A transmembrane form of annexin XII detected by site-directed spin labeling

An eggshell-localised annexin plays a key role in the coordination of the life cycle of a plant-parasitic nematode with its host

Host-specific plant pathogens must coordinate their life cycles with the availability of a host plant. Although this is frequently achieved through a response to specific chemical cues derived from the host plant, little is known about the molecular basis of the response to such cues and how these are used to trigger activation of the life cycle. In host-specific plant-parasitic cyst nematodes, unhatched juvenile nematodes lie dormant in the eggshell until chemical cues from a suitable host plant are detected and the hatching process is initiated. The molecular mechanisms by which hatch is linked to the presence of these chemical cues is unknown. We have identified a novel annexin-like protein that is localised to the eggshell of the potato cyst nematode Globodera rostochiensis. This annexin is unique in having a short peptide insertion that structural modelling predicts is present in one of the calcium-binding sites of this protein. Host-induced gene silencing of the annexin impacts the ability of the nematode to regulate and control permeability of the eggshell. We show that in the presence of the chemicals that induce hatching annexin lipid binding capabilities change, providing the first molecular link between a nematode eggshell protein and host-derived cues. This work demonstrates how a protein from a large family has been recruited to play a critical role in the perception of the presence of a host and provides a new potential route for control of cyst nematodes that impact global food production.

Role of the IgG4-related cholangitis autoantigen annexin A11 in cholangiocyte protection

BACKGROUND & AIMS

Annexin A11 was identified as autoantigen in IgG4-related cholangitis (IRC), a B-cell driven disease. Annexin A11 modulates calcium-dependent exocytosis, a crucial mechanism for insertion of proteins into their target membranes. Human cholangiocytes form an apical 'biliary bicarbonate umbrella' regarded as defense against harmful hydrophobic bile acid influx. The bicarbonate secretory machinery comprises the chloride/bicarbonate exchanger AE2 and the chloride channel ANO1. We aimed to investigate the expression and function of annexin A11 in human cholangiocytes and a potential role of IgG1/IgG4-mediated autoreactivity against annexin A11 in the pathogenesis of IRC.

METHODS

Expression of annexin A11 in human liver was studied by immunohistochemistry and immunofluorescence. In human control and ANXA11 knockdown H69 cholangiocytes, intracellular pH, AE2 and ANO1 surface expression, and bile acid influx were examined using ratio microspectrofluorometry, cell surface biotinylation, and 22,23-3H-glycochenodeoxycholic acid permeation, respectively. The localization of annexin A11-mEmerald and ANO1-mCherry was investigated by live-cell microscopy in H69 cholangiocytes after incubation with IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies or disease control serum.

RESULTS

Annexin A11 was strongly expressed in human cholangiocytes, but not hepatocytes. Knockdown of ANXA11 led to reduced plasma membrane expression of ANO1, but not AE2, alkalization of intracellular pH and uncontrolled bile acid influx. High intracellular calcium conditions led to annexin A11 membrane shift and colocalization with ANO1. Incubation with IRC patient serum inhibited annexin A11 membrane shift and reduced ANO1 surface expression.

CONCLUSION

Cholangiocellular annexin A11 mediates apical membrane abundance of the chloride channel ANO1, thereby supporting biliary bicarbonate secretion. Insertion is inhibited by IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies. Anti-annexin A11 autoantibodies may contribute to the pathogenesis of IRC by weakening the 'biliary bicarbonate umbrella'.

LAY SUMMARY

We previously identified annexin A11 as a specific autoantigen in immunoglobulin G4-related cholangitis (IRC), a B-cell driven disease affecting the bile ducts. Human cholangiocytes are protected against harmful hydrophobic bile acid influx by a defense mechanism referred to as the 'biliary bicarbonate umbrella'. We found that annexin A11 is required for the formation of a robust bicarbonate umbrella. Binding of patient-derived annexin A11 autoantibodies inhibits annexin A11 function, possibly contributing to bile duct damage by weakening the biliary bicarbonate umbrella in patients with IRC.

Zooming into the Dark Side of Human Annexin-S100 Complexes: Dynamic Alliance of Flexible Partners

Annexins and S100 proteins form two large families of Ca²⁺-binding proteins. They are quite different both structurally and functionally, with S100 proteins being small (10–12 kDa) acidic regulatory proteins from the EF-hand superfamily of Ca²⁺-binding proteins, and with annexins being at least three-fold larger (329 ± 12 versus 98 ± 7 residues) and using non-EF-hand-based mechanism for calcium binding. Members of both families have multiple biological roles, being able to bind to a large cohort of partners and possessing a multitude of functions. Furthermore, annexins and S100 proteins can interact with each other in either a Ca²⁺-dependent or Ca²⁺-independent manner, forming functional annexin-S100 complexes. Such functional polymorphism and binding indiscrimination are rather unexpected, since structural information is available for many annexins and S100 proteins, which therefore are considered as ordered proteins that should follow the classical “one protein–one structure–one function” model. On the other hand, the ability to be engaged in a wide range of interactions with multiple, often unrelated, binding partners and possess multiple functions represent characteristic features of intrinsically disordered proteins (IDPs) and intrinsically disordered protein regions (IDPRs); i.e., functional proteins or protein regions lacking unique tertiary structures. The aim of this paper is to provide an overview of the functional roles of human annexins and S100 proteins, and to use the protein intrinsic disorder perspective to explain their exceptional multifunctionality and binding promiscuity.

Annexin A2 extracellular translocation and virus interaction: A potential target for antivirus-drug discovery

Annexin A2 is a membrane scaffolding and binding protein, which mediated various cellular events. Its functions are generally affected by cellular localization. In the cytoplasm, they interacted with different phospholipid membranes in Ca²⁺ -dependent manner and play vital roles including actin binding, remodeling and dynamics, cytoskeletal rearrangement, and lipid-raft microdomain formation. However, upon cell exposure to certain stimuli, annexin A2 translocates to the external leaflets of the plasma membrane where annexin A2 was recently reported to serve as a virus receptor, play an important role in the formation of virus replication complex, or implicated in virus assembly and budding. Here, we review some of annexin A2 roles in virus infections and the potentiality of targeting annexin A2 in the design of novel and promising antivirus agent that may have a broader consequence in virus therapy.

An alternative N-terminal fold of the intestine-specific annexin A13a induces dimerization and regulates membrane-binding

Annexin proteins function as Ca²⁺-dependent regulators of membrane trafficking and repair that may also modulate membrane curvature. Here, using high-resolution confocal imaging, we report that the intestine-specific annexin A13 (ANX A13) localizes to the tips of intestinal microvilli and determined the crystal structure of the ANX A13a isoform to 2.6 Å resolution. The structure revealed that the N terminus exhibits an alternative fold that converts the first two helices and the associated helix-loop-helix motif into a continuous α-helix, as stabilized by a domain-swapped dimer. We also found that the dimer is present in solution and partially occludes the membrane-binding surfaces of annexin, suggesting that dimerization may function as a means for regulating membrane binding. Accordingly, as revealed by in vitro binding and cellular localization assays, ANX A13a variants that favor a monomeric state exhibited increased membrane association relative to variants that favor the dimeric form. Together, our findings support a mechanism for how the association of the ANX A13a isoform with the membrane is regulated.

