Crystal structure of bovine annexin VI in a calcium-bound state

Calcium phosphate formation and deposition in ischemic neurons

Ischemic stroke causes acute brain calcium phosphate (CaP) deposition, a process involving primarily the injured neurons. Whereas the adverse impact of CaP deposition on the brain structure and function has been recognized, the underlying mechanisms remain poorly understood. This investigation demonstrated that the neuron-expressed, plasma membrane-associated Ca2+-binding proteins annexin (Anx) A2, AnxA5, AnxA6, and AnxA7 contributed to neuronal CaP deposition in the mouse model of ischemic stroke. These Anxs were released from the degraded plasma membrane of the ischemic neurons and were able to form Anx/CaP complexes, a nanostructure capable of binding to the β actin filaments via Anx-actin interaction to cause neuronal CaP deposition prior to brain infarction. Anx administration to the healthy mouse brain caused brain CaP deposition and infarction. Monomeric β actin was able to block competitively Anx binding to β actin filaments and prevent ischemic stroke- and Anx administration-induced brain CaP deposition and infarction. Administration of siRNAs specific to the four Anx mRNAs alleviated brain CaP deposition and infarction. These observations support the role of Anxs in CaP formation and deposition in ischemic neurons.

AnnexinA6: a potential therapeutic target gene for extracellular matrix mineralization

The mineralization of the extracellular matrix (ECM) is an essential and crucial process for physiological bone formation and pathological calcification. The abnormal function of ECM mineralization contributes to the worldwide risk of developing mineralization-related diseases; for instance, vascular calcification is attributed to the hyperfunction of ECM mineralization, while osteoporosis is due to hypofunction. AnnexinA6 (AnxA6), a Ca2+-dependent phospholipid-binding protein, has been extensively reported as an essential target in mineralization-related diseases such as osteoporosis, osteoarthritis, atherosclerosis, osteosarcoma, and calcific aortic valve disease. To date, AnxA6, as the largest member of the Annexin family, has attracted much attention due to its significant contribution to matrix vesicles (MVs) production and release, MVs-ECM interaction, cytoplasmic Ca2+ influx, and maturation of hydroxyapatite, making it an essential target in ECM mineralization. In this review, we outlined the recent advancements in the role of AnxA6 in mineralization-related diseases and the potential mechanisms of AnxA6 under normal and mineralization-related pathological conditions. AnxA6 could promote ECM mineralization for bone regeneration in the manner described previously. Therefore, AnxA6 may be a potential osteogenic target for ECM mineralization.

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.

Annexin Animal Models-From Fundamental Principles to Translational Research

Routine manipulation of the mouse genome has become a landmark in biomedical research. Traits that are only associated with advanced developmental stages can now be investigated within a living organism, and the in vivo analysis of corresponding phenotypes and functions advances the translation into the clinical setting. The annexins, a family of closely related calcium (Ca²⁺)- and lipid-binding proteins, are found at various intra- and extracellular locations, and interact with a broad range of membrane lipids and proteins. Their impacts on cellular functions has been extensively assessed in vitro, yet annexin-deficient mouse models generally develop normally and do not display obvious phenotypes. Only in recent years, studies examining genetically modified annexin mouse models which were exposed to stress conditions mimicking human disease often revealed striking phenotypes. This review is the first comprehensive overview of annexin-related research using animal models and their exciting future use for relevant issues in biology and experimental medicine.

Structure Determination of the Transactivation Domain of p53 in Complex with S100A4 Using Annexin A2 as a Crystallization Chaperone

To fully understand the environmental factors that influence crystallization is an enormous task, therefore crystallographers are still forced to work "blindly" trying as many crystallizing conditions and mutations to improve crystal packing as possible. Numerous times these random attempts simply fail even when using state-of-the-art techniques. As an alternative, crystallization chaperones, having good crystal-forming properties, can be invoked. Today, the almost exclusively used such protein is the maltose-binding protein (MBP) and crystallographers need other widely applicable options. Here, we introduce annexin A2 (ANXA2), which has just as good, if not better, crystal-forming ability than the wild-type MBP. Using ANXA2 as heterologous fusion partner, we were able to solve the atomic resolution structure of a challenging crystallization target, the transactivation domain (TAD) of p53 in complex with the metastasis-associated protein S100A4. p53 TAD forms an asymmetric fuzzy complex with the symmetric S1004 and could interfere with its function.

