Calcium channel activity of purified human synexin and structure of the human synexin gene

Inhibition of annexin A7 suppresses senescence-associated heterochromatin foci formation and senescence through the AMPK/mTOR pathway in human dermal fibroblasts

Senescence-associated heterochromatin foci (SAHF) is often used as a biological marker for senescent cells, but the regulation of its formation process is unclear. To find a new modulator of SAHF, we screened our chemical small molecules and found 7-amino-2,3,4,5-tetrahedrobenzo[b][1,4] oxazepin-3-ol (ABO) that was identified as an inhibitor of annexin A7 GTPase (ANXA7) dramatically suppressed the aggregation of heterochromatin protein (HP1γ), an indicator of SAHF. To understand its action mechanism, we first observed the changes in the karyoplasmic ratio of ANXA7 because HP1γ mainly located in the nucleus. The results showed that ABO elevated the protein level of ANXA7 in the nucleus. Therefore, we raised a hypothesis that ANXA7 interacted with HP1γ and regulated its phosphorylation, which is closely related to the formation of SAHF. The co-immunoprecipitation and Western blot experiment results showed that ANXA7 had no direct interaction with HP1γ, however, the phosphorylation of HP1γ was increased by ABO, which suggested that ANXA7 indirectly regulated HP1γ phosphorylation. Then, based on our previous discovery of ANXA7 interacting with AMP-activated protein kinase (AMPK), we investigated the effect of the AMPK/mammalian target of rapamycin (mTOR) signaling pathway on ABO-increased phosphorylation of HP1γ. We found that ABO decreased AMPK phosphorylation and increased the phosphorylation level and activity of mTOR. In the presence of an AMPK activator or mTOR inhibitor, ABO could not increase HP1γ phosphorylation. As a result, ABO inhibited the senescence of human dermal fibroblasts (HDFs). In this study, we found that ANXA7 was a new regulator of SAHF, it could regulate the formation of SAHF through the AMPK/mTOR pathway. The data suggested that ABO could be used as a powerful tool to inhibit the replicative senescence of HDFs.

A Dominant-Negative Mutant of ANXA7 Impairs Calcium Signaling and Enhances the Proliferation of Prostate Cancer Cells by Downregulating the IP3 Receptor and the PI3K/mTOR Pathway

Annexin A7/ANXA7 is a calcium-dependent membrane fusion protein with tumor suppressor gene (TSG) properties, which is located on chromosome 10q21 and is thought to function in the regulation of calcium homeostasis and tumorigenesis. However, whether the molecular mechanisms for tumor suppression are also involved in the calcium- and phospholipid-binding properties of ANXA7 remain to be elucidated. We hypothesized that the 4 C-terminal endonexin-fold repeats in ANXA7 (GX(X)GT), which are contained within each of the 4 annexin repeats with 70 amino acids, are responsible for both calcium- and GTP-dependent membrane fusion and the tumor suppressor function. Here, we identified a dominant-negative triple mutant (DNTM/DN-ANXA7J) that dramatically suppressed the ability of ANXA7 to fuse with artificial membranes while also inhibiting tumor cell proliferation and sensitizing cells to cell death. We also found that the [DNTM]ANA7 mutation altered the membrane fusion rate and the ability to bind calcium and phospholipids. In addition, in prostate cancer cells, our data revealed that variations in phosphatidylserine exposure, membrane permeabilization, and cellular apoptosis were associated with differential IP3 receptor expression and PI3K/AKT/mTOR modulation. In conclusion, we discovered a triple mutant of ANXA7, associated with calcium and phospholipid binding, which leads to the loss of several essential functions of ANXA7 pertinent to tumor protection and highlights the importance of the calcium signaling and membrane fusion functions of ANXA7 for preventing tumorigenesis.

Annexin A protein family: Focusing on the occurrence, progression and treatment of cancer

The annexin A (ANXA) protein family is a well-known tissue-specific multigene family that encodes Ca²⁺ phospholipid-binding proteins. A considerable amount of literature is available on the abnormal expression of ANXA proteins in various malignant diseases, including cancer, atherosclerosis and diabetes. As critical regulatory molecules in cancer, ANXA proteins play an essential role in cancer progression, proliferation, invasion and metastasis. Recent studies about their structure, biological properties and functions in different types of cancers are briefly summarised in this review. We further discuss the use of ANXA as new class of targets in the clinical diagnosis and treatment of cancer.

Structure of the ALS Mutation Target Annexin A11 Reveals a Stabilising N-Terminal Segment

The functions of the annexin family of proteins involve binding to Ca²⁺, lipid membranes, other proteins, and RNA, and the annexins share a common folded core structure at the C terminus. Annexin A11 (AnxA11) has a long N-terminal region, which is predicted to be disordered, binds RNA, and forms membraneless organelles involved in neuronal transport. Mutations in AnxA11 have been linked to amyotrophic lateral sclerosis (ALS). We studied the structure and stability of AnxA11 and identified a short stabilising segment in the N-terminal end of the folded core, which links domains I and IV. The crystal structure of the AnxA11 core highlights main-chain hydrogen bonding interactions formed through this bridging segment, which are likely conserved in most annexins. The structure was also used to study the currently known ALS mutations in AnxA11. Three of these mutations correspond to buried Arg residues highly conserved in the annexin family, indicating central roles in annexin folding. The structural data provide starting points for detailed structure-function studies of both full-length AnxA11 and the disease variants being identified in ALS.

Expression of Metazoan Annexins in Yeast Provides Protection Against Deleterious Effects of the Biofuel Isobutanol

The ability of microorganisms to produce biofuels by fermentation is adversely affected by the perturbing effects of the hydrophobic biofuel on plasma membrane structure. It is demonstrated here that heterologous expression of metazoan, calcium-dependent, membrane-binding proteins of the annexin class can reduce deleterious effects of isobutanol on Saccharomyces cerevisiae viability and complex membrane functions. Therefore, expression of annexins in industrial strains of yeast or bacteria may prove beneficial in biofuel production.

Annexins induce curvature on free-edge membranes displaying distinct morphologies

Annexins are a family of proteins characterized by their ability to bind anionic membranes in response to Ca²⁺-activation. They are involved in a multitude of cellular functions including vesiculation and membrane repair. Here, we investigate the effect of nine annexins (ANXA1-ANXA7, ANXA11, ANXA13) on negatively charged double supported membrane patches with free edges. We find that annexin members can be classified according to the membrane morphology they induce and matching a dendrogam of the annexin family based on full amino acid sequences. ANXA1 and ANXA2 induce membrane folding and blebbing initiated from membrane structural defects inside patches while ANXA6 induces membrane folding originating both from defects and from the membrane edges. ANXA4 and ANXA5 induce cooperative roll-up of the membrane starting from free edges, producing large rolls. In contrast, ANXA3 and ANXA13 roll the membrane in a fragmented manner producing multiple thin rolls. In addition to rolling, ANXA7 and ANXA11 are characterized by their ability to form fluid lenses localized between the membrane leaflets. A shared feature necessary for generating these morphologies is the ability to induce membrane curvature on free edged anionic membranes. Consequently, induction of membrane curvature may be a significant property of the annexin protein family that is important for their function.