Detection of Curvature-Radius-Dependent Interfacial pH/Polarity for Amphiphilic Self-Assemblies: Positive versus Negative Curvature

It is possible that a defined curvature at the membrane interface controls its pH/polarity to exhibit specific bioactivity. By utilizing an interface-interacting spiro-rhodamine pH probe and the Schiff base polarity probe, we have shown that the pH deviation from the bulk phase to the interface (ΔpH)/interfacial dielectric constant (κ(i)) for amphiphilic self-assemblies can be regulated by the curvature geometry (positive/negative) and its radius. According to ¹H NMR and fluorescence anisotropy investigations, the probes selectively interact with an anionic interfacial Stern layer. The ΔpH/κ(i) values for the Stern layer are estimated by UV-vis absorption and fluorescence studies. For the anionic sodium bis-2-ethylhexyl-sulfosuccinate (AOT) inverted micellar (IM) negative interface, the highly restricted water and proton penetration into the Stern layer owing to tight surfactant packing or a reduced water-exposed headgroup area may be responsible for the much lower ΔpH ≈ -0.45 and κ(i) ≈ 28 in comparison to ∼-2.35 and ∼44, respectively, for the anionic sodium dodecyl sulfate (SDS) micellar positive interface with a close similar Stern layer. With increasing AOT IM water-pool radius (1.7-9.5 nm) or [water]/[AOT] ratio ( w₀) (8.0-43.0), the ΔpH and κ(i) increase maximally up to ∼-1.22 and ∼45, respectively, due to a greater water-exposed headgroup area. However, the unchanged ΔpH ≈ -0.65 and κ(i) ≈ 53.0 within radii ∼3.5-8.0 nm for the positive interface of a mixed Triton X-100 (TX-100)/SDS (4:1) micelle justify its packing flexibility. Interestingly, the continuously increasing ΔpH trend for IM up to its largest possible water-pool radius of ∼9.5 nm may rationalize the increase in ΔpH (∼-1.4 to -1.6) with the change in the curvature radii (∼15 to 50 nm) for sodium 1,2-dimyristoyl- sn-glycero-3-phosphorylglycerol (DMPG)/1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) (2:1) large unilamellar vesicles (LUV) owing to its negative interface. Whereas, similar to the micellar positive interface, the unchanged ΔpH at the positive LUV interface was confirmed by fluorescence microscopic studies with giant unilamellar vesicles of identical lipids composition. The present study offers a unique and simple method of monitoring the curvature-radius-dependent interfacial pH/polarity for biologically related membranes.

Annexins-Coordinators of Cholesterol Homeostasis in Endocytic Pathways

The spatiotemporal regulation of calcium (Ca2+) storage in late endosomes (LE) and lysosomes (Lys) is increasingly recognized to influence a variety of membrane trafficking events, including endocytosis, exocytosis, and autophagy. Alterations in Ca2+ homeostasis within the LE/Lys compartment are implicated in human diseases, ranging from lysosomal storage diseases (LSDs) to neurodegeneration and cancer, and they correlate with changes in the membrane binding behaviour of Ca2+-binding proteins. This also includes Annexins (AnxA), which is a family of Ca2+-binding proteins participating in membrane traffic and tethering, microdomain organization, cytoskeleton interactions, Ca2+ signalling, and LE/Lys positioning. Although our knowledge regarding the way Annexins contribute to LE/Lys functions is still incomplete, recruitment of Annexins to LE/Lys is greatly influenced by the availability of Annexin bindings sites, including acidic phospholipids, such as phosphatidylserine (PS) and phosphatidic acid (PA), cholesterol, and phosphatidylinositol (4,5)-bisphosphate (PIP2). Moreover, the cytosolic portion of LE/Lys membrane proteins may also, directly or indirectly, determine the recruitment of Annexins to LE. Strikingly, within LE/Lys, AnxA1, A2, A6, and A8 differentially contribute to cholesterol transport along the endocytic route, in particular, cholesterol transfer between LE and other compartments, positioning Annexins at the centre of major pathways mediating cellular cholesterol homeostasis. Underlying mechanisms include the formation of membrane contact sites (MCS) and intraluminal vesicles (ILV), as well as the modulation of LE-cholesterol transporter activity. In this review, we will summarize the current understanding how Annexins contribute to influence LE/Lys membrane transport and associated functions.

When a transmembrane channel isn't, or how biophysics and biochemistry (mis)communicate

Annexins are a family of soluble proteins that bind to acidic phospholipids such as phosphatidylserine in a calcium-dependent manner. The archetypical member of the annexin family is annexin A5. For many years, its function remained unknown despite the availability of a high-resolution structure. This, combined with the observations of specific ion conductance in annexin-bound membranes, fueled speculations about the possible membrane-spanning forms of annexins that functioned as ion channels. The channel hypothesis remained controversial and did not gather sufficient evidence to become accepted. Yet, it continues to draw attention as a framework for interpreting indirect (e.g., biochemical) data. The goal of the mini-review is to examine the data on annexin-lipid interactions from the last ~30 years from the point of view of the controversy between the two lines of inquiry: the well-characterized peripheral assembly of the annexins at membranes vs. their putative transmembrane insertion. In particular, the potential role of lipid rearrangements induced by annexin binding is highlighted.

Topical Ocular Delivery of TGF-β1 to the Back of the Eye: Implications in Age-Related Neurodegenerative Diseases

Dysregulation of the transforming growth factor-β1 (TGF-β1)/selected small mother against decapentaplegic (SMAD) pathway can be implicated in development of age-related macular degeneration (AMD), and the delivery of TGF-β1 could be beneficial for AMD. We developed a new ophthalmic formulation of TGF-β1 assessing the ocular pharmacokinetic profile of TGF-β1 in the rabbit eye. Small unilamellar vesicles (SUV) loaded with TGF-β1 were complemented with Annexin V and Ca²⁺, and the vitreous bioavailability of TGF-β1 was assessed after topical ocular administration by a commercial ELISA kit. We detected high levels of TGF-β1 (C_max 114.7 ± 12.40 pg/mL) in the vitreous after 60 min (T_max) from the topical application of the liposomal suspension. Ocular tolerability was also assessed by a modified Draize’s test. The new formulation was well tolerated. In conclusion, we demonstrated that the novel formulation was able to deliver remarkable levels of TGF-β1 into the back of the eye after topical administration. Indeed, this TGF-β1 delivery system may be useful in clinical practice to manage ophthalmic conditions such as age-related macular degeneration, skipping invasive intraocular injections.