Recombinant annexin A6 promotes membrane repair and protects against muscle injury

Membrane repair is essential to cell survival. In skeletal muscle, injury often associates with plasma membrane disruption. Additionally, muscular dystrophy is linked to mutations in genes that produce fragile membranes or reduce membrane repair. Methods to enhance repair and reduce susceptibility to injury could benefit muscle in both acute and chronic injury settings. Annexins are a family of membrane-associated Ca2+-binding proteins implicated in repair, and annexin A6 was previously identified as a genetic modifier of muscle injury and disease. Annexin A6 forms the repair cap over the site of membrane disruption. To elucidate how annexins facilitate repair, we visualized annexin cap formation during injury. We found that annexin cap size positively correlated with increasing Ca2+ concentrations. We also found that annexin overexpression promoted external blebs enriched in Ca2+ and correlated with a reduction of intracellular Ca2+ at the injury site. Annexin A6 overexpression reduced membrane injury, consistent with enhanced repair. Treatment with recombinant annexin A6 protected against acute muscle injury in vitro and in vivo. Moreover, administration of recombinant annexin A6 in a model of muscular dystrophy reduced serum creatinine kinase, a biomarker of disease. These data identify annexins as mediators of membrane-associated Ca2+ release during membrane repair and annexin A6 as a therapeutic target to enhance membrane repair capacity.

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.

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.

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 a6 modulates chick cranial neural crest cell emigration

The vertebrate neural crest is a population of migratory cells that originates in the dorsal aspect of the embryonic neural tube. These cells undergo an epithelial-to-mesencyhmal transition (EMT), delaminate from the neural tube and migrate extensively to generate an array of differentiated cell types. Elucidating the gene regulatory networks involved in neural crest cell induction, migration and differentiation are thus crucial to understanding vertebrate development. To this end, we have identified Annexin A6 as an important regulator of chick midbrain neural crest cell emigration. Annexin proteins comprise a family of calcium-dependent, membrane-binding molecules that mediate a variety of cellular and physiological processes including cell adhesion, migration and invasion. Our data indicate that Annexin A6 is expressed in the proper spatio-temporal pattern in the chick midbrain to play a potential role in neural crest cell ontogeny. To investigate Annexin A6 function, we have depleted or overexpressed Annexin A6 in the developing midbrain neural crest cell population. Our results show that knock-down or overexpression of Annexin A6 reduces or expands the migratory neural crest cell domain, respectively. Importantly, this phenotype is not due to any change in cell proliferation or cell death but can be correlated with changes in the size of the premigratory neural crest cell population and with markers associated with EMT. Taken together, our data indicate that Annexin A6 plays a pivotal role in modulating the formation of cranial migratory neural crest cells during vertebrate development.

Crystallization and preliminary X-ray diffraction studies of the catalytic domain of a novel chitinase, a member of GH family 23, from the moderately thermophilic bacterium Ralstonia sp. A-471

Chitinase from the moderately thermophilic bacterium Ralstonia sp. A-471 (Ra-ChiC) is divided into two domains: a chitin-binding domain (residues 36-80) and a catalytic domain (residues 103-252). Although the catalytic domain of Ra-ChiC has homology to goose-type lysozyme, Ra-ChiC does not show lysozyme activity but does show chitinase activity. The catalytic domain with part of an interdomain loop (Ra-ChiC(89-252)) was crystallized under several different conditions using polyethylene glycol as a precipitant. The crystals diffracted to 1.85 Å resolution and belonged to space group P6(1)22 or P6(5)22, with unit-cell parameters a = b = 100, c = 243 Å. The calculated Matthews coefficient was approximately 3.2, 2.4 or 1.9 Å(3) Da(-1) assuming the presence of three, four or five Ra-ChiC(89-252) molecules in the asymmetric unit, respectively.