Lineage-specific splicing of a brain-enriched alternative exon promotes glioblastoma progression

Tissue-specific alternative splicing is critical for the emergence of tissue identity during development, yet the role of this process in malignant transformation is undefined. Tissue-specific splicing involves evolutionarily conserved, alternative exons that represent only a minority of the total alternative exons identified. Many of these conserved exons have functional features that influence signaling pathways to profound biological effect. Here, we determined that lineage-specific splicing of a brain-enriched cassette exon in the membrane-binding tumor suppressor annexin A7 (ANXA7) diminishes endosomal targeting of the EGFR oncoprotein, consequently enhancing EGFR signaling during brain tumor progression. ANXA7 exon splicing was mediated by the ribonucleoprotein PTBP1, which is normally repressed during neuronal development. PTBP1 was highly expressed in glioblastomas due to loss of a brain-enriched microRNA (miR-124) and to PTBP1 amplification. The alternative ANXA7 splicing trait was present in precursor cells, suggesting that glioblastoma cells inherit the trait from a potential tumor-initiating ancestor and that these cells exploit this trait through accumulation of mutations that enhance EGFR signaling. Our data illustrate that lineage-specific splicing of a tissue-regulated alternative exon in a constituent of an oncogenic pathway eliminates tumor suppressor functions and promotes glioblastoma progression. This paradigm may offer a general model as to how tissue-specific regulatory mechanisms can reprogram normal developmental processes into oncogenic ones.

Annexin A7 trafficking to alveolar type II cell surface: possible roles for protein insertion into membranes and lamellar body secretion

A role for annexin A7 (A7) is postulated in the obligatory fusion between lamellar bodies and the plasma membrane during surfactant secretion in alveolar type II cells. This study investigated if surfactant secretagogues increase cell surface A7, which could support A7 insertion into plasma membrane as annexin proteins reportedly lack membrane penetration ability. In vivo trafficking of A7 to cell surface was determined by immuno-staining after non-permeabilizing fixation of alveolar type II cells. Stimulation with various secretagogues increased protein kinase-dependent staining for A7 and ABCA3 in comparison to control cells. Biotin-labeling of surface proteins showed ~4% of total A7 in control cells, which increased ~3-4 folds in stimulated type II cells. Increased cell surface A7 was also observed by protein cross-linking studies showing ~70kDa A7-adduct in the membranes but not in the cytosol fraction of PMA- or A23187-stimulated cells. In vitro phosphorylation increased the Ca(2+)-dependent binding of recombinant A7 to lung plasma membranes; and subsequent cross-linking showed increased levels of ~70kDa A7-adduct. PMA-stimulation of type II cells increased A7 trafficking to lipid rafts suggesting that the latter are involved in A7 trafficking to the cell surface. However, in vitro membrane insertion of recombinant A7 and its tryptophan mutants as determined by fluorescence quenching with doxylPC suggested only shallow membrane insertion by A7. Together, our studies support in vivo association between surfactant secretion and cell surface A7 occurring by insertion into plasma membrane and by fusion of A7 containing lamellar bodies.

Secretagogues of lung surfactant increase annexin A7 localization with ABCA3 in alveolar type II cells

Membrane fusion between the lamellar bodies and plasma membrane is an obligatory event in the secretion of lung surfactant. Previous studies have postulated a role for annexin A7 (A7) in membrane fusion during exocytosis in some cells including alveolar type II cells. However, the intracellular trafficking of A7 during such fusion is not described. In this study, we investigated association of endogenous A7 with lamellar bodies in alveolar type II cells following treatment with several secretagogues of lung surfactant. Biochemical studies with specific antibodies showed increased membrane-association of cell A7 in type II cells stimulated with agents that increase secretion through different signaling mechanisms. Immuno-fluorescence studies showed increased co-localization of A7 with ABCA3, the lamellar body marker protein. Because these agents increase surfactant secretion through activation of PKC and PKA, we also investigated the effects of PKC and PKA inhibitors, bisindolylmaleimideI (BisI) and H89, respectively, on A7 partitioning. Western blot analysis showed that these inhibitors prevented secretagogue-mediated A7 increase in the membrane fractions. These inhibitors also blocked increased co-localization of A7 with ABCA3 in secretagogue-treated cells, as revealed by immuno-fluorescence studies. In vitro studies with recombinant A7 showed phosphorylation with PKC and PKA. The cell A7 was also phosphorylated in cells treated with surfactant secretagogues. Thus, our studies demonstrate that annexin A7 relocates to lamellar bodies in a phosphorylation-dependent manner. We suggest that activation of protein kinase promotes phosphorylation and membrane-association of A7 presumably to facilitate membrane fusion during lung surfactant secretion.

Cluster analysis and phylogenetic relationship in biomarker identification of type 2 diabetes and nephropathy

Cluster analysis of DNA microarray data that uses statistical algorithms to arrange the genes according to similarity in patterns of gene expression and the output displayed graphically is described in this article. Hierarchical clustering is a multivariate tool often used in phylogenetics, comparative genomics to relate the evolution of species. The patterns seen in microarray expression data can be interpreted as indications of the status of the genes responsible for nephropathy in peripheral blow cells of type 2 diabetes (T2DN). Out of 415 genes totally expressed in the 3 DNA chips it was concluded that only 116 genes expressed in T2DN and in that only 50 are functional genes. These 50 functional genes are responsible for diabetic nephropathy; of these 50, some of the genes which are more expressed and responsible are AGXT: Alanine-glyoxylate aminotransferase, RHOD: Ras homolog gene family, CAPN6: Calpain 6, EFNB2: Ephrin-B2, ANXA7: Annexin A7, PEG10: Paternally expressed 10, DPP4: Dipeptidyl-peptidase 4 (CD26, adenosine deaminase complexing protein 2), ENSA: Endosulfine alpha, IGFBP2: Insulin-like growth factor binding protein 2, 36kDa, CENPB: Centromere protein B, 80kDa, MLL3: Myeloid/lymphoid or mixed-lineage leukemia 3, BDNF: Brain-derived neurotrophic factor, EIF4A2: Eukaryotic translation initiation factor 4A, isoform 2, PPP2R1A: Protein phosphatase 2 (formerly 2A), regulatory subunit A, alpha isoform. Fifty genes and their nucleotide sequences are taken from NCBI and a phylogenetic tree is constructed using CLUSTAL W and the distances are closer to each other concluding that based on the sequence similarity and evolution the genes are expressed similarly. Literature survey is done for each gene in OMIM and the genes responsible for diabetic nephropathy are listed.

Novel protein ligands of the annexin A7 N-terminal region suggest pro-beta helices engage one another with high specificity

The N-terminal regions of annexins A7 (synexin) and A11 consist of an extended series of short sequence repeats rich in tyrosine, proline, and glycine that provide binding sites for other proteins that may be recruited to membranes by the annexins and that may modulate the calcium and membrane binding activities of the annexin core domains. In this study two new ligands for the annexin A7 N terminal region were identified by yeast two hybrid screening: the TNFalpha receptor regulatory protein SODD (Suppressor Of Death Domains) and KIAA0280, a protein of unknown function. Strikingly, the sites of interaction of these proteins with the annexin also contain sequence repeats similar to those present in the N-termini of annexins A7 and A11. It was also found that the annexin A7 N-terminal region interacts with itself in the two hybrid assay. These results suggest that sequence repeats of this nature form novel structures, called YP pro-beta helices, that are characterized by an ability to interact with one another. Specificity of interactions between the pro-beta helices in different proteins may be encoded by the variations of residues and lengths of the sequence repeats.