Annexins: Ca2+ Effectors Determining Membrane Trafficking in the Late Endocytic Compartment

Despite the discovery of annexins 40 years ago, we are just beginning to understand some of the functions of these still enigmatic proteins. Defined and characterized by their ability to bind anionic membrane lipids in a Ca²⁺-dependent manner, each annexin has to be considered a multifunctional protein, with a multitude of cellular locations and diverse activities. Underlying causes for this considerable functional diversity include their capability to associate with multiple cytosolic and membrane proteins. In recent years, the increasingly recognized establishment of membrane contact sites between subcellular compartments opens a new scenario for annexins as instrumental players to link Ca²⁺ signalling with the integration of membrane trafficking in many facets of cell physiology. In this chapter, we review and discuss current knowledge on the contribution of annexins in the biogenesis and functioning of the late endocytic compartment, affecting endo- and exocytic pathways in a variety of physiological consequences ranging from membrane repair, lysosomal exocytosis, to cell migration.

Two translation initiation codons direct the expression of annexin VI 64kDa and 68kDa isoforms

Annexin A6 is a multicompetent, multifunctional protein involved in several biological processes within and outside of the cell. Whereas HeLa cells express annexin A6 only as a 68/67-kDa doublet, indicating alternative splicing (Smith PD et al. (1994) Proc Natl Acad Sci USA 91, 2713-2717), the GMO2784 human fibroblast cell line expresses two additional isoforms at 64 and 58kDa. In both cell lines, annexin A6 is located intracellularly and on the plasma membrane. In vitro eukaryotic protein synthesis of pIRESneoAnxA6 cDNA and pIRESneoAnxA6/Met^1- or Met^33- using a reticulocyte lysate coupled transcription/translation system revealed that this gene contains two translation start codons, Met¹ and Met³³. Immunoprecipitation of the products obtained from the transcription/translation system using various anti-annexin A6 antibodies confirmed the presence of several isoforms and suggested that this protein might be present in different configurations.

Targeting acidity in diseased tissues: mechanism and applications of the membrane-inserting peptide, pHLIP

pHLIPs are a family of soluble ∼36 amino acid peptides, which bind to membrane surfaces. If the environment is acidic, a pHLIP folds and inserts across the membrane to form a stable transmembrane helix, thus preferentially locating itself in acidic tissues. Since tumors and other disease tissues are acidic, pHLIPs' low-pH targeting behavior leads to applications as carriers for diagnostic and surgical imaging agents. The energy of membrane insertion can also be used to promote the insertion of modestly polar, normally cell-impermeable cargos across the cell membrane into the cytosol of targeted cells, leading to applications in tumor-targeted delivery of therapeutic molecules. We review the biochemical and biophysical basis of pHLIPs' unique properties, diagnostic and therapeutic applications, and the principles upon which translational applications are being developed.

Dilution of dipolar interactions in a spin-labeled, multimeric metalloenzyme for DEER studies

The metallo-β-lactamases (MβLs), which require one or two Zn(II) ions in their active sites for activity, hydrolyze the amide bond in β-lactam-containing antibiotics, and render the antibiotics inactive. All known MβLs contain a mobile element near their active sites, and these mobile elements have been implicated in the catalytic mechanisms of these enzymes. However little is known about the dynamics of these elements. In this study, we prepared a site-specific, double spin-labeled analog of homotetrameric MβL L1 with spin labels at positions 163 and 286 and analyzed the sample with DEER (double electron electron resonance) spectroscopy. Four unique distances were observed in the DEER distance distribution, and these distances were assigned to the desired intramolecular dipolar coupling (between spin labels at positions 163 and 286 in one subunit) and to intermolecular dipolar couplings. To rid the spin-labeled analog of L1 of the intermolecular couplings, spin-labeled L1 was "diluted" by unfolding/refolding the spin-labeled enzyme in the presence of excess wild-type L1. DEER spectra of the resulting, spin-diluted enzyme revealed a single distance corresponding to the desire intramolecular dipolar coupling.

Annexin-phospholipid interactions. Functional implications

Annexins constitute an evolutionary conserved multigene protein superfamily characterized by their ability to interact with biological membranes in a calcium dependent manner. They are expressed by all living organisms with the exception of certain unicellular organisms. The vertebrate annexin core is composed of four (eight in annexin A6) homologous domains of around 70 amino acids, with the overall shape of a slightly bent ring surrounding a central hydrophilic pore. Calcium- and phospholipid-binding sites are located on the convex side while the N-terminus links domains I and IV on the concave side. The N-terminus region shows great variability in length and amino acid sequence and it greatly influences protein stability and specific functions of annexins. These proteins interact mainly with acidic phospholipids, such as phosphatidylserine, but differences are found regarding their affinity for lipids and calcium requirements for the interaction. Annexins are involved in a wide range of intra- and extracellular biological processes in vitro, most of them directly related with the conserved ability to bind to phospholipid bilayers: membrane trafficking, membrane-cytoskeleton anchorage, ion channel activity and regulation, as well as antiinflammatory and anticoagulant activities. However, the in vivo physiological functions of annexins are just beginning to be established.

Annexin VI is a mannose-6-phosphate-independent endocytic receptor for bovine β-glucuronidase

Endocytosis and transport of bovine liver β-glucuronidase to lysosomes in human fibroblasts are mediated by two receptors: the well-characterized cation-independent mannose 6-phosphate receptor (IGF-II/Man6PR) and an IGF-II/Man6PR-independent receptor, which recognizes a Ser-Trp*-Ser sequence present on the ligand. The latter receptor was detergent extracted from bovine liver membranes and purified. LC/ESI-MS/MS analysis revealed that this endocytic receptor was annexin VI (AnxA6). Several approaches were used to confirm this finding. First, the binding of bovine β-glucuronidase to the purified receptor from bovine liver membranes and His-tagged recombinant human AnxA6 protein was confirmed using ligand-blotting assays. Second, western blot analysis using antibodies raised against IGF-II/Man6PR-independent receptor as well as commercial antibodies against AnxA6 confirmed that the receptor and AnxA6 were indeed the same protein. Third, double immunofluorescence experiments in human fibroblasts confirmed a complete colocalization of the bovine β-glucuronidase and the AnxA6 receptor on the plasma membrane. Lastly, two cell lines were stably transfected with a plasmid containing the cDNA for human AnxA6. In both transfected cell lines, an increase in cell surface AnxA6 and in mannose 6-phosphate-independent endocytosis of bovine β-glucuronidase was detected. These results indicate that AnxA6 is a novel receptor that mediates the endocytosis of the bovine β-glucuronidase.

