECV304 and EA.hy926 cells with EGTA, followed by a return to normal medium, resulted in an influx of Ca(2+) and an increase in adhering Jurkat cells. Oxidised low-density lipoprotein induced a procoagulant response in cultured ECV304 cells and increased the number of adhering Jurkat cells, but adhesion was not inhibited by pretreating ECV304 cells with annexin V. PS was not significantly exposed on untreated Jurkat cells, as determined by flow cytometry with annexin V-FITC. However, after adhesion to thrombin-treated ECV304 cells for 10 min followed by detachment in 1 mM EDTA, there was a marked exposure of PS on the Jurkat cells. Binding of annexin V-FITC to the detached cells was inhibited by pretreating them with unlabelled annexin V. Contact with thrombin-treated ECV304 cells thus induced the exposure of PS on Jurkat cells and, as Jurkat cells were unable to adhere to thrombin-treated ECV304 cells in the presence of EGTA, the adhesion of the two cell types may involve a Ca(2+) bridge between PS on both cell surfaces. The number of T cells from normal, human peripheral blood that adhered to ECV304 cells was not increased by treating the latter with thrombin. However, findings made with several T cell lines were generally, but not completely, consistent with the possibility that adhesion to surface PS on endothelial cells may be a feature of T cells that express both CD4(+) and CD8(+) antigens. Possible implications for PS-dependent adhesion of T cells to endothelial cells in metastasis, and early in atherogenesis, are discussed.

Calcium-dependent self-association of annexin II: a possible implication in exocytosis

Alveolar type II cells secrete lung surfactant through exocytosis of lamellar bodies. We previously showed that the annexin II tetramer (Anx IIt) mediates the fusion of lamellar bodies with liposomes. The present study examined the possible involvement of membrane proteins in this process. Pre-treatment of lamellar bodies with trypsin and alpha-chymotrypsin reduced Anx IIt-mediated membrane fusion. With the use of an Anx IIt-conjugated Sepharose column, three Anx IIt-binding proteins with molecular weights of 67,000, 36,000 and 34,000 were isolated froM the Triton X-100 extract of bovine lung tissue membranes. These proteins were identified as annexins VI, II and IV by Western blot. The interaction of Anx IIt with annexins II and IV was confirmed by ligand blot assay. An EGTA-resistant membrane-bound annexin II was present in lung type II cells. Anx IIt preferentially hound to membranous annexin II compared with cytosolic annexin II of type II cells. With the use of immunofluorescence, annexin II was found to translocate from cytoplasm to plasma membranes in type II cells upon stimulation with phorbol 12-myristate 13-acetate. These results suggest that cytosolic annexin II may bind to membranous annexin II and form a protein-protein bridge to bring two membranes together.

Cholangiocyte-specific rat liver proteins identified by establishment of a two-dimensional gel protein database

The liver is composed of a variety of cells that form a functional unit involved in uptake, synthesis, metabolism, and secretion. Until recently, most studies examining liver function did not analyze the specific proteins expressed or functions performed by the multiple individual cell types that constitute the hepatic mass. In the last decade, novel isolation methods have been developed that allow the purification of liver cell populations highly enriched in one type of liver cell. Here, we present a detailed two-dimensional (2-D) protein map of rat bile duct epithelial cells (i.e., cholangiocytes) using a recently developed isolation procedure. In addition, we identify 27 major cholangiocyte proteins either by comparison to maps of known rat liver proteins (based on pI and Mr) or by tryptic digestion and microsequencing. Finally, we compare the relative abundance of individual proteins present in cholangiocytes to whole liver as well as hepatocyte-specific proteins. Our results show that cholangiocytes express a unique array of individual proteins. The cholangiocyte 2-D protein pattern is markedly different from that of isolated rat hepatocytes or whole rat liver, with high levels of proteins previously known to be expressed by cholangiocytes (e.g., cytokeratins, actins) as well as protein not previously demonstrated to be expressed at high levels (e.g., annexin V, selenium binding protein). We conclude that this cholangiocyte-derived, 2-D protein map will be a crucial resource for studies directed at our understanding of cholangiocyte physiology and pathobiology.