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

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

Purification and characterization of a new anticoagulant protein, PP27, from placenta

It has long been known that water extracts of placenta hominis (Jahage in Korean) are effective for treating immunological and vascular diseases and is a major constituent of traditional oriental medicines. We report herein on the isolation and purification of a new type of anticoagulant protein, PP27, from human placenta. PP27 ran as a single band on SDS-PAGE with a molecular mass (Mr) of 27 kDa under denaturing conditions and chromatography on a calibrated Sepharose 4B column indicated a molecular mass of 23 kDa, a value that is similar to those of other PP4 enzymes reported to date. The isoelectric point of PP27 was pI 5.2. PP27, at doses higher than 10 microg/ml, inhibited platelet activating factor (PAF)-induced platelet activation in a dose-dependent manner. The protein was found to inhibit the coagulation time in a concentration-dependent manner. PP27, which acts as a vascular anticoagulant of annexin type, inhibits the blood clotting process by virtue of its binding of essential lipids, which is dependent on the presence of Ca2+ ions. In the presence of Ca2+ ions, PP27 combines with platelet membranes and neutralizes their procoagulant effect. Coagulation, triggered by the addition of thromboplastin/lipid mixtures, is extinguished by PP27.

Partial truncation of the NH2-terminus affects physical characteristics and membrane binding, aggregation, and fusion properties of annexin A7

Annexin A7 (synexin, annexin VII), a member of the annexin family of proteins, causes aggregation of membranes in a Ca2+-dependent manner and has been suggested to promote membrane fusion during exocytosis of lung surfactant, catecholamines, and insulin. Although annexin A7 (A7) was one of the first annexin proteins described, limited studies of its physical characteristics or of structural domains affecting any of its proposed functions have been conducted. As postulated for other annexin proteins, the unique NH2-domain possibly determines the functional specificity of A7. Therefore, we evaluated the effects of segmental deletions in the NH2-terminus on several characteristics associated with the COOH-terminus of A7. The COOH-terminus contains the only tryptophan residue, and all potential trypsin sites, and the Ca2+ and phospholipid binding sites. Recombinant rat A7 and its deletion mutants were expressed using constructs based on the cDNA sequence obtained by screening a rat lung cDNA library. Ca2+ increased the tryptophan fluorescence of A7 and caused a small red shift in the emission maximum (lambdamax), which was further increased in presence of phospholipid vesicles (PLV). NH2-terminal deletions of 29, 51, and 109 residues affected the peak width of fluorescence and lambdamax, surface-exposure of tryptophan residue, and caused a smaller Ca2+-dependent red shift in lambdamax of membrane-bound protein in comparison to A7. Limited proteolysis with trypsin showed that Ca2+ increased the proteolysis of all proteins, but the deletions also affected the pattern of proteolysis. The presence of PLV protected against Ca2+-dependent increase in proteolysis of all proteins. The deletion of first 29 residues also caused decreased membrane binding, aggregation, and fusion, when compared with A7. Collectively, these results suggest that specific NH2-terminus domains can alter those properties of A7 that are normally associated with the COOH-terminus. We speculate that interactions between the NH2- and COOH-termini are required for membrane binding, and aggregation and fusion properties of annexin A7.

Changes in intracellular calcium concentration in response to hypertonicity in bovine articular chondrocytes

Intracellular calcium concentration ([Ca2+]i) in articular chondrocytes changes during mechanical challenges associated with joint movements, because of the fluctuation of the extracellular osmotic environment during joint loading. Matrix synthesis by chondrocytes is modulated by loading patterns, possibly mediated by variations in intracellular composition, including [Ca2+]i. The present study has employed the Ca(2+)-sensitive fluoroprobe Fura-2 to determine the effects of hypertonic shock on intracellular Ca2+ concentration ([Ca2+]i) and to characterise the mechanisms involved in the response for isolated bovine articular chondrocytes. In cells subjected to a hypertonic shock, [Ca2+]i rapidly increased by approximately 300%, reaching a maximal value within 50 s following the hypertonic shock with a recovery of more than 90% towards the initial [Ca2+]i within 5 min. The effect was inhibited by removal of extracellular Ca2+ ions, but not by thapsigargin, indicating that the rise in [Ca2+]i is only a result of influx from the extracellular medium. The rise was insensitive to inhibitors of L-type voltage-activated Ca2+ channels, TRPV channels or stretch-activated cation channels. Non-specific inhibitors of Ca2+ channels like CdCl2, NiCl2, LaCl3 and ZnCl2 significantly attenuated the response, although the extent in which CdCl2 and NiCl2 (both of them inhibitors of annexin-mediated Ca2+ fluxes) inhibited the response was significantly greater. The rise was also sensitive to KBR7943, inhibitor of NCE reverse mode and trifluoperazine, inhibitor of the activity of annexins. Hypertonic shock also produced also hyperpolarisation of chondrocytes (Em measured by means of Di-BA-C4(3), a membrane potential sensitive dye), which was inhibited by TEA-Cl and BaCl, but was not affected by changing the extracellular solution to Ca(2+)-free HBS. Inhibition of hyperpolarisation completely abolished the [Ca2+]i rise following hypertonic shock. Treatment with retinoic acid, which can increase the activity of annexins as Ca2+ transport pathways caused a significant increase in [Ca2+]i. The recovery of [Ca2+] was inhibited by benzamil and was dependent on extracellular Na+, but was unaffected by Na-orthovanadate, an inhibitor of plasma Ca(2+)-ATPase. We conclude that in response to hypertonic shock, NCE reverse mode and annexins are the pathways responsible for the [Ca2+]i increase, while forward mode operation of NCE is responsible for the subsequent extrusion of Ca2+ and recovery of [Ca2+]i towards initial values.

Loss of the F-actin binding and vesicle-associated protein comitin leads to a phagocytosis defect

Comitin is an F-actin binding and membrane-associated protein from Dictyostelium discoideum, which is present on Golgi and vesicle membranes and changes its localization in response to agents affecting the cytoskeleton. To investigate its in vivo functions we have generated knockout mutants by gene replacement. Based on comitin's in vitro functions we examined properties related to vesicular transport and microfilament function. Whereas cell growth, pinocytosis, secretion, chemotaxis, motility, and development were unaltered, comitin-lacking cells were impaired in the early steps of phagocytosis of Saccharomyces cerevisiae particles and of Escherichia coli, whereas uptake of latex beads was unaffected. Furthermore, the lack of comitin positively affected survival of pathogenic bacteria. Mutant cells also showed an altered response to hyperosmotic shock in comparison to the wild type. The redistribution of comitin during hyperosmotic shock in wild-type cells and its presence on early phagosomes suggest a direct involvement of comitin in these processes.