The unconventional secretory machinery of fibroblast growth factor 2

Unconventional secretory proteins represent a subpopulation of extracellular factors that are exported from eukaryotic cells by mechanisms that do not depend on the endoplasmic reticulum and the Golgi complex. Various pathways have been implicated in unconventional secretion including those involving intracellular membrane-bound intermediates and others that are based on direct protein translocation across plasma membranes. Interleukin 1β (IL1β) and fibroblast growth factor 2 (FGF2) are classical examples of unconventional secretory proteins with IL1β believed to be present in intracellular vesicles prior to secretion. By contrast, FGF2 represents an example of a non-vesicular mechanism of unconventional secretion. Here, the author discusses the current knowledge about the molecular machinery being involved in FGF2 secretion. To reveal both differential and common requirements, this review further aims at a comprehensive comparison of this mechanism with other unconventional secretory processes. In particular, a potentially general role of tyrosine phosphorylation as a regulatory signal in unconventional protein secretion will be discussed.

Structural origins of nitroxide side chain dynamics on membrane protein α-helical sites

Understanding the structure and dynamics of membrane proteins in their native, hydrophobic environment is important to understanding how these proteins function. EPR spectroscopy in combination with site-directed spin labeling (SDSL) can measure dynamics and structure of membrane proteins in their native lipid environment; however, until now the dynamics measured have been qualitative due to limited knowledge of the nitroxide spin label's intramolecular motion in the hydrophobic environment. Although several studies have elucidated the structural origins of EPR line shapes of water-soluble proteins, EPR spectra of nitroxide spin-labeled proteins in detergents or lipids have characteristic differences from their water-soluble counterparts, suggesting significant differences in the underlying molecular motion of the spin label between the two environments. To elucidate these differences, membrane-exposed α-helical sites of the leucine transporter, LeuT, from Aquifex aeolicus, were investigated using X-ray crystallography, mutational analysis, nitroxide side chain derivatives, and spectral simulations in order to obtain a motional model of the nitroxide. For each crystal structure, the nitroxide ring of a disulfide-linked spin label side chain (R1) is resolved and makes contacts with hydrophobic residues on the protein surface. The spin label at site I204 on LeuT makes a nontraditional hydrogen bond with the ortho-hydrogen on its nearest neighbor F208, whereas the spin label at site F177 makes multiple van der Waals contacts with a hydrophobic pocket formed with an adjacent helix. These results coupled with the spectral effect of mutating the i ± 3, 4 residues suggest that the spin label has a greater affinity for its local protein environment in the low dielectric than on a water-soluble protein surface. The simulations of the EPR spectra presented here suggest the spin label oscillates about the terminal bond nearest the ring while maintaining weak contact with the protein surface. Combined, the results provide a starting point for determining a motional model for R1 on membrane proteins, allowing quantification of nitroxide dynamics in the aliphatic environment of detergent and lipids. In addition, initial contributions to a rotamer library of R1 on membrane proteins are provided, which will assist in reliably modeling the R1 conformational space for pulsed dipolar EPR and NMR paramagnetic relaxation enhancement distance determination.

Fluorescence spectroscopy in thermodynamic and kinetic analysis of pH-dependent membrane protein insertion

Experimental determination of the free energy stabilizing the structure of membrane proteins in their native lipid environment is undermined by a lack of appropriate methods and suitable model systems. Here, we demonstrate how fluorescence correlation spectroscopy can be used to characterize thermodynamics of pH-triggered bilayer insertion of nonconstitutive membrane proteins (e.g., bacterial toxins, colicins). The experimental design is guided by the appropriate thermodynamic scheme which considers two independent processes: pH-dependent formation of a membrane-competent form and its insertion into the lipid bilayer. Measurements of a model protein annexin B12 under conditions of lipid saturation demonstrate that protonation leading to the formation of the membrane-competent state occurs near membrane interface. Lipid titration experiments demonstrate that the free energy of transfer to the intermediate interfacial state is especially favorable, while the free energy of final insertion is modulated by interplay of hydrophobic and electrostatic interactions on the bilayer interface. The general principles of kinetic measurements along the insertion pathway containing interfacial intermediate are discussed and practical examples emphasizing appropriate fitting and normalization procedures are presented.

Fluorescent nanocrystals reveal regulated portals of entry into and between the cells of Hydra

Initially viewed as innovative carriers for biomedical applications, with unique photophysical properties and great versatility to be decorated at their surface with suitable molecules, nanoparticles can also play active roles in mediating biological effects, suggesting the need to deeply investigate the mechanisms underlying cell-nanoparticle interaction and to identify the molecular players. Here we show that the cell uptake of fluorescent CdSe/CdS quantum rods (QRs) by Hydra vulgaris, a simple model organism at the base of metazoan evolution, can be tuned by modifying nanoparticle surface charge. At acidic pH, amino-PEG coated QRs, showing positive surface charge, are actively internalized by tentacle and body ectodermal cells, while negatively charged nanoparticles are not uptaken. In order to identify the molecular factors underlying QR uptake at acidic pH, we provide functional evidence of annexins involvement and explain the QR uptake as the combined result of QR positive charge and annexin membrane insertion. Moreover, tracking QR labelled cells during development and regeneration allowed us to uncover novel intercellular trafficking and cell dynamics underlying the remarkable plasticity of this ancient organism.

The annexins: spatial and temporal coordination of signaling events during cellular stress

Annexins are a family of structurally related, Ca2+-sensitive proteins that bind to negatively charged phospholipids and establish specific interactions with other lipids and lipid microdomains. They are present in all eukaryotic cells and share a common folding motif, the "annexin core", which incorporates Ca2+- and membrane-binding sites. Annexins participate in a variety of intracellular processes, ranging from the regulation of membrane dynamics to cell migration, proliferation, and apoptosis. Here we focus on the role of annexins in cellular signaling during stress. A chronic stress response triggers the activation of different intracellular pathways, resulting in profound changes in Ca2+ and pH homeostasis and the production of lipid second messengers. We review the latest data on how these changes are sensed by the annexins, which have the ability to simultaneously interact with specific lipid and protein moieties at the plasma membrane, contributing to stress adaptation via regulation of various signaling pathways.

Measurement of annexin V uptake and lactadherin labeling for the quantification of apoptosis in adherent Tca8113 and ACC-2 cells

Phosphatidylserine (PS) exposure occurs during the cell death program and fluorescein-labeled lactadherin permits the detection of PS exposure earlier than annexin V in suspended cell lines. Adherent cell lines were studied for this apoptosis-associated phenomenon to determine if PS probing methods are reliable because specific membrane damage may occur during harvesting. Apoptosis was induced in the human tongue squamous carcinoma cell line (Tca8113) and the adenoid cystic carcinoma cell line (ACC-2) by arsenic trioxide. Cells were harvested with a modified procedure and labeled with lactadherin and/or annexin V. PS exposure was localized by confocal microscopy and apoptosis was quantified by flow cytometry. The detachment procedure without trypsinization did not induce cell damage. In competition binding experiments, phospholipid vesicles competed for more than 95 and 90% of lactadherin but only about 75 and 70% of annexin V binding to Tca8113 and ACC-2 cells. These data indicate that PS exposure occurs in three stages during the cell death program and that fluorescein-labeled lactadherin permitted the detection of early PS exposure. A similar pattern of PS exposure has been observed in two malignant cell lines with different adherence, suggesting that this pattern of PS exposure is common in adherent cells. Both lactadherin and annexin V could be used in adherent Tca8113 and ACC-2 cell lines when an appropriate harvesting procedure was used. Lactadherin is more sensitive than annexin V for the detection of PS exposure as the physical structure of PS in these blebs and condensed apoptotic cell surface may be more conducive to binding lactadherin than annexin V.