Lipocortin 1 (annexin 1) in patches associated with the membrane of a lung adenocarcinoma cell line and in the cell cytoplasm

Lipocortin 1 (annexin I) is a calcium- and phospholipid-binding annexin protein which can be externalised from cells despite the lack of a signal sequence. To determine its cellular distribution lipocortin 1 in A549 human lung adenocarcinoma cells was localised by light- and electron-microscopic immunocytochemistry and by cell fractionation and western blotting. Lipocortin 1 immunoreactivity is concentrated in prominent patches associated with the plasma membrane. The intensity of these patches varied with the confluence and duration of the culture and was not detectably diminished by an EDTA wash before fixation. Tubulin and cytokeratin 8 were colocalized with lipocortin 1 in the patches. Within the cells lipocortin 1 was distributed throughout the cytoplasm. Electron microscopy revealed prominent immunoreactivity along the plasma membrane with occasional large clusters of gold particles in contact with the membrane surface of the cells; within the cytoplasm the membrane of some vesicle/vacuole structures and some small electron-dense bodies was immunoreactive, but no immunogold particles were associated with the multilamellar bodies. Subcellular fractionation, extraction and western blotting showed that lipocortin 1 in the membrane pellet was present as two distinct fractions; one, intimately associated with the lipid bilayer, which behaved like an integral membrane protein and one loosely attached which behaved like a peripheral membrane protein. The results show that a substantial amounts of lipocortin 1 is concentrated in focal structures associated with and immediately beneath the plasma membrane. These might form part of the mechanism by which lipocortin 1 is released from the cells.

Relocation of annexin V to platelet membranes is a phosphorylation-dependent process

Annexins are a family of calcium-binding proteins that have been implicated in a wide range of intracellular processes. We have previously reported that stimulation of platelets with agents that increase intracellular [Ca2+] induces the relocation of annexin V to membranes, and that this annexin V may be binding to a 50 kDa protein located within platelet membranes. We report here, using an in vitro reconstitution system, that the relocation of annexin V to membranes is enhanced by ATP. We also demonstrate that when adenosine 5'-[gamma-thio]-triphosphate, which can replace ATP in phosphorylation reactions, is substituted for ATP, the amount of annexin V that binds to membranes is further increased. In separate experiments using intact cells, we show that the protein phosphatase inhibitor okadaic acid mimics the action of the physiological agonist thrombin, in that it induces annexin V to bind to membranes and that the addition of the protein kinase inhibitor staurosporine inhibits A23187-induced relocation of annexin V. In addition, alkaline phosphatase, when added to isolated membranes, was found to remove endogenous annexin V from the membranes. Furthermore, immunoprecipitation of 33P-labelled proteins indicated that annexin V may form a multi-protein complex including phosphoproteins of 25, 50 and 83 kDa. Taken together these observations suggest that, following physiological activation, the phosphorylation of one or more proteins is responsible for the tight association of annexin V with platelet membranes and the subsequent regulation of membrane localized processes.

Localization of annexin V in the adult and neonatal heart

Annexins are a major family of intracellular Ca2+-binding proteins which have been implicated in a variety of cellular functions. Several conflicting reports have been published on the location of annexin V in the heart. In this paper we have used confocal microscopy to demonstrate that annexin V is associated with the sarcolemma and intercalated discs of cardiac myocytes in sections of adult porcine and rat heart. In addition, we have used confocal microscopy of isolated rat myocytes to show that this association is stable even after cells were treated with the intracellular calcium chelator BAPTA-AM, to reduce cytosolic calcium levels to very low levels. This demonstrates that annexin V associates tightly with the sarcolemma and suggests that components in addition to phospholipid are involved in binding annexin V to the membrane. Furthermore, we show that, in sections of the neonatal rat left ventricle, annexin V has a different subcellular location than that observed in the terminally differentiated adult myocyte. In these differentiating neonatal cells, annexin V is also located in the nucleoplasm and at the periphery of the nucleus. These results demonstrate that the subcellular location of annexin V is differentially regulated and suggest that annexin V regulates calcium-dependent processes at both the sarcolemma and the nucleus.