Proteolytic signals in the primary structure of annexins

Annexins are a superfamily of calcium-dependent membrane-associated proteins which interact with phospholipids. The primary structure of Annexins I, III, VII, VIII and XI contain a region enriched in proline, glutamate, serine and threonine (PEST sequences) towards the N-terminal end while annexins II, V and VI possess PEST regions somewhat distal to the N-terminus. These PEST sequences are believed to be the signals for rapid intracellular degradation. Annexin I is known to be cleaved by calpain near its PEST region suggesting that its PEST region might be a possible calpain recognition site. Western blot analysis of annexins V and XI in rat lung homogenates suggest that these proteins are resistant to proteolysis by calpain. Annexin V was found to be stable to intrinsic lung proteases in the presence of either Ca2+ or EGTA while annexin XI was found to be partially degraded by intrinsic lung proteases in the presence of EGTA. Eight of the 10 known mammalian annexins also contain a pentapeptide sequence that is biochemically related to the KFERQ motif which is a known signal that targets protein for lysosomal proteolysis. Our data suggest that the annexins may be regulated by limited proteolysis, most likely at their N-terminal end, while most, if not all, of them might be degraded by the lysosomal pathway.

Biophysical and molecular properties of annexin-formed channels

The annexins are water soluble proteins possessing a hydrophilic surface, which belong to a family of proteins which (a) bind ('annex') both calcium and phospholipids, and (b) form voltage-dependent calcium channels within planar lipid bilayers. Annexins types are diverse (94 annexins in 45 species) and they belong to an enormous multigene family that ranges throughout all eukaryotic kingdoms. Although the structure of these proteins is now well known their functional and physiological roles remain largely unknown and circumstantial. Various experimental approaches provided evidence that annexins function as Ca(2+) channels that could act as regulators of membrane fusion. The identity of annexins is derived from the conserved 34 kDa C-terminal domain which comprises four repeats - except for annexin VI, with eight repeats - of a sequence of approximately seventy amino acids, which holds the area known as the 'endonexin fold', with its identifying GXGTDE. Annexins have been placed into three subgroups of (1) tetrad core and short amino terminal, (2) tetrad core and long amino terminal, and (3) octad core and short amino terminal. The repeats are highly conserved, each forming a compact alpha-helical domain comprising five alpha-helices wound in a right-handed superhelix. Four domains are formed, arranged in a nearly flat and cyclical array, with domains I and IV, and II and III respectively forming two tightly organised modules with almost twofold symmetry. A hydrophilic pore lies at the centre of the molecule, forming a prominent ion channel coated with charged and highly conserved residues. The annexin molecule is slightly curved, with both a convex and a concave face. The cation/anion permeability ratios and the selectivity sequence of the ion channels formed by several annexins confirm the selectivity of the annexins for Ca(2+) over other divalent cations, and reveals the importance of structural sites, e.g. amino acid positions 17, 78, 95 and 112 for the identification of the ion channel's position, function and regulation. Some are sensitive to low doses of the phenothiazine drugs, trifluoperazine (an anti-schizophrenia drug) and promethazine (anti nausea drug) La(3+) and Cd(2+), (blockers of voltage-gated Ca(2+) channels) nifedipine (an inhibitor of non-activating Ca(2+) channels). There are two main competing models used to explain in vitro ion channel activity of annexins: one involves changes in the conductance of ion via electrostatic disturbance of the membrane surface; the other involves a much more extensive alteration in protein structure and a correspondingly deeper penetration into the membrane.

The sorcin-annexin VII calcium-dependent interaction requires the sorcin N-terminal domain

Surface plasmon resonance experiments show that at neutral pH the stability of the complex between sorcin and annexin VII (synexin) increases dramatically between 3 and 6 microM calcium; at the latter cation concentration the K(D) value is 0.63 microM. In turn, the lack of complex formation between the sorcin Ca(2+) binding domain (33-198) and synexin maps the annexin binding site to the N-terminal region of the sorcin polypeptide chain. Annexin VII likewise employs the N-terminal domain, more specifically the first 31 amino acids, to interact with sorcin [Brownawell, A.M. and Creutz, C.E. (1997) J. Biol. Chem. 272, 22182-22190]. The interaction may involve similar structural motifs in the two proteins, namely GGYY and GYGG in sorcin and GYPP in synexin.

bicaudal encodes the Drosophila beta NAC homolog, a component of the ribosomal translational machinery*

bicaudal was the first Drosophila mutation identified as producing mirror-image pattern duplications along the anteroposterior axis of the embryo. However the mutation has been little studied due to its low penetrance and suppressibility. We undertook cloning of the bicaudal locus together with studies of the mutation's effects on key elements of the posterior embryonic patterning pathway. Our mapping studies place the bicaudal mutation within a approximately 2 kb region, 3′ to the protein coding sequence of the Drosophila homolog of beta NAC, a subunit of Nascent polypeptide Associated Complex (NAC). Genomic DNA encoding beta NAC completely rescues the bicaudal phenotype. The lethal phenotype of Enhancer of Bicaudal, E(Bic), a mutation hypothesized to affect the bicaudal locus, is also completely rescued by the beta NAC locus. We further demonstrate that the E(Bic) mutation is caused by a P element insertion into the transcribed region of the beta NAC gene. NAC is among the first ribosome-associated entities to bind the nascent polypeptide after peptide bond formation. In contrast to other bicaudal-embryo-producing mutations, bicaudal leads to ectopic translation of mRNA for the posterior determinant nanos, without affecting the localization of mRNA for its upstream regulator, oskar, in the embryo. These findings suggest that repression of nanos mRNA translation occurs on the ribosome and involves a role for beta NAC.

Annexin VII as a novel marker for invasive phenotype of malignant melanoma

Both F10 and BL6 sublines of B16 mouse melanoma cells are metastatic after intravenous injection, but only BL6 cells are metastatic after subcutaneous injection. While examining the genetic difference between the two sublines, we found a marked reduction of annexin VII expression in BL6 cells. In addition, fusion cell clones of both sublines were as poorly metastatic as F10 cells after subcutaneous injection, and contained the annexin VII message as abundantly as F10 cells. Hence, we examined whether the annexin VII expression was correlated with the less malignant phenotype of clinical cases by immunohistochemistry. Immunoreactivities to anti-annexin VII antibody in melanoma cells were evaluated quantitatively by using skin mast cells as an internal positive control. Eighteen patients with malignant melanoma were divided into two groups: lymph node metastasis-negative and positive groups. The ratio of numbers of patients positive versus negative to the antibody was significantly larger in the former than in the latter group. These results not only indicated that annexin VII serves as a marker for less invasive phenotype of malignant melanoma, but also suggested a possible role of annexin VII in tumor suppression.

Expression and localisation of annexin VII (synexin) isoforms in differentiating myoblasts

Annexin VII exists in a 47 kDa and a 51 kDa isoform with the 51 kDa protein being the only isoform present in skeletal muscle. Expression of the 51 kDa isoform during myogenesis and localization was studied in cells after conversion into myogenic cells by transduction with MyoD and in mouse and human myogenic cell lines. MyoD expression in NIH3T3 and C3H10T1/2 fibroblasts led to disappearance of the mRNA specific for the 47 kDa isoform and appearance of the 51 kDa isoform-specific mRNA. The overall amount of annexin VII protein was reduced in myogenic converted cells. Both in undifferentiated and differentiated cells annexin VII was localized by immunofluorescence microscopy to punctate structures which were distributed all over the cell. A GFP annexin VII fusion protein showed a similar distribution. Cell fractionation studies indicated that annexin VII is equally distributed between cytosol and membrane fractions in undifferentiated cells, while in differentiated cells it is exclusively present in the membrane fraction. By sucrose gradient centrifugation of postnuclear supernatants we identified two distinct annexin VII-containing membrane populations that cofractionated with caveolin 3- and sorcin-containing membranes.