Annexin A6-Linking Ca(2+) signaling with cholesterol transport

Annexin A6 (AnxA6) belongs to a conserved family of Ca(2+)-dependent membrane-binding proteins. Like other annexins, the function of AnxA6 is linked to its ability to bind phospholipids in cellular membranes in a dynamic and reversible fashion, in particular during the regulation of endocytic and exocytic pathways. High amounts of AnxA6 sequester cholesterol in late endosomes, thereby lowering the levels of cholesterol in the Golgi and the plasma membrane. These AnxA6-dependent redistributions of cellular cholesterol pools give rise to reduced cytoplasmic phospholipase A2 (cPLA(2)) activity, retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. In addition to regulating cholesterol and caveolin distribution, AnxA6 acts as a scaffold/targeting protein for several signaling proteins, the best characterized being the Ca(2+)-dependent membrane targeting of p120GAP to downregulate Ras activity. AnxA6 also stimulates the Ca(2+)-inducible involvement of PKC in the regulation of HRas and possibly EGFR signal transduction pathways. The ability of AnxA6 to recruit regulators of the EGFR/Ras pathway is likely potentiated by AnxA6-induced actin remodeling. Accordingly, AnxA6 may function as an organizer of membrane domains (i) to modulate intracellular cholesterol homeostasis, (ii) to create a scaffold for the formation of multifactorial signaling complexes, and (iii) to regulate transient membrane-actin interactions during endocytic and exocytic transport. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Annexin-A6 presents two modes of association with phospholipid membranes. A combined QCM-D, AFM and cryo-TEM study

Annexins are soluble proteins that bind to biological membranes in a Ca(2+)-dependent manner. Annexin-A6 (AnxA6) is unique in the annexin family as it consists of the repeat of two annexin core modules, while all other annexins consist of a single module. AnxA6 has been proposed to participate in various membrane-related processes, including endocytosis and exocytosis, yet the molecular mechanism of association of AnxA6 with biological membranes, especially its ability to aggregate membranes, is still unclear. To address this question, we studied the association of AnxA6 with model phospholipid membranes by combining the techniques of quartz crystal microbalance with dissipation monitoring (QCM-D), (cryo-) transmission electron microscopy (TEM) and atomic force microscopy (AFM). The properties of membrane binding and membrane aggregation of AnxA6 were compared to two reference systems, annexin A5 (AnxA5), which is the annexin prototype, and a chimerical AnxA5-dimer molecule, which is able to aggregate two membranes in a symmetrical manner. We show that AnxA6 presents two modes of association with lipid membranes depending on Ca(2+)-concentration. At low Ca(2+)-concentration ( approximately 60-150microM), AnxA6 binds to membranes via its two coplanar annexin modules and is not able to associate two separate membranes. At high Ca(2+)-concentration ( approximately 2mM), AnxA6 molecules are able to bind two adjacent phospholipid membranes and present a conformation similar to the AnxA6 3D crystallographic structure. Possible biological implications of these novel membrane-binding properties of AnxA6 are discussed.

S100-annexin complexes--structural insights

Annexins and S100 proteins represent two large, but distinct, calcium-binding protein families. Annexins are made up of a highly alpha-helical core domain that binds calcium ions, allowing them to interact with phospholipid membranes. Furthermore, some annexins, such as annexins A1 and A2, contain an N-terminal region that is expelled from the core domain on calcium binding. These events allow for the interaction of the annexin N-terminus with target proteins, such as S100. In addition, when an S100 protein binds calcium ions, it undergoes a structural reorientation of its helices, exposing a hydrophobic patch capable of interacting with its targets, including the N-terminal sequences of annexins. Structural studies of the complexes between members of these two families have revealed valuable details regarding the mechanisms of the interactions, including the binding surfaces and conformation of the annexin N-terminus. However, other S100-annexin interactions, such as those between S100A11 and annexin A6, or between dicalcin and annexins A1, A2 and A5, appear to be more complicated, involving the annexin core region, perhaps in concert with the N-terminus. The diversity of these interactions indicates that multiple forms of recognition exist between S100 proteins and annexins. S100-annexin interactions have been suggested to play a role in membrane fusion events by the bridging together of two annexin proteins, bound to phospholipid membranes, by an S100 protein. The structures and differential interactions of S100-annexin complexes may indicate that this process has several possible modes of protein-protein recognition.