Heme-independent soluble and membrane-associated peroxidase activity of a Zea mays annexin preparation

Annexins are cytosolic proteins capable of reversible, Ca(2+)-dependent membrane binding or insertion. Animal annexins form and regulate Ca(2+)-permeable ion channels and may therefore participate in signaling. Zea mays (maize) annexins (ZmANN33 and ZmANN35) have recently been shown to form a Ca(2+)-permeable conductance in planar lipid bilayers and also exhibit in vitro peroxidase activity. Peroxidases form a superfamily of intra- or extracellular heme-containing enzymes that use H(2)O(2) as the electron acceptor in a number of oxidative reactions. Maize annexin peroxidase activity appears independent of heme and persists after membrane association, the latter suggesting a role in reactive oxygen species signaling.

Double cushions preserve transmembrane protein mobility in supported bilayer systems

Supported lipid bilayers (SLBs) have been widely used as model systems to study cell membrane processes because they preserve the same 2D membrane fluidity found in living cells. One of the most significant limitations of this platform, however, is its inability to incorporate mobile transmembrane species. It is often postulated that transmembrane proteins reconstituted in SLBs lose their mobility because of direct interactions between the protein and the underlying substrate. Herein, we demonstrate a highly mobile fraction for a transmembrane protein, annexin V. Our strategy involves supporting the lipid bilayer on a double cushion, where we not only create a large space to accommodate the transmembrane portion of the macromolecule but also passivate the underlying substrate to reduce nonspecific protein-substrate interactions. The thickness of the confined water layer can be tuned by fusing vesicles containing polyethyleneglycol (PEG)-conjugated lipids of various molecular weights to a glass substrate that has first been passivated with a sacrificial layer of bovine serum albumin (BSA). The 2D fluidity of these systems was characterized by fluorescence recovery after photobleaching (FRAP) measurements. Uniform, mobile phospholipid bilayers with lipid diffusion coefficients of around 3 x 10(-8) cm2/s and percent mobile fractions of over 95% were obtained. Moreover, we obtained annexin V diffusion coefficients that were also around 3 x 10(-8) cm2/s with mobile fractions of up to 75%. This represents a significant improvement over bilayer platforms fabricated directly on glass or using single cushion strategies.

The N-terminal domain of annexin 2 serves as a secondary binding site during membrane bridging

Annexin A2 (AnxA2) is a Ca(2+)- and acidic phospholipid-binding protein involved in many cellular processes. It undergoes Ca(2+)-mediated membrane bridging at neutral pH and has been demonstrated to be involved in an H(+)-mediated mechanism leading to a novel AnxA2-membrane complex structure. We used fluorescence techniques to characterize this H(+)-dependent mechanism at the molecular level; in particular, the involvement of the AnxA2 N-terminal domain. This domain was labeled at Cys-8 either with acrylodan or pyrene-maleimide fluorescent probes. Steady-state and time-resolved fluorescence analysis for acrylodan and fluorescence quenching by doxyl-labeled phospholipids revealed direct interaction between the N-terminal domain and the membrane. The absence of pyrene excimer suggested that interactions between N termini are not involved in the H(+)-mediated mechanism. These findings differ from those previously observed for the Ca(2+)-mediated mechanism. Protein titration experiments showed that the protein concentration for half-maximal membrane aggregation was twice for Ca(2+)-mediated compared with H(+)-mediated aggregation, suggesting that AnxA2 was able to bridge membranes either as a dimer or as a monomer, respectively. An N-terminally deleted AnxA2 was 2-3 times less efficient than the wild-type protein for H(+)-mediated membrane aggregation. We propose a model of AnxA2-membrane assemblies, highlighting the different roles of the N-terminal domain in the H(+)- and Ca(2+)-mediated membrane bridging mechanisms.

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.

Annexins: multifunctional components of growth and adaptation

Plant annexins are ubiquitous, soluble proteins capable of Ca(2+)-dependent and Ca(2+)-independent binding to endomembranes and the plasma membrane. Some members of this multigene family are capable of binding to F-actin, hydrolysing ATP and GTP, acting as peroxidases or cation channels. These multifunctional proteins are distributed throughout the plant and throughout the life cycle. Their expression and intracellular localization are under developmental and environmental control. The in vitro properties of annexins and their known, dynamic distribution patterns suggest that they could be central regulators or effectors of plant growth and stress signalling. Potentially, they could operate in signalling pathways involving cytosolic free calcium and reactive oxygen species.

Determining the topology of integral membrane peptides using EPR spectroscopy

This paper reports on the development of a new structural biology technique for determining the membrane topology of an integral membrane protein inserted into magnetically aligned phospholipid bilayers (bicelles) using EPR spectroscopy. The nitroxide spin probe, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC), was attached to the pore-lining transmembrane domain (M2delta) of the nicotinic acetylcholine receptor (AChR) and incorporated into a bicelle. The corresponding EPR spectra revealed hyperfine splittings that were highly dependent on the macroscopic orientation of the bicelles with respect to the static magnetic field. The helical tilt of the peptide can be easily calculated using the hyperfine splittings gleaned from the orientational dependent EPR spectra. A helical tilt of 14 degrees was calculated for the M2delta peptide with respect to the bilayer normal of the membrane, which agrees well with previous 15N solid-state NMR studies. The helical tilt of the peptide was verified by simulating the corresponding EPR spectra using the standardized MOMD approach. This new method is advantageous because: (1) bicelle samples are easy to prepare, (2) the helical tilt can be directly calculated from the orientational-dependent hyperfine splitting in the EPR spectra, and (3) EPR spectroscopy is approximately 1000-fold more sensitive than 15N solid-state NMR spectroscopy; thus, the helical tilt of an integral membrane peptide can be determined with only 100 microg of peptide. The helical tilt can be determined more accurately by placing TOAC spin labels at several positions with this technique.