Transcription, biochemistry and localization of nematode annexins

The transcription of three annexin genes in the nematode, Caenorhabditis elegans, was detected by reverse transcriptase/polymerase chain reaction amplification of messenger RNAs. The highest level of expression was from the nex-1 gene, with lower levels detected for the nex-2 and nex-3 genes. The expression of nex-1 was reduced in the Dauer larval stage relative to the other annexins, correlating with the absence of the spermathecal valves, a major site of nex-1 protein localization. Recombinant nex-1 protein was expressed in yeast, isolated by calcium-dependent binding to acidic phospholipids, and its membrane binding and aggregating activities characterized using bovine chromaffin granules as a representative intracellular substrate. Binding to granule membranes was promoted by calcium with half-maximal binding seen at 630 microM calcium. Chromaffin granule aggregation was similarly promoted by the nex-1 protein at 630 microM calcium. This low sensitivity to calcium suggests the annexin can only be activated in vivo near the plasma membrane or other sources of calcium. Sequences including the nex-1 promoter were fused to the gene for green fluorescent protein and this construct was introduced into nematodes by microinjection. Examination of transgenic offspring revealed specific nex-1 promoter activity in the pharynx, the hypodermal cells, the vulva, and the spermathecal valve, locations in which the annexin may function in collagen secretion/deposition and membrane-membrane interactions. A sensitive anti-nex-1 antibody labelled with rhodamine was injected into body cavities of the nematode but did not detect extracellular nex-1 protein. Therefore, this annexin is apparently cytosolic and may function on the cytoplasmic side of the plasma membrane of the spermathecal valve to chaperon the folding of this membrane during the opening and closing of the valve.

Functional identification of alpha 1-giardin as an annexin of Giardia lamblia

A protein with a relative molecular mass of 31 kDa was specifically extracted by EGTA from a detergent-insoluble fraction of Giardia lamblia. N-terminal sequencing showed this protein to be identical to alpha 1-giardin, a component of the ventral disc which, based on its predicted amino acid sequence, has been classified as annexin XIX. Purified alpha 1-giardin associated with multilamellar phosphatidyl serine-containing vesicles in a Ca(2+)-dependent manner, confirming that it is a functional annexin. Molecular modelling of the amino acid sequence of the giardial annexin into the X-ray structure of annexin V suggests that the Ca(2+)-binding sites, which, as in other annexins, are all located on the convex surface of the molecule, are of the low-affinity type III.

Calcium-dependent binding of sorcin to the N-terminal domain of synexin (annexin VII)

The annexins are characterized by their ability to bind phospholipid membranes in a Ca2+-dependent manner. Sequence variability between the N-terminal domains of the family members may contribute to the specific cellular function of each annexin. To identify proteins that interact with the N-terminal domain of synexin (annexin VII), a fusion protein was constructed composed of glutathione S-transferase fused to amino acids 1-145 of human synexin. Affinity chromatography using this construct identified sorcin as a Ca2+-dependent synexin-binding protein. Overlay assays confirmed the interaction. The glutathione S-transferase construct associates with recombinant sorcin over the range of pCa2+ = 4.7-3.1 with no binding observed at pCa2+ = 5.4. Overlay assays using deletion constructs of the synexin N-terminal domain mapped the sorcin binding site to the N-terminal 31 amino acids of the synexin protein. Additionally, synexin forms a complex with sorcin and recruits this protein to chromaffin granule membranes in a Ca2+-dependent manner. Sorcin is able to inhibit synexin-mediated chromaffin granule aggregation in a manner saturable with increasing sorcin concentrations, but does not influence the Ca2+ sensitivity of synexin-mediated granule aggregation. Therefore, the interaction between sorcin and synexin may serve to regulate the functions of these proteins on membrane surfaces in a Ca2+-dependent manner.

Crystal structure of the C-terminal tetrad repeat from synexin (annexin VII) of Dictyostelium discoideum

Synexin (annexin VII) is a cytosolic Ca(2+)-binding protein that promotes membrane fusion and forms voltage-regulated ion channels in artificial and natural membranes. The crystal structure of the C-terminal tetrad repeat from recombinant synexin (annexin VII) of Dictyostelium discoideum was solved to 2.45 A resolution. The protein crystallized in a dimeric form with two molecules joined face-to-face by their convex sides. Mainly hydrogen bonds and van der Waals contacts are involved in dimer formation, while not Ca2+ is bound to the conserved Ca(2+)-binding sites. The truncated N terminus is folded into a short antiparallel beta-sheet, from which the side-chain of Tyr111 penetrates sideways into the central, hydrophilic pore and may directly affect the ion channel activity. In order to investigate the structure of the missing N-terminal domain, we synthesized a 37-membered peptide of the N-terminal tail, (GYPPQQ)6G. CD and NMR studies showed a random coil conformation of the peptide in solution, suggesting for the synexin N terminus the lack of a well-ordered, three-dimensional fold.

Antisense-mediated regulation of Annexin VII gene expression during the transition from growth to differentiation in Dictyostelium discoideum

Annexin VII is believed to be required for proper Ca(2+)-homeostasis in Dictyostelium discoideum cells. As was previously reported, the expression of Annexin VII gene increased during the transition of D. discoideum Ax-2 cells from growth to differentiation. We have casually cloned an interesting gene, Quit3, by the differential plaque hybridization. Quit3 had no coding region, and was expressed more predominantly in the growth phase than in the differentiation phase. Unexpectedly, this gene was found to encode the complementary sequence of Annexin VII. Therefore, it is most likely that the Quit3 mRNA may regulate the Annexin VII synthesis by the natural antisense transcript via an antisense RNA-RNA interaction, thus resulting in striking increase of Annexin VII production in the phase-shift of cells from growth to differentiation. Since Annexin VII is known to be coded for by a single gene in Dictyostelium, the antisense RNA seemed to be encoded in the same genetic locus as the Annexin VII mRNA.

Identification of an ion channel activity of the Vpu transmembrane domain and its involvement in the regulation of virus release from HIV-1-infected cells

HIV-1 Vpu catalyzes two independent functions, degradation of the virus receptor CD4 in the endoplasmic reticulum and enhancement of virus release from the cell surface. These activities are confined to distinct structural domains of Vpu, the cytoplasmic tail and the transmembrane (TM) anchor, respectively. It was recently reported that Vpu forms cation-selective ion channels in lipid bilayers. Here we report that this property of Vpu is a characteristic of its TM anchor. Expression of full-length Vpu in Xenopus oocytes increases membrane conductance. The Vpu-induced conductance is selective to monovalent cations over anions, does not discriminate Na+ over K+ and shows marginal permeability to divalent cations. Notably, introduction of the scrambled TM sequence into full-length Vpu abrogates its capacity to increase membrane conductance in oocytes and to promote virus release from infected cells. Reconstitution of synthetic Vpu fragments in lipid bilayers identified an ion channel activity for a sequence corresponding to the TM domain of Vpu. In contrast, a peptide with the same amino acid composition but with a scrambled sequence does not form ion channels. Our findings therefore suggest that the ability of Vpu to increase virus release from infected cells may be correlated with an ion channel activity of the TM domain, thereby providing a potential target for drug intervention based on the development of Vpu-specific channel blockers.