In vitro modeling of matrix vesicle nucleation: synergistic stimulation of mineral formation by annexin A5 and phosphatidylserine

Annexins A5, A2, and A6 (Anx-A5, -A2, and -A6) are quantitatively major proteins of the matrix vesicle nucleational core that is responsible for mineral formation. Anx-A5 significantly activated the induction and propagation of mineral formation when incorporated into synthetic nucleation complexes made of amorphous calcium phosphate (ACP) and Anx-A5 or of phosphatidylserine (PS) plus ACP (PS-CPLX) and Anx-A5. Incorporation of Anx-A5 markedly shortened the induction time, greatly increasing the rate and overall amount of mineral formed when incubated in synthetic cartilage lymph. Constructed by the addition of Ca(2+) to PS, emulsions prepared in an intracellular phosphate buffer matched in ionic composition to the intracellular fluid of growth plate chondrocytes, these biomimetic PS-CPLX nucleators had little nucleational activity. However, incorporation of Anx-A5 transformed them into potent nucleators, with significantly greater activity than those made from ACP without PS. The ability of Anx-A5 to enhance the nucleation and growth of mineral appears to stem from its ability to form two-dimensional crystalline arrays on PS-containing monolayers. However, some stimulatory effect also may result from its ability to exclude Mg(2+) and HCO(-)(3) from nucleation sites. Comparing the various annexins for their ability to activate PS-CPLX nucleation yields the following: avian cartilage Anx-A5 > human placental Anx-A5 > avian liver Anx-A5 > or = avian cartilage Anx-A6 >> cartilage Anx-A2. The stimulatory effect of human placental Anx-A5 and avian cartilage Anx-A6 depended on the presence of PS, since in its absence they either had no effect or actually inhibited the nucleation activity of ACP. Anx-A2 did not significantly enhance mineralization.

Apo and calcium-bound crystal structures of Alpha-11 giardin, an unusual annexin from Giardia lamblia

Alpha-11 giardin is a member of the multi-gene alpha-giardin family in the intestinal protozoan, Giardia lamblia. This gene family shares an ancestry with the annexin super family, whose common characteristic is calcium-dependent binding to membranes that contain acidic phospholipids. Several alpha giardins are highly expressed during parasite-induced diarrhea in humans. Despite being a member of a large family of proteins, little is known about the function and cellular localization of alpha-11 giardin, although giardins are often associated with the cytoskeleton. It has been shown that Giardia exhibits high levels of alpha-11 giardin mRNA transcript throughout its life cycle; however, constitutive over-expression of this protein is lethal to the parasite. Determining the three-dimensional structure of an alpha-giardin is essential to identifying functional domains shared in the alpha-giardin family. Here we report the crystal structures of the apo and Ca(2+)-bound forms of alpha-11 giardin, the first alpha giardin to be characterized structurally. Crystals of apo and Ca(2+)-bound alpha-11 giardin diffracted to 1.1 A and 2.93 A, respectively. The crystal structure of selenium-substituted apo alpha-11 giardin reveals a planar array of four tandem repeats of predominantly alpha-helical domains, reminiscent of previously determined annexin structures, making this the highest-resolution structure of an annexin to date. The apo alpha-11 giardin structure also reveals a hydrophobic core formed between repeats I/IV and II/III, a region typically hydrophilic in other annexins. Surprisingly, the Ca(2+)-bound structure contains only a single calcium ion, located in the DE loop of repeat I and coordinated differently from the two types of calcium sites observed in previous annexin structures. The apo and Ca(2+)-bound alpha-11 giardin structures assume overall similar conformations; however, Ca(2+)-bound alpha-11 giardin crystallized in a lower-symmetry space group with four molecules in the asymmetric unit. Vesicle-binding studies suggest that alpha-11 giardin, unlike most other annexins, does not bind to vesicles composed of acidic phospholipids in a calcium-dependent manner.

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.