The nuclear location of annexin V in the human osteosarcoma cell line MG-63 depends on serum factors and tyrosine kinase signaling pathways

Serum starvation of MG-63 cells increases their doubling time from 24 h to 4 days. Cells grown in medium containing 10% fetal calf serum contain high levels of annexin V in the cell nucleus, whereas growth for 4 days in the absence of serum results in loss of nuclear annexin V from 72 +/- 4% of cells. Many of the cells which still have nuclear annexin V under these conditions seem to have recently finished dividing. Refeeding cells with medium containing serum restores annexin V to nuclei within 5 h. Charcoal treatment removes factors from serum that are required to allow annexin V to return to the nucleus. Protein synthesis is not required for annexin V to return to nuclei since inhibition of protein synthesis with cycloheximide does not prevent the serum-induced return of annexin V to nuclei. This, and other evidence, indicates that the presence of annexin V in nuclei reflects translocation rather than catabolism and resynthesis. Inhibition of tyrosine kinase activities with genistein attenuates the relocation of annexin V from the cytoplasm to the nucleus. Thus, the nuclear location of annexin V is controlled by signaling pathways involving serum factors and tyrosine kinases. The results argue for an important role for annexin V in the cell nucleus.

Annexin II binds to the membrane of A549 cells in a calcium-dependent and calcium-independent manner

We investigated the nature of annexin II binding to the biological membranes using a lung epithelium-derived cell line A549. The cytosolic and membrane fractions of A549 cells were separated in the presence of 5 mM EGTA. Both fractions contain annexin II monomer and tetramer as evaluated by western blots using specific monoclonal antibodies against p36 and p11 subunits of annexin II. A substantial amount of annexin II was associated with the membrane fraction even after extensive washing with EGTA buffer, indicating the presence of two pools of annexin II. The EGTA-resistant membrane-bound annexin II could be partially extracted by 1% Triton X-100 or 60 mM n-octyl-beta-D-glucopyranoside, and completely by 30 mM CHAPS or 0.1% deoxycholate. This fraction of annexin II was also extracted by 0.1 M Na2CO3, pH 11 and partitioned into the aqueous phase after being treated with Triton X-114, demonstrating that the EGTA-resistant annexin II is a peripheral membrane protein. When the cells were lysed in varying concentrations of Ca2+, annexin II translocated from cytosolic fraction to membrane fraction at 4-25 microM Ca2+. To identify proteins closely associated with annexin II the membrane fraction was treated with the bifunctional chemical cross-linker disulfosuccinimidyl tartarate, followed by western blot analysis using anti-p36 or anti-p11 antibodies. We find that both p36 and p11 were cross-linked to a 51 kDa protein. In addition, p11 also binds to several proteins with molecular mass of 91, 65, 40 and 36 kDa. Our results suggest that annexin II may bind to the A549 cell membranes via specific membrane-associated proteins.

Phosphatidylserine-mediated adhesion of T-cells to endothelial cells

Phosphatidylserine was exposed on the surface of human umbilical endothelial cells (ECV304) a few minutes after adding thrombin in vitro, as monitored by prothrombinase assays with and without annexin V. Jurkat T cells adhered to the thrombin-treated cells. The adhesion was inhibited by annexin V, indicating that it was mediated by exposed phosphatidylserine on the endothelial cells.

Ca2+ concentration during binding determines the manner in which annexin V binds to membranes

Annexins are a family of calcium-binding proteins that have been implicated in a wide range of intracellular processes. We have previously reported that stimulation of platelets with thrombin can induce the association of intracellular annexin V with membranes in two distinct ways. First, in such a way that it can be eluted from the membrane with EGTA and secondly in a manner such that it is tightly bound to the membrane and requires the non-ionic detergent Triton X-100 for its solubilization. We report that exposure of platelets to the calcium ionophore A23187 mimics the relocation induced by stimulation with thrombin. In separate experiments we demonstrate that a calcium ion concentration [Ca2+] of 0.8 microM is sufficient for maximum binding of the EGTA-resistant form to membranes. In contrast a higher [Ca2+] was required to induce maximal binding of the annexin V which could be extracted with EGTA. We demonstrate that following temperature-induced phase separation in Triton X-114, the membrane-associated annexin V partitions predominantly into the aqueous phase. We also show that the isoelectric point of annexin V does not change following membrane association. These observations suggest that a covalent modification, of annexin V itself, is not responsible for its association with the membrane. Millimolar [Ca2+] is required for maximal binding of purified annexin V to phospholipid vesicles. We show that binding to phospholipids can be reversed entirely by subsequent treatment with EGTA. This suggests that the EGTA-resistant form of annexin V is binding to a membrane component other than phosphatidylserine. Annexin V has previously been shown to bind to protein kinase C. Relocation of annexin V to membranes paralleled that of protein kinase C in thrombin-stimulated cells but not in cells treated with A23187, suggesting that these proteins are not functionally linked in platelet activation. Using bifunctional cross-linking reagents we have identified an 85 kDa complex containing annexin V. This may represent an association between annexin V and an annexin V-binding protein with a molecular mass of approximately 50 kDa.