Human placental Fc gamma-binding proteins in the maternofetal transfer of IgG

Annexin II, a member of the annexin family of Ca2+ and phospholipid binding proteins, is present in human placenta. Placental annexin II has low affinity FcR activity, and is present as a heterotetramere on syncytiotrophoblast apical cell membrane extracellular surface. In addition to annexin II, transmembraneous leukocyte FcRIII is present on syncytiotrophoblast apical membrane. Either one, or both molecules may mediate the binding of IgG and thereby facilitate its transport through the syncytiotrophoblast layer. However, the presence of other maternal plasma proteins in syncytiotrophoblasts that are not transported to the human fetus is suggestive of nonspecific fluid phase endocytosis. The MHC class I like FcR, similar to the receptor found in neonatal rodent intestine, FcRn, is present intracellularly in human syncytiotrophoblasts, as is its light chain beta 2-microglobulin. The hFcRn is not detected on the apical plasma membrane. The placental hFcRn co-localizes with IgG in syncytiotrophoblast granules. It is likely that hFcRn binds and transcytoses IgG through the syncytiotrophoblast. Protected transfer of IgG may occur within syncytiotrophoblast endocytotic vesicles prior to release in the villous stroma and subsequent translocation into the lumen of fetal stem vessels by uptake and transport in endothelial caveolae.

Membrane fusion protein synexin (annexin VII) as a Ca2+/GTP sensor in exocytotic secretion

Exocytotic membrane fusion and secretion are promoted by the concerted action of GTP and Ca2+, although the precise site(s) of action in the process are not presently known. However, the calcium-dependent membrane fusion reaction driven by synexin (annexin VII) is an in vitro model for this process, which we have now found to be further activated by GTP. The mechanism of fusion activation depends on the unique ability of synexin to bind and hydrolyze GTP in a calcium-dependent manner, both in vitro and in vivo in streptolysin O-permeabilized chromaffin cells. The required [Ca2+] for GTP binding by synexin is in the range of 50-200 microM, which is known to occur at exocytotic sites in chromaffin cells, neurons, and other cell types. Previous immunolocalization studies place synexin at exocytotic sites in chromaffin cells, and we conclude that synexin is an atypical G protein that may be responsible for both detecting and mediating the Ca2+/GTP signal for exocytotic membrane fusion.

Novel isoforms of synexin in Xenopus laevis: multiple tandem PGQM repeats distinguish mRNAs in specific adult tissues and embryonic stages

Synexin (annexin VII) is a calcium-dependent, phospholipid-binding and membrane fusion protein in the annexin gene family, which forms calcium channels and may play a role in exocytotic secretion. We report here the cloning and characterization of five novel isoforms of cDNAs encoding Xenopus synexin from brain, oocyte and stage 24 cDNA libraries. The most prevalent Xenopus synexin has 1976 bp of cDNA sequence, which contains a 1539 bp open reading frame of 512 amino acids encoding a 54 kDa protein. This Xenopus protein is 6 kDa larger than the previously reported human and mouse synexins with which it shares approx. 73% identity in the C-terminal region and approx. 44% identity in the N-terminal region. Further studies with PCR revealed the molecular basis of the substantial divergence in the Xenopus synexin's N-terminal domain. The domain equivalent to the mammalian tissue-specific cassette exon occurs at a different position and is variable in size and sequence. The most interesting observation relates to the occurrence of different forms of synexin due to the varying numbers of tandem PGQM repeats that are expressed differently in different adult tissues and embryonic stages. For these reasons we have labelled this set of unique isoforms annexin VIIb, referring to mammalian forms, which lack the PGQM tandem repeats, as annexin VIIa. In spite of these differences from annexin VIIa, the form of recombinant annexin VIIb with three PGQM repeats was found to be catalytically active. We interpret these results to indicate that the actual calcium and phospholipid binding sites are conserved in Xenopus, and that the variations observed between members of the synexin gene family in the regulatory domain clearly point towards the tissue- and stage-specific roles of individual members, possibly involving the exocytotic process.

Linking microfilaments to intracellular membranes: the actin-binding and vesicle-associated protein comitin exhibits a mannose-specific lectin activity

Comitin is a 24 kDa actin-binding protein from Dictyostelium discoideum that is located primarily on Golgi and vesicle membranes. We have probed the molecular basis of comitin's interaction with both actin and membranes using a series of truncation mutants obtained by expressing the appropriate cDNA in Escherichia coli. Comitin dimerizes in solution; its principle actin-binding activity is located between residues 90 and 135. The N-terminal 135 'core' residues of comitin contain a 3-fold sequence repeat that is homologous to several monocotyledon lectins and which retains key residues that determine these lectins' three-dimensional structure and mannose binding. These repeats of comitin appear to mediate its interaction with mannose residues in glycoproteins or glycolipids on the cytoplasmic surface of membrane vesicles from D.discoideum, and comitin can be released from membranes with mannose. Our data indicate that comitin binds to vesicle membranes via mannose residues and, by way of its interaction with actin, links these membranes to the cytoskeleton.

Sperm protein (sp50) binds to acrosome and plasma membranes in a Ca(2+)-dependent manner: possible role in acrosome reaction

Annexins are a family of Ca(2+)-binding proteins involved in the exocytotic process. The presence and the role of annexins in mammalian spermatozoa have not been well established. Two annexin-like proteins were obtained from guinea pig testis, a doublet of Mr 31-33 kD (p31/33) and a protein of Mr 50 kD (p50). Both proteins were able to bind to erythrocyte ghosts in a Ca(2+)-dependent fashion. Polyclonal antibodies against p31/33 reacted with two major proteins, Mrs 50 kD (sp50) and 42 kD (sp42), from mature and immature guinea pig spermatozoa. p50 and sp50 are likely the native proteins from testis and spermatozoa, respectively, and they are seemingly related. By immunofluorescence, sp50 was only found in the acrosome region of immature and capacitated and noncapacitated spermatozoa, and its location was intracellular. In spermatozoa undergoing acrosome reaction, sp50 was detected in the whole acrosome, while in spermatozoa that had undergone acrosome reaction sp50 was not detected. However, in the protein pattern of acrosome reaction vesicles, anti-p31/33 antibody revealed diffuse bands of Mr 35-38 kD. sp50 was able to bind to plasma membrane fragments and acrosome outer membrane from demembranated sperm in a Ca(2+)-dependent fashion. The presence of sp50 in the acrosome region, its distribution throughout the acrosome membrane just before the acrosome reaction, and its ability to bind both plasma and outer acrosome membranes in a Ca(2+)-dependent manner suggest that sp50 may participate in the acrosome reaction mechanism in guinea pig spermatozoa.