Effects of mutagenesis of W343 in human annexin A6 isoform 1 on its interaction with GTP: nucleotide-induced oligomer formation and ion channel activity

Accumulated experimental evidence suggests that annexin A6 (AnxA6) is involved in ion transport in various tissues. Such a biological function is related either to the modulation of ion transport systems by AnxA6 or to the ion channel activity of the protein. While AnxA6 channel activity at low pH seems to be associated with a large conformational transition in the protein, the mechanism of GTP-induced ion channel formation remains obscure. This activity is not accompanied by changes in protein structure. The existence of a domain binding the phosphate groups of GTP in AnxA6 [Bandorowicz-Pikula, J., Kirilenko, A., van Deursen, R., Golczak, M., Kuhnel, M., Lancelin, J. M., Pikula, S., and Buchet, R. (2003) Biochemistry 42, 9137-9146] may provide some clues about the molecular mechanisms of GTP-induced ion channel formation. In addition, we observed that one of the AnxA6 tryptophan residues, W192 or W343, may be involved in GTP binding. Therefore, we created several site-directed mutants of AnxA6 in which selected amino acid residues within a consensus sequence of a putative nucleotide-binding domain of AnxA6 were replaced with other amino acid residues without affecting the overall structure of protein as examined by circular dichroism and infrared spectroscopies. Their properties were analyzed and compared to those of the native protein. In contrast to mutant W192S and wild-type annexin, mutant W343S neither bound GTP nor exhibited GTP-induced ion channel activity. In addition, we detected the likely formation of AnxA6 trimers in the presence of GTP. The ability of mutant W343S to form trimers was significantly impaired. Our findings suggest that W343 participates in the formation of AnxA6 trimers. We hypothesize that such trimers could lead to a functional unit of the GTP-induced ion channels formed by the annexin molecules.

New reagents for phosphatidylserine recognition and detection of apoptosis

The phospholipid bilayer surrounding animal cells is made up of four principle phospholipid components, phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and sphingomyelin (SM). These four phospholipids are distributed between the two monolayers of the membrane in an asymmetrical fashion, with PC and SM largely populating the extracellular leaflet and PE and PS restricted primarily to the inner leaflet. Breakdown in this transmembrane phospholipid asymmetry is a hallmark of the early to middle stages of apoptosis. The consequent appearance of PS on the extracellular membrane leaflet is commonly monitored using dye-labeled Annexin V, a 36 kDa, Ca2+-dependent PS binding protein. Substitutes for Annexin V are described, including small molecules, nanoparticles, cationic liposomes, and other proteins that can recognize PS in a membrane surface. Particular attention is given to the use of these reagents for detecting apoptosis.

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.

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.

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.

Annexins expressed on the cell surface serve as receptors for adhesion to immobilized fetuin-A

Fetuin-A is a major constituent of the fetal bovine serum used extensively in cell culture media. We hereby present data demonstrating that breast carcinoma cells can adhere to immobilized fetuin-A in a calcium-dependent fashion. Interestingly, the cells can also divide and attain confluency under these conditions. Using a proteomic approach, we have identified annexin-II and -VI as the putative cell surface receptors for fetuin-A in the presence of Ca2+ ions. Biotinylation of cell surface proteins followed by immunoprecipitation revealed that annexin-VI was expressed on the extracytoplasmic surface of the cell membranes. Finally, to demonstrate that annexin-II and -VI were the adhesive receptors for fetuin-A, siRNA knockdown of expression of the annexins significantly reduced the calcium-mediated adhesion. Interestingly, we demonstrated that the tumor cells could also adhere to immobilized fetuin-A in the presence of magnesium ions, and that this adhesion was most likely mediated by integrins because neutralizing antibodies against beta1 integrins substantially reduced the adhesion. Our studies suggest that the expression of annexin-II and -VI and possibly other members of the family mediate novel adhesion and signaling mechanisms in tumor cells.