Collagen binding activity of recombinant and N-terminally modified annexin V (anchorin CII)

We have cloned the full coding cDNA sequence of chicken annexin V and of a mutant lacking 8 amino acid residues of the N-terminal tail for prokaryotic expression. Both proteins were synthesized in Escherichia coli upon induction with isopropyl thio-beta-D-galactoside, and were purified following two different protocols: one based on the ability of these proteins to interact reversibly with liposomes in the presence of calcium, and the other based on two sequential ion-exchange chromatographic steps. Spectroscopical analysis of recombinant annexin V revealed that binding of calcium did not change the circular dichroism spectra indicating no significant changes on the secondary structure; however, a conformational change affecting the exposition to the solvent of the tryptophan residue 187 was detected by analysis of fluorescence emission spectra. Recombinant annexin V binds with high affinity to collagen types II and X, and with lower affinity to collagen type I in a calcium-independent manner. Heat denaturing of collagen decreases this interaction while pepsin-treatment of collagen almost completely abolishes annexin V binding. Mutated annexin V interacts with collagen in a similar way as the nonmutated recombinant protein, indicating that the N-terminal tail of annexin V is not essential for collagen binding.

Characterization and immunolocalization of rat liver annexin VI

Annexin VI has been purified to homogeneity from rat liver and monospecific antibodies have been produced. The antibodies have been used for immunoblot analysis of rat tissues. Annexin VI is present in most tissues, with particularly high concentrations in liver, spleen, muscle, and intestine. In liver, annexin VI constitutes approximately 0.25% of total cellular protein. Immunohistochemical studies have located annexin VI on plasma membranes of hepatocytes with enhanced concentration on bile canaliculi. Annexin VI binds in a Ca(2+)-dependent manner to a sub-cellular fraction containing membranes. In the presence of physiological concentrations of ATP, the free Ca2+ concentration required for half-maximal binding of annexin VI to membranes is significantly reduced. While annexin VI binds in vitro to membranes in the presence of Ca2+, in rat liver about 31% of the annexin VI is associated with membranes in a Ca(2+)-independent manner and its solubilization requires the presence of Triton X-100. However, studies using Triton X-114 showed no increase in the hydrophobicity of this fraction of the protein compared to the purified EGTA-soluble annexin VI.

Highly polarized expression of carbohydrate-binding protein p33/41 (annexin IV) on the apical plasma membrane of epithelial cells in renal proximal tubules

p33/41 is a Ca(2+)-dependent carbohydrate-binding protein and is identical to annexin IV, a member of the annexin protein family. The localization of p33/41 in bovine kidney specimens was investigated immunohistochemically by use of specific polyclonal antibodies. The most interesting finding on immunostaining was that p33/41 was highly concentrated in the apical plasma membrane of the epithelial cells in the proximal tubules contrary to the distribution throughout the cytoplasm in the papillary ducts and papilla epithelium. The enrichment of p33/41 in the apical membrane was confirmed by immunoblotting of the brush border membrane fraction prepared from a kidney homogenate. Sequential extraction with EDTA and Triton X-100, and a partition experiment with Triton X-114 revealed that most p33/41 associates with the renal brush border membrane in a Ca(2+)-independent manner and is integrated into the membrane like intrinsic membrane proteins.

The sub-cellular localization of annexin V in cultured chick-embryo fibroblasts

The Ca(2+)- and phospholipid-binding protein, annexin V, has been shown by an immune assay to represent 0.4% of total cell protein in cultured chick-embryo fibroblasts. Immunofluorescent localization studies indicate that in primary cultures the protein is abundant in the cytoplasm of the cells and also extends into the nucleus. Nuclear staining is no longer detectable, however, in approx. 25% of the cells following sub-culture. Sub-populations of annexin V are associated with cytoskeletal structures and with the inner face of the plasma membrane in a Ca(2+)-independent manner. In addition, we report results indicating the secretion of annexin V from this cell type.