Identification, localization, and functional implications of an abundant nematode annexin

Cultures of the nematode C. elegans were examined for the presence of calcium-dependent, phospholipid-binding proteins of the annexin class. A single protein of apparent mass on SDS-polyacrylamide gels of 32 kD was isolated from soluble extracts of nematode cultures on the basis of its ability to bind to phospholipids in a calcium-dependent manner. After verification of the protein as an annexin by peptide sequencing, an antiserum to the protein was prepared and used to isolate a corresponding cDNA from an expression library in phage lambda gt11. The encoded protein, herein referred to as the nex-1 annexin, has a mass of 35 kD and is 36-42% identical in sequence to 10 known mammalian annexins. Several unique modifications were found in the portions of the sequence corresponding to calcium-binding sites. Possible phosphorylation sites in the NH2-terminal domain of the nematode annexin correspond to those of mammalian annexins. The gene for this annexin (nex-1) was physically mapped to chromosome III in the vicinity of the dpy-17 genetic marker. Two other annexin genes (nex-2 and nex-3) were also identified in chromosome III sequences reported by the nematode genomic sequencing project (Sulston, J., Z. Du, K. Thomas, R. Wilson, L. Hillier, R. Staden, N. Halloran, P. Green, J. Thierry-Mieg, L. Qiu, et al. 1992. Nature (Lond.). 356:37-41). The nex-1 annexin was localized in the nematode by immunofluorescence and by electron microscopy using immunogold labeling. The protein is associated with membrane systems of the secretory gland cells of the pharynx, with sites of cuticle formation in the grinder in the pharynx, with yolk granules in oocytes, with the uterine wall and vulva, and with membrane systems in the spermathecal valve. The presence of the annexin in association with the membranes of the spermathecal valve suggests a novel function of the protein in the folding and unfolding of these membranes as eggs pass through the valve. The localizations also indicate roles for the annexin corresponding to those proposed in mammalian systems in membrane trafficking, collagen deposition, and extracellular matrix formation.

Annexins II, IV, V and VI relocate in response to rises in intracellular calcium in human foreskin fibroblasts

Annexins are a family of proteins implicated in a number of cellular processes involving calcium. We studied annexins I, II, IV, V and VI and found that they are all present in human foreskin fibroblasts and, from immunocytochemical studies, have distinct locations in the cell. Only annexin IV and annexin V have unstructured cytoplasmic staining patterns consistent with predominantly cytosolic locations. Annexin VI partially colocalizes with the endoplasmic reticulum. In contrast, annexins I and II are both associated with the plasma membrane with annexin II having a very homogeneous staining compared with the punctate pattern observed for annexin I. Annexins I, IV and V are all present in the nucleus at higher concentrations than in the cytoplasm. Treatment of cells with the calcium ionophore A23187 to raise intracellular calcium, results in relocations of annexin II, IV, V and VI. Intranuclear annexins IV and V relocate to the nuclear membrane whereas the cytosolic pools of these annexins relocate to the plasma membrane. Annexin II relocates to granular structures at the plasma membrane whereas annexin VI relocates to a more homogeneous distribution on the plasma membrane. These results are consistent with an important role for annexins in mediating the calcium signal at the plasma membrane and within the nuclei of fibroblasts.

cDNA cloning and tissue-specific regulation of expression of rat calcium-binding protein 65/67. Identification as a homologue of annexin VI

We isolated a cDNA encoding the rat membrane-associated 65/67-kDa calcium-binding protein, CBP 65/67, from a lambda ZAP II cDNA-expression library of rat liver by immunoscreening using monospecific polyclonal anti-(CBP 65/67) antibodies and monoclonal anti-(CBP 65/67) IgG. The product of this cDNA expressed in Escherichia coli was confirmed as CBP 65/67 both by immunostaining and by comparison of the molecular mass with the CBP 65/67 isolated from rat liver by SDS/PAGE. The cDNA sequence and the deduced amino acid sequence of CBP 65/67 both show a high degree of identity to human p68 and human calelectrin, which belong to a family of calcium-dependent, membrane-associated, phospholipid-binding proteins, called annexins. This means that CBP 65/67 is a homolog of the two human proteins just mentioned above. We are not aware that a rat annexin VI has previously been isolated and sequenced. The mRNA expression of CBP 65/67 in different rat organs during development was investigated by Northern blot analysis. In adult tissues, high mRNA levels of CBP 65/67 were found in lung, heart, muscle, spleen and especially in thymus and pancreas, whereas in liver, kidney, intestine, stomach and brain only low levels of CBP 65/67 mRNA could be detected. The amount of mRNA during tissue development in kidney, stomach and muscle showed only slight changes. In contrast, a significant increase of CBP 65/67 expression was observed in liver, lung, heart and brain. In most of the organs investigated, the level of mRNA correlated closely with the level of protein expression, indicating that the expression of CBP 65/67 in most organs is controlled primarily at the transcriptional level.

The in vivo role of annexin VII (synexin): characterization of an annexin VII-deficient Dictyostelium mutant indicates an involvement in Ca(2+)-regulated processes

Dictyostelium discoideum cells harbor two annexin VII isoforms of 47 and 51 kDa which are present throughout development. In immunofluorescence and cell fractionation studies annexin VII was found in the cytoplasm and on the plasma membrane. In gene disruption mutants lacking both annexin VII isoforms growth, pinocytosis, phagocytosis, chemotaxis and motility were not significantly impaired under routine laboratory conditions, and the cells were able to complete the developmental cycle on bacterial plates. On non-nutrient agar plates development was delayed by three to four hours and a significant number of aggregates was no longer able to form fruiting bodies. Exocytosis as determined by measuring extracellular cAMP phosphodiesterase, alpha-fucosidase and alpha-mannosidase activity was unaltered, the total amounts of these enzymes were however lower in the mutant than in the wild type. The mutant cells were markedly impaired when they were exposed to low Ca2+ concentrations by adding EGTA to the nutrient medium. Under these conditions growth, motility and chemotaxis were severely affected. The Ca2+ concentrations were similar in mutant and wild-type cells both under normal and Ca2+ limiting conditions; however, the distribution was altered under low Ca2+ conditions in SYN-cells. The data suggest that annexin VII is not required for membrane fusion events but rather contributes to proper Ca2+ homeostasis in the cell.

Expression and localization of annexin VII (synexin) in muscle cells

Annexin VII (synexin) is a member of the annexin family of proteins, which are characterized by Ca(2+)-dependent binding to phospholipids. We used PCR to isolate from a lambda gt11-mouse fibroblast library annexin VII cDNA fragments corresponding to the two isoforms found in both humans and Dictyostelium discoideum. The two isoforms of 47 kDa and 51 kDa differed by 22 amino acids inserted into the proximal third of the hydrophobic N terminus. Annexin VII-specific polypeptides expressed in Escherichia coli were used to generate isoform-specific monoclonal antibodies. Expression of the two isoforms during myogenesis was followed in the myogenic cell lines BC3H1 and L6. Only the 47 kDa isoform was present in undifferentiated L6 or BC3H1 myoblasts. The 51 kDa isoform appeared after myogenesis had been induced and in striated muscle only the 51 kDa isoform was observed. Immunofluorescence showed that annexin VII was located in the cytosol of mononucleated and fused polynucleated cultured cells, whereas in striated muscle, annexin VII was located preferentially at the plasma membrane and the transverse tubules. However, there was also some residual cytosolic staining, which was more abundant in type II (fast twitch) than in type I (slow twitch) fibers. Permeabilization of L6 cells with digitonin in the presence of 5 mM EGTA led to a release of annexin VII from the cells, which paralleled the loss of cytosolic lactate dehydrogenase (LDH) at low detergent concentrations (50 microM). In the presence of 100 microM extracellular Ca2+, annexin VII remained bound to the plasma membrane even in the presence of high digitonin concentrations. Incubation with the Ca(2+)-specific ionophore A23187 and 100 microM extracellular Ca2+ led to a redistribution of annexin VII from the cytosol to the plasma membrane after 30 minutes of incubation. The results obtained indicate a developmentally and Ca(2+)-regulated localization and expression of annexin VII and raise the possibility that annexin VII may play a role in excitation-contraction coupling in skeletal muscle.