A putative consensus sequence for the nucleotide-binding site of annexin A6

Reaction-induced infrared difference spectroscopy (RIDS) has been used to investigate the nature of interactions of human annexin A6 (ANXA6) with nucleotides. RIDS results for ANXA6, obtained after the photorelease of GTP-gamma-S, ATP, or P(i) from the respective caged compounds, were identical, suggesting that the interactions between the nucleotide and ANXA6 were dominated by the phosphate groups. Phosphate-induced structural changes in ANXA6 were small and affected only seven or eight amino acid residues. The GTP fluorescent analogue, 2'(3')-O-(2,4,6-trinitrophenyl)guanosine 5'-triphosphate (TNP-GTP), quenched tryptophan fluorescence of ANXA6 when bound to the protein. A binding stoichiometry of 1 mol of nucleotide/mol ANXA6 was established with a K(D) value of 2.8 microM for TNP-GTP. The bands observed on RIDS of ANXA6 halves (e.g., N-terminal half, ANXA6a, and C-terminal half, ANXA6b) were similar to those of the whole molecule. However, their amplitudes were smaller by a factor of 2 compared to those of whole ANXA6. TNP-GTP bound to both fragments of ANXA6 with a stoichiometry of 0.5 mol/mol. However, the binding affinities of ANXA6a and ANXA6b differed from that of ANXA6. Simulated molecular modeling revealed a nucleotide-binding site which was distributed in two distinct domains. Residues K296, Y297, K598, and K644 of ANXA6 were less than 3 A from the bound phosphate groups of either GTP or ATP. The presence of two identical sequences in ANXA6 with the F-X-X-K-Y-D/E-K-S-L motif, located in the middle of ANXA6, at residues 293-301 (within ANXA6a) and at 641-649 (within ANXA6b), suggested that the F-X-X-K-Y-D/E-K-S-L motif was the putative sequence in ANXA6 for nucleotide binding.

Structural and dynamic changes in human annexin VI induced by a phosphorylation-mimicking mutation, T356D

Phosphorylation of some members of the annexin family of proteins may play a significant role in controlling their calcium-dependent interactions with membranes. Recent electron microscopic studies of annexin VI revealed that the protein's two core domains exhibit a great degree of flexibility and are able to undergo a relative conformational change that could potentially initiate contacts between membranes [Avila-Sakar, A. J., et al. (2000) J. Struct. Biol. 130, 54-62]. To assess the possibility of a regulatory role of phosphorylation in this behavior, the crystal structure of a phosphorylation-mimicking mutant (T356D in the flexible connector region of human annexin VI) was determined to 2.65 A resolution. When the mutant is compared to the wild-type annexin VI, subtle differences are seen at the site of the mutation, while larger changes are evident in one of the calcium-binding loops and in the presence of five calcium ions. Furthermore, biochemical studies provide evidence for additional conformational differences between the T356D and wild-type solution structures. Fluorescence emission and acrylamide quenching suggest a higher level of solvent exposure of Trp-343 in the connector region of T356D in the presence of calcium. Comparisons of retardation coefficients in native gel electrophoresis reveal that T356D has a more extended shape, while proteolytic studies show a greater accessibility of a trypsin cleavage site inside the linker region, indicating a conformation more open than the wild-type form. These data provide insights into a possible regulatory mechanism leading to a higher degree of flexibility and possibly a higher calcium binding affinity of annexin VI upon phosphorylation.

Interfacial basic cluster in annexin V couples phospholipid binding and trimer formation on membrane surfaces

Annexin V is an abundant eukaryotic protein that binds phospholipid membranes in a Ca(2+)-dependent manner. In the present studies, site-directed mutagenesis was combined with x-ray crystallography and solution liposome binding assays to probe the functional role of a cluster of interfacial basic residues in annexin V. Four mutants were investigated: R23E, K27E, R61E, and R149E. All four mutants exhibited a significant reduction in adsorption to phospholipid membranes relative to the wild-type protein, and the R23E mutation was the most deleterious. Crystal structures of wild-type and mutant proteins were similar except for local changes in salt bridges involving basic cluster residues. The combined data indicate that Arg(23) is a major determinant for interfacial phospholipid binding and participates in an intermolecular salt bridge that is key for trimer formation on the membrane surface. Together, crystallographic and solution data provide evidence that the interfacial basic cluster is a locus where trimerization is synergistically coupled to membrane phospholipid binding.