Annexin B12 is a sensor of membrane curvature and undergoes major curvature-dependent structural changes

The regulation of membrane curvature plays an important role in many membrane trafficking and fusion events. Recent studies have begun to identify some of the proteins involved in controlling and sensing the curvature of cellular membranes. A mechanistic understanding of these processes is limited, however, as structural information for the membrane-bound forms of these proteins is scarce. Here, we employed a combination of biochemical and biophysical approaches to study the interaction of annexin B12 with membranes of different curvatures. We observed selective and Ca(2+)-independent binding of annexin B12 to negatively charged vesicles that were either highly curved or that contained lipids with negative intrinsic curvature. This novel curvature-dependent membrane interaction induced major structural rearrangements in the protein and resulted in a backbone fold that was different from that of the well characterized Ca(2+)-dependent membrane-bound form of annexin B12. Following curvature-dependent membrane interaction, the protein retained a predominantly alpha-helical structure but EPR spectroscopy studies of nitroxide side chains placed at selected sites on annexin B12 showed that the protein underwent inside-out refolding that brought previously buried hydrophobic residues into contact with the membrane. These structural changes were reminiscent of those previously observed following Ca(2+)-independent interaction of annexins with membranes at mildly acidic pH, yet they occurred at neutral pH in the presence of curved membranes. The present data demonstrate that annexin B12 is a sensor of membrane curvature and that membrane curvature can trigger large scale conformational changes. We speculate that membrane curvature could be a physiological signal that induces the previously reported Ca(2+)-independent membrane interaction of annexins in vivo.

The annexins of Dictyostelium

Annexins are a highly conserved ubiquitous family of Ca2+- and phospholipid-binding proteins present in nearly all eukaryotic cells. Analysis of the Dictyostelium genome revealed the presence of two annexin genes, the annexin C1 gene (nxnA) giving rise to two isoforms of 47 and 51 kDa (previously synexin), and the annexin C2 gene (nxnB) coding for a 56-kDa protein with 33% sequence identity to annexin C1. Annexin C2 is expressed at very low and constant levels throughout development. Quantification by real-time PCR indicated that it is present in about 35-fold lower amounts compared to annexin C1. We have used a GFP-tagged annexin C2 to study its cellular distribution and dynamics. In cell fractionation studies, annexin C2 cofractionates with annexin C1 and is enriched in the 100,000 g pellet. Like annexin C1, GFP-AnxC2 stains the plasma membrane. In addition it is present in the perinuclear region and overlaps to some degree with the Golgi apparatus, whereas annexin C1 is present on intracellular membranes resembling endosomal membranes and in the nucleus. Annexin C2 is not observed in the nucleus. An annexin C1 mutant (SYN-) which shows a defect during multicellular development can be rescued by full-length annexin C1, whereas overexpression of GFP-AnxC2 did not rescue the developmental defect The data support the concept that annexins, although having a highly conserved structure, participate in different functions in a cell.

Myocyte apoptosis during acute myocardial infarction in rats is related to early sarcolemmal translocation of annexin A5 in border zone

Annexin A5 is a Ca2+-dependent phospholipid binding protein well known for its high phosphatidylserine affinity. In vitro, translocation to sarcolemma and externalization of endogenous annexin A5 in the cardiomyocyte has recently been demonstrated to exert a proapoptotic effect. To determine whether these in vitro findings occurred in vivo, we performed myocardial infarction (MI) and studied the time course of apoptosis and annexin A5 localization (0.5 to 8 h) in the border zone around the infarcted area. This zone that was defined as Evans blue unstained and triphenyltetrazolium chloride (TTC) stained, represented 42.3 ± 5.5% of the area at risk and showed apoptotic characteristics (significant increases in caspase 3 activity 2.3-fold at 0.5 h; P < 0.05), transferase-mediated dUTP nick-end labeling-positive cardiomyocytes (15.8 ± 0.8% at 8 h), and DNA ladder. When compared with sham-operated rats, we found that in this area, annexin A5 was translocated to the sarcolemma as early as 0.5 h after MI and that translocation increased with time. Moreover, the amount of annexin A5 was unchanged in the border zone and decreased in the infarcted area after 1 h (77.1 ± 4.8%; P < 0.01 vs. perfused area), suggesting a release in the latter but not in the former. In conclusion, we demonstrated that annexin A5 translocation is an early and rapid event of the whole border zone, likely due to Ca2+increase. Part of this translocation occurred in areas where apoptosis was later detected and suggests that in vivo as in vitro annexin A5 might be involved in the regulation of early apoptotic events during cardiac pathological situations.

Lactadherin binding and phosphatidylserine expression on cell surface-comparison with annexin A5

BACKGROUND

Transbilayer movement of anionic phospholipids from the inner to the outer leaflet of the plasma membrane occurs during platelet activation, red cell senescence, and apoptosis. The anionic phospholipid-binding protein, annexin A5, has been used to detect the presence of phosphatidylserine on the outer leaflet of the cell membrane. Lactadherin, a glycoprotein secreted by macrophages, binds to phosphatidylserine on apoptotic cells and promote their clearance by macrophages.

METHODS

The authors isolated and labeled lactadherin and annexin A5 with FITC and compared their ability to detect phosphatidylserine expression by flow cytometry.

RESULTS

FITC-lactadherin induced greater shift in the histogram and a higher mean fluorescence intensity than FITC-annexin A5 when platelets were activated with thrombin (0.1 unit/mL) or Ca(2+) ionophore A23187 (1 microM). Similarly, lactadherin was more sensitive in detecting phosphatidylserine in red cells induced to express phosphatidylserine. Also, in HL 60 cells undergoing apoptosis, lactadherin detected phosphatidylserine expression earlier than annexin A5. In patients with disseminated intravascular coagulation, lactadherin detected phosphatidylserine-expressing platelets in most patients, whereas under similar conditions, FITC-annexin A5 could not.

CONCLUSIONS

The authors' studies show that FITC-lactadherin is a better probe than annexin A5 in detecting phosphatidylserine-expressing activated platelets, red cells, and apoptotic cells.

Lifetime fluorescence method for determining membrane topology of proteins

Recently, we introduced a sensitive method for determining the bilayer topology (cis- or trans-leaflet location) of single-site cysteine-linked 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) fluorescent labels on membrane proteins. It uses a novel quencher, LysoUB, composed of a single acyl chain attached to a UniBlue chromophore. In its original version, the method relied on the comparison of steady-state fluorescence measurements of membrane-inserted proteins in samples with different distributions of the LysoUB in cis- and trans-leaflets of the lipid bilayer. Here we modify the method to take advantage of the fluorescence lifetime methodology, which allows us to simplify sample manipulation and, as a result, increase the reliability of topology determination. We tested the method using three model systems with artificially created all-cis, all-trans, and isotropic distribution of NBD. Because the quenching efficiency is higher when LysoUB and NBD are in the same leaflet, introduction of the quencher into the cis-leaflet results in a predictably different amount of quenching for these three model systems. Indeed, the addition of 2% LysoUB into the all-cis NBD model system causes strong reduction of the longest lifetime (from 8.1 to 4.9 ns), whereas the same addition of LysoUB results in marginal quenching (from 8.7 to 8.5 ns) in the case of all-trans NBD. This difference provides a good basis for topology determination using time-resolved fluorescence quenching.