Calcium and membrane-binding properties of monomeric and multimeric annexin II

The calcium-dependent interaction of several annexins with membranes was studied. A novel technique was developed that allowed estimation of calcium binding to aggregating systems. This consisted of immobilized phospholipids on phenyl-Sepharose. Proteins associated with this affinity gel in the presence of radioactive calcium and were eluted with the same buffer containing excess EGTA. This produced an elution profile with a peak of excess calcium. Protein extinction coefficients were estimated in order to quantitate the protein more accurately. Association of annexin II with the membrane was of very high affinity and involved a calcium stoichiometry of 11 +/- 1 at 12.5 microM free Ca2+ and 10 +/- 2 at 50 microM free Ca2+. (AII)2(p11)2, a heterotetramer of two annexin II and two p11 subunits, bound 12 +/- 1 Ca2+ at 12.5 microM Ca2+ and 15 +/- 1 Ca2+ at 50 microM Ca2+. These stoichiometries showed a pattern of "all or none" calcium binding where the number of calcium ions bound to a protein-membrane complex was virtually independent of the free calcium concentration or the density of protein on the membrane. (AII)2(p11)2 contains two annexin II subunits so that calcium stoichiometry was not directly related to the number of potential sites. (AII)2(p11)2 required less calcium to support membrane binding than did annexin II. Thus, dimerization of the membrane binding unit may be needed for annexins to function at intracellular calcium levels. Annexin VI contains twice as many putative calcium binding units as annexin V and the same pattern of behavior occurred for this pair of proteins. At 25 microM free calcium, (AII)2(p11)2 alone bound no detectable calcium (< 0.1 mol of calcium/mol of protein) and annexin II bound only 0.3-0.6 calcium ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and characterization of a Dictyostelium discoideum annexin

The annexins are calcium-dependent phospholipid-binding proteins. Recently the gene encoding the homologue of a mammalian annexin has been identified in Dictyostelium discoideum. Analysis of cDNA and genomic clones showed that the transcript for Dictyostelium annexin is alternatively spliced (Greenwood, M. and Tsang, A. (1991) Biochim. Biophys. Acta 1088, 429-432; Döring, V., Schleicher, M and Noegel, A. (1991) J. Biol. Chem. 266, 17509-17515). Here, we showed that the Dictyostelium annexin DNA hybridized to two populations of transcripts. We used a recombinant annexin polypeptide to raise polyclonal antibody. Immunoblot analysis revealed that the antibody recognized two polypeptides of 48 kDa and 54 kDa in developing D. discoideum cells. The molecular sizes of these polypeptides correspond well with the expected sizes of the alternatively spliced products. The 48-kDa and 54-kDa polypeptides were purified by isoelectric focusing to more than 70% homogeneity. The partially purified proteins were found to associate with phosphatidylserine vesicles in a calcium-dependent manner. These results suggest that the 48- and 54-kDa polypeptides are the products of alternative splicing of the annexin transcripts. During development the two polypeptides accumulate at different rates to about 60 times the level detected in vegetative cells. On the other hand, RNA blot analysis showed that the level of the annexin transcripts in multicellular aggregates was about 5 times that of vegetative cells.

Annexin 3 is associated with cytoplasmic granules in neutrophils and monocytes and translocates to the plasma membrane in activated cells

Annexins are soluble proteins capable of binding to phospholipid membranes in a calcium-dependent manner. Annexin 3, a 33 kDa protein mainly expressed in neutrophils, aggregates granules in cell-free assays, and a 36 kDa variant of this protein, specifically expressed in monocytes, has recently been identified. To obtain further information on these proteins, we defined their subcellular localization in resting and activated cells by immunofluorescence microscopy. Both proteins were associated with cytoplasmic granules in resting cells. We obtained evidence to indicate that, in neutrophils which possess a heterogenous granule population, annexin 3 was more likely to be associated with the specific granules. In cells activated with phorbol 12-myristate 13-acetate or opsonized zymosan, the 33 kDa and 36 kDa proteins translocated to the plasma or the phagosome membrane. Upon stimulation with A23187, annexin 3 translocated to the plasma membrane only in neutrophils. We also report that while annexin 3 was associated with restricted membranes in intact cells, it binds indiscriminately to every membrane fraction in cell-free assay. In conclusion, association of both forms of annexin 3 with granules suggests that these proteins could be implicated in processes of granule fusion.

The expression of different annexins in the fish embryo is developmentally regulated

The expression of annexins, a family of Ca2+/phospholipid-binding proteins, was analyzed by biochemical and immunological criteria in the fish Misgurnus fossilis (loach), which is a good model system to study embryonic events. Five different annexins (loach annexins A to E) are present as a maternal pool in the unfertilized eggs. Only annexin E is newly synthesized in the early embryo. Its synthesis, already apparent at mid-blastula, decreases in later stages when two additional annexins (F and G) appear. They are present among the newly synthesized polypeptides of mid-gastrula and later embryonic stages and are also found in loach larvae. The developmentally controlled expression of several annexins indicates a specific role of these proteins at certain embryonic stages.

Genomic organization and chromosomal localization of the mouse synexin gene

We have isolated and characterized the gene encoding mouse synexin, which consists of 14 exons and spans approximately 30 kbp of genomic DNA. The protein's unique N-terminal domain is encoded by six exons, and the C-terminal tetrad repeat, the site of the membrane-fusion and ion-channel domain, is encoded by seven exons. The first exon encodes the 5'-untranslated region. Analysis of synexin-gene expression in different mouse tissues shows that mRNA with exon 6 is only present in brain, heart and skeletal muscle. mRNA lacking exon 6 is expressed in all tissues we have examined. The initiation site for transcription was determined by primer-extension analysis and S1 nuclease mapping. Sequence analysis of the 1.3 kb 5'-flanking region revealed that the promoter has a TATA box located at position -25 and a number of potential promoter and regulatory elements. A CCAAT motif was not observed but CCATT is located in an appropriate position for the CCAAT motif upstream from the transcription-initiation start site. In addition, the 5'-flanking region contains two sets of palindromic sequences. Finally, we have determined that the functional synexin gene (Anx7) is located on mouse chromosome 14 and that a pseudogene (Anx7-ps1) is located on chromosome 10.

Differential tissue expression of Annexin VIII in human

The expression of Annexins V and VIII by human lung, liver, kidney, skin, heart, uterus, spleen and skeletal muscle was investigated by ELISA. All investigated tissues contained Annexin V. Its level varied with the tissue from around 5 microgram (skin) to approximately 120 micrograms (spleen) per g of wet tissue. Contradistinctionally Annexin VIII expression was less ubiquitous and less abundant. Only lung, skin, liver, and kidney expressed Annexin VIII. Its levels were approximately 100-fold less then the Annexin V levels. Immunohistochemical analysis of lung sections revealed Annexin VIII presence exclusively in the endothelia. Annexin V and VIII levels of cultured human umbilical vein endothelial cells, human arterial smooth muscle cells, human lung fibroblasts and HeLa cells were measured by ELISA. All cell types expressed Annexin V whereas only HeLa cells had detectable levels of Annexin VIII. The results indicate a tissue specific expression of Annexin VIII by lung endothelium, suggesting a highly specialised function.