Cholesterol modulates the membrane binding and intracellular distribution of annexin 6

Annexins are Ca(2+)- and phospholipid-binding proteins that are widely expressed in mammalian tissues and that bind to different cellular membranes. In recent years its role in membrane traffic has emerged as one of its predominant functions, but the regulation of its intracellular distribution still remains unclear. We demonstrated that annexin 6 translocates to the late endocytic compartment in low density lipoprotein-loaded CHO cells. This prompted us to investigate whether cholesterol, one of the major constituents of low density lipoprotein, could influence the membrane binding affinity and intracellular distribution of annexin 6. Treatment of crude membranes or early and late endosomal fractions with digitonin, a cholesterol-sequestering agent, displayed a strong reduction in the binding affinity of a novel EDTA-resistant and cholesterol-sensitive pool of annexin 6 proteins. In addition, U18666A-induced accumulation of cholesterol in the late endosomal compartment resulted in a significant increase of annexin 6 in these vesicles in vivo. This translocation/recruitment correlates with an increased membrane binding affinity of GST-annexin 6 to late endosomes of U18666A-treated cells in vitro. In conclusion, the present study shows that changes in the intracellular distribution and concentration of cholesterol in different subcellular compartments participate in the reorganization of intracellular pools of Ca(2+)-dependent and -independent annexin 6.

A new consensus sequence for phosphatidylserine recognition by annexins

Annexins are abundant and ubiquitous proteins that bind, by their four structurally identical domain cores, to phosphatidylserine-containing membranes in the presence of Ca2+. Using molecular simulation and mutagenesis, we have identified a new phosphatidylserine-binding site in annexin V domain 1 and established its structure. The residues involved in this site constitute a consensus sequence highly conserved in all annexins. Remarkably, this consensus sequence is exclusively found in domains 1 or 2, sometimes in both, but never in domains 3 and 4. Such a pattern actually delineates three classes of annexins, shedding new light on the role played by the four-domain core of annexins that could encode specific information discriminating the different annexins that compete within a given cell for membrane binding. Our findings thus provide new strategies for understanding the regulation of the cellular functions of annexins.

An improved hydrogen bond potential: impact on medium resolution protein structures

A new semi-empirical force field has been developed to describe hydrogen-bonding interactions with a directional component. The hydrogen bond potential supports two alternative target angles, motivated by the observation that carbonyl hydrogen bond acceptor angles have a bimodal distribution. It has been implemented as a module for a macromolecular refinement package to be combined with other force field terms in the stereochemically restrained refinement of macromolecules. The parameters for the hydrogen bond potential were optimized to best fit crystallographic data from a number of protein structures. Refinement of medium-resolution structures with this additional restraint leads to improved structure, reducing both the free R-factor and over-fitting. However, the improvement is seen only when stringent hydrogen bond selection criteria are used. These findings highlight common misconceptions about hydrogen bonding in proteins, and provide explanations for why the explicit hydrogen bonding terms of some popular force field sets are often best switched off.

GTP-induced membrane binding and ion channel activity of annexin VI: is annexin VI a GTP biosensor?

Annexin VI (AnxVI) formed ion channels in planar lipid bilayers that were induced by the addition of millimolar guanosine 5'-triphosphate (GTP) at pH 7.4 and that were not accompanied by a penetration of the protein into the membrane hydrophobic region. GTP-influenced interactions of AnxVI with Ca2+/liposomes produced small structural alterations as revealed by circular dichroism and infrared spectroscopies. Guanosine 5'-3-O-(thio)-triphosphate (GTPgammaS) binding to AnxVI, promoted by the photorelease of GTPgammaS from GTPgammaS[1-(4,5-dimethoxy-2-nitrophenyl)-ethyl] (caged-GTPgammaS), affected three to four amino acid residues of AnxVI in the presence of Ca2+/liposomes, while about eight or nine amino acid residues were altered in their absence. This suggested that the nucleotide-binding site overlapped the lipid-binding domain of AnxVI. The binding of the fluorescent GTP analog, 2'-(or 3')-O-(2,4,6-trinitrophenyl)guanosine 5'-triphosphate (TNP-GTP) to AnxVI was optimal in the presence of Ca2+/liposomes, with a dissociation constant (K(d)) of 1 microM and stoichiometry of 1. TNP-GTP promoted fluorescence resonance energy transfer from tryptophan residues to the nucleotide. Ion conductance and fluorescence measurements of the C- and N-terminal fragments of AnxVI indicated distinct GTP-binding properties, suggesting that the existence of the GTP-induced ion channel activity of AnxVI is associated with the flexibility of the two halves of the protein. Such structural flexibility could contribute to a molecular mechanism of AnxVI acting as a GTP biosensor.