Structure-function relationship in annexin A13, the founder member of the vertebrate family of annexins

Annexin A13 is considered the original progenitor of the 11 other members of vertebrate annexins, a superfamily of calcium/phospholipid-binding proteins. It is highly tissue-specific, being expressed only in intestinal and kidney epithelial cells. Alternative splicing generates two isoforms, both of which bind to rafts. In view of the lack of structural information supporting the physiological role of this annexin subfamily, we have cloned, expressed and purified human annexin A13b to investigate its structural and functional properties. The N-terminus of annexin A13b: (i) destabilizes the conserved protein core, as deduced from the low melting temperature in the absence (44 degrees C) or presence of calcium (55 degrees C), and (ii) impairs calcium-dependent binding to acidic phospholipids, requiring calcium concentrations >400 microM. Truncation of the N-terminus restores thermal stability and decreases the calcium requirement for phospholipid binding, confirming its essential role in the structure-function relationship of this annexin. Non-myristoylated annexin A13b only binds to acidic phospholipids at high calcium concentrations. We show for the first time that myristoylation of annexin A13b enables the direct binding to phosphatidylcholine, raft-like liposomes and acidic phospholipids in a calcium-independent manner. The conformational switch induced by calcium binding, from a 'closed' to an 'open' conformation with exposure of Trp227, can be mimicked by a decrease in pH, a process that may be relevant for membrane interactions. Our studies confirm that the common structural and functional characteristics that are dependent on the protein core of vertebrate annexins are likely to be common conserved features, whereas their variable N-termini confer distinct functional properties on annexins, as we report for myristoylation of annexin A13b.

Angiomotin Regulates Endothelial Cell-Cell Junctions and Cell Motility

We have previously identified angiomotin by its ability to bind to and mediate the anti-angiogenic properties of angiostatin. In vivo and in vitro data indicate an essential role of angiomotin in endothelial cell motility. Here we show that angiostatin binds angiomotin on the cell surface and provide evidence for a transmembrane model for the topology of both p80 and p130 angiomotin isoforms. Immunofluorescence analysis shows that angiomotin co-localized with ZO-1 in cell-cell contacts in endothelial cells in vitro and in angiogenic blood vessels of the postnatal mouse retina in vivo. Transfection of p80 as well as p130 angiomotin in Chinese hamster ovary cells resulted in junctional localization of both isoforms. Furthermore, p130 angiomotin could recruit ZO-1 to actin stress fibers. The p130 but not p80 isoform could be coprecipitated with MAGI-1b, a component of endothelial tight junctions. Paracellular permeability, as measured by diffusion of fluorescein isothiocyanate-dextran, was reduced by p80 and p130 angiomotin expression with 70 and 88%, respectively, compared with control. Angiostatin did not have any effect on cell permeability but inhibited the migration of angiomotin-expressing cells in the Boyden chamber assay. We conclude that angiomotin, in addition to controlling cell motility, may play a role in the assembly of endothelial cell-cell junctions.

Annexins: linking Ca2+ signalling to membrane dynamics

Eukaryotic cells contain various Ca(2+)-effector proteins that mediate cellular responses to changes in intracellular Ca(2+) levels. A unique class of these proteins - annexins - can bind to certain membrane phospholipids in a Ca(2+)-dependent manner, providing a link between Ca(2+) signalling and membrane functions. By forming networks on the membrane surface, annexins can function as organizers of membrane domains and membrane-recruitment platforms for proteins with which they interact. These and related properties enable annexins to participate in several otherwise unrelated events that range from membrane dynamics to cell differentiation and migration.

A helical hairpin region of soluble annexin B12 refolds and forms a continuous transmembrane helix at mildly acidic pH

Annexins are soluble proteins that are best known for their ability to undergo reversible Ca(2+)-dependent binding to the surface of phospholipid bilayers. Recent studies, however, have shown that annexins also reversibly bind to membranes in a Ca(2+)-independent manner at mildly acidic pH. We investigated the structural changes that occur upon pH-dependent membrane binding by performing a nitroxide scan on the helical hairpin encompassing helices A and B in the fourth repeat of annexin B12. Residues 251-273 of annexin B12 were replaced, one at a time, with cysteine and then labeled with a nitroxide spin label. Electron paramagnetic resonance (EPR) mobility and accessibility analyses of soluble annexin B12 derivatives were in excellent agreement with the known crystal structure of annexin B12. However, EPR studies of annexin B12 derivatives bound to membranes at pH 4.0 indicated major structural changes in the scanned region. The helix-loop-helix structure present in the soluble protein was converted into a continuous transmembrane alpha-helix that was exposed to the hydrophobic core of the bilayer on one side and exposed to an aqueous pore on the other side. Asp-264 was on the hydrophobic membrane-exposed face of the amphipathic transmembrane helix, thereby suggesting that protonation of its carboxylate group stabilized the transmembrane form. Inspection of the amino acid sequence of annexin B12 revealed several other helical hairpin regions that might refold and form continuous amphipathic transmembrane helices in response to protonation of Asp or Glu switch residues on or near the hydrophobic face of the helix.

Structure of human annexin a6 at the air-water interface and in a membrane-bound state

We postulate the existence of a pH-sensitive domain in annexin A6 (AnxA6), on the basis of our observation of pH-dependent conformational and orientation changes of this protein and its N- (AnxA6a) and C-terminal (AnxA6b) halves in the presence of lipids. Brewster angle microscopy shows that AnxA6, AnxA6a, and AnxA6b in the absence of lipids accumulate at the air-water interface and form a stable, homogeneous layer at pH below 6.0. Under these conditions polarization modulation IR absorption spectroscopy reveals significant conformational changes of AnxA6a whereas AnxA6b preserves its alpha-helical structure. The orientation of protein alpha-helices is parallel with respect to the interface. In the presence of lipids, polarization modulation IR reflection absorption spectroscopy experiments suggest that AnxA6a incorporates into the lipid/air interface, whereas AnxA6b is adsorbed under the lipid monolayer. In this case AnxA6a regains its alpha-helical structures. At a higher pressure of the lipid monolayer the average orientation of the alpha-helices of AnxA6a changes from flat to tilted by 45 degrees with respect to normal to the membrane interface. For AnxA6b no such changes are detected, even at a high pressure of the lipid monolayer-suggesting that the putative pH-sensitive domain of AnxA6 is localized in the N-terminal half of the protein.

Annexin A1 interaction with a zwitterionic phospholipid monolayer: a fluorescence microscopy study

We present the results of a fluorescence microscopy study of the interaction of annexin A1 with dipalmitoylphosphatidylcholine (DPPC) monolayers as a function of the lipid monolayer phase and the pH of the aqueous subphase. We show that annexin A1-DPPC interaction depends strongly on the domain structure of the DPPC monolayer and only weakly on the subphase pH. Annexin A1 is found to be line active, with preferential adsorption at phase boundaries. Also, annexin A1 is found to form networks in the presence of a domain structure in the monolayer. Our results point toward an important contribution of the unique N-terminal domain to the organization of the protein at the interface.

An Annexin 2 Phosphorylation Switch Mediates p11-dependent Translocation of Annexin 2 to the Cell Surface

Annexin 2 is a profibrinolytic co-receptor for plasminogen and tissue plasminogen activator that stimulates activation of the major fibrinolysin, plasmin, at cell surfaces. In human subjects, overexpression of annexin 2 in acute promyelocytic leukemia leads to a bleeding diathesis reflective of excessive cell surface annexin 2-dependent generation of plasmin (Menell, J. S., Cesarman, G. M., Jacovina, A. T., McLaughlin, M. A., Lev, E. A., and Hajjar, K. A. (1999) N. Engl. J. Med. 340, 994-1004). In addition, mice completely deficient in annexin 2 display fibrin accumulation within blood vessels and impaired clearance of injury-induced thrombi (Ling Q., Jacovina, A.T., Deora, A.B., Febbraio, M., Simantov, R., Silverstein, R. L., Hempstead, B. L., Mark, W., and Hajjar, K. A. (2004) J. Clin. Investig. 113, 38-48). Here, we show that endothelial cell annexin 2, a protein that lacks a typical signal peptide, translocates from the cytoplasm to the extracytoplasmic plasma membrane in response to brief temperature stress both in vitro and in vivo in the absence of cell death or cell lysis. This regulated response is independent of new protein or mRNA synthesis and does not require the classical endoplasmic reticulum-Golgi pathway. Temperature stress-induced annexin 2 translocation is dependent on both expression of protein p11 (S100A10) and tyrosine phosphorylation of annexin 2 because annexin 2 release is completely eliminated on depletion of p11, inactivation of tyrosine kinase, or mutation of tyrosine 23. Translocation of annexin 2 to the cell surface dramatically increases tissue plasminogen activator-dependent plasminogen activation potential and may represent a novel stress-induced protein secretion pathway.

Structure and Dynamics of a Helical Hairpin that Mediates Calcium-dependent Membrane Binding of Annexin B12

A wealth of high-resolution structural data has accumulated for soluble annexins, but only limited information is available for the biologically important membrane-bound proteins. To investigate the structural and dynamic changes that occur upon membrane binding, we analyzed the electron paramagnetic resonance (EPR) mobility and accessibility parameters of a continuous 30-residue nitroxide scan encompassing helices D and E in repeat 2 of annexin B12 (residues 134-163) while the protein was bound to phospholipid vesicles in the presence of Ca(2+). A comparison of these data to those from a previously published study of the protein in solution (Isas, J. M., Langen, R., Haigler, H. T., and Hubbell, W. L. (2002) Biochemistry 41, 1464-1473) showed that the overall backbone fold for the scanned region did not change upon membrane binding. However, side-chains in the loop between the D and E helices were highly dynamic in solution but became essentially frozen in the EPR time scale upon binding to membranes. Accessibility measurements clearly established that side-chains in this loop were exposed to the hydrophobic core of the bilayer and provide the first evidence that a D-E loop directly participates in the Ca(2+)-dependent binding of annexins to membranes. Other localized changes showed that the D-helix became much less dynamic after membrane binding and identified quaternary contact sites in the membrane-bound homo-trimer. Finally, immobilization of the D-E loop upon contact with phospholipid suggests that the bilayer, which is normally very mobile on the EPR time scale, is immobilized in the head-group region by the annexin B12. This suggests that annexin B12 alters membrane structure in a manner that may be biologically significant.

Redox regulation of annexin 2 and its implications for oxidative stress-induced renal carcinogenesis and metastasis

Ferric nitrilotriacetate (Fe-NTA) induces oxidative renal damage leading to a high incidence of renal cell carcinoma (RCC) in rats. Differential display analysis of such RCCs revealed elevated expression of annexin 2 (Anx2), a substrate for kinases and a receptor for tissue-type plasminogen activator and plasminogen. We conducted this study to clarify the significance of Anx2 in Fenton reaction-based carcinogenesis. Messenger RNA and protein levels of Anx2 were increased time-dependently in the rat kidney after Fe-NTA administration as well as in LLC-PK1 cells after exposure to H2O2. The latter was inhibited by pretreatment with N-acetylcysteine, pyrrolidine dithiocarbamate or catalase. Immunohistochemistry revealed negligible staining in the normal renal proximal tubules, but strong staining in regenerating proximal tubules, karyomegalic cells and RCCs. Metastasizing RCCs showed higher Anx2 protein levels. Anx2 was phosphorylated at serine and tyrosine residues in these cells and coimmunoprecipitated with phosphorylated actin. Overexpression of Anx2 induced a higher cell proliferation rate in LLC-PK1 cells. In contrast, a decrease in proliferation leading to apoptosis was observed after Anx2 antisense treatment to cell lines established from Fe-NTA-induced RCCs. These results suggest that Anx2 is regulated by redox status, and that persistent operation of this adaptive mechanism plays a role in the proliferation and metastasis of oxidative stress-induced cancer.

Novel organization and properties of annexin 2-membrane complexes

Annexin 2 belongs to the annexin family of proteins that bind to phospholipid membranes in a Ca(2+)-dependent manner. Here we show that, under mild acidic conditions, annexin 2 binds to and aggregates membranes containing anionic phospholipids, a fact that questions the mechanism of its interaction with membranes via Ca(2+) bridges only. The H(+) sensitivity of annexin 2-mediated aggregation is modulated by lipid composition (i.e. cholesterol content). Cryo-electron microscopy of aggregated liposomes revealed that both the monomeric and the tetrameric forms of the protein form bridges between the liposomes at acidic pH. Monomeric annexin 2 induced two different organizations of the membrane junctions. The first resembled that obtained at pH 7 in the presence of Ca(2+). For the tetramer, the arrangement was different. These bridges seemed more flexible than the Ca(2+)-mediated junctions allowing the invagination of membranes. Time-resolved fluorescence analysis at mild acidic pH and the measurement of Stokes radius revealed that the protein undergoes conformational changes similar to those induced by Ca(2+). Labeling with the lipophilic probe 3-(trifluoromethyl)-3-(m-[(125)I]iodophenyl)diazirine indicated that the protein has access to the hydrophobic part of the membrane at both acidic pH in the absence of Ca(2+) and at neutral pH in the presence of Ca(2+). Models for the membrane interactions of annexin 2 at neutral pH in the presence of Ca(2+) and at acidic pH are discussed.