N- and C-terminal halves of human annexin VI differ in ability to form low pH-induced ion channels

Human recombinant annexin VI (AnxVI) or its N- (AnxVIA) and C-terminal (AnxVIB) fragments were expressed in E. coli. Their ability to form voltage-dependent ion channels in membranes, induced by low pH, was measured to verify the hypothesis that, upon acidification, the hydrophobicity of AnxVI at a specific domain significantly increases allowing the AnxVI interaction with lipids in a Ca(2+)-independent manner. By theoretically analyzing changes in protein hydrophobicity, we found that hydrophobicity of AnxVIA significantly differed from that of AnxVIB at low pH. These predictions were confirmed experimentally by using planar lipid bilayers and liposome pull-down assay. We found striking difference between AnxVIA and AnxVIB in the ion channel activity, as well as in the membrane binding, suggesting that the halves of AnxVI maybe functionally different. Moreover, we calculated and predicted that the ion channel activity at low pH should appear in other human annexins, as AnxII, AnxV (as known), AnxVIII, and AnxXIII. The possibility that AnxVI acts as cytosolic component of a transmembrane pH-sensing mechanism is proposed.

Conformational states of annexin VI in solution induced by acidic pH

Acidic pH-induced folding of annexin (Anx)VI in solution was investigated in order to study the mechanism of formation of ion channels by the protein in membranes. Using 2-(p-toluidino)naphthalene-6-sulfonic acid as a hydrophobic probe, it was demonstrated that AnxVI exerts a large change in hydrophobicity at acidic pH. Moreover, circular dichroism spectra indicated that the native state of AnxVI changes at acidic pH towards a state characterized by a significant loss of alpha-helix content and appearance of new beta-structures. These changes are reversible upon an increase of pH. It is postulated that the structural folding of AnxVI could explain how a soluble protein may undergo transition into a molecule able to penetrate the membrane hydrophobic region. The physiological significance of these observations is discussed.

Selective degradation of annexins by chaperone-mediated autophagy

Annexins are a family of proteins that bind phospholipids in a calcium-dependent manner. Analysis of the sequences of the different members of the annexin family revealed the presence of a pentapeptide biochemically related to KFERQ in some annexins but not in others. Such sequences have been proposed to be a targeting sequence for chaperone-mediated autophagy, a lysosomal pathway of protein degradation that is activated in confluent cells in response to removal of serum growth factors. We demonstrate that annexins II and VI, which contain KFERQ-like sequences, are degraded more rapidly in response to serum withdrawal, while annexins V and XI, without such sequences, are degraded at the same rate in the presence and absence of serum. Using isolated lysosomes, only the annexins containing KFERQ-like sequences are degraded by chaperone mediated-autophagy. Annexins V and XI could associate with lysosomes but did not enter the lysosomes and were not proteolytic substrates. Furthermore, four annexins containing KFERQ-like sequences, annexins I, II, IV, and VI, are enriched in lysosomes with high chaperone-mediated autophagy activity as expected for substrate proteins. These results provide striking evidence for the importance of KFERQ motifs in substrates of chaperone-mediated autophagy.

Membrane-bound 3D structures reveal the intrinsic flexibility of annexin VI

Several quasi-ordered arrays and three two-dimensional crystal forms of annexin VI were obtained on artificial lipid monolayers. Three-dimensional reconstructions of the crystal forms exhibit marked differences in the orientations of the two lobes, revealing flexibility of the linker between the two lobes of annexin VI. Evidence is presented that the lobes may bind the monolayer in a parallel orientation, or an antiparallel orientation, in which the second lobe is turned away from the monolayer. It is hypothesized that annexin VI may also adopt several conformations in vivo, underlying different functional roles.

Mapping the site of interaction between annexin VI and the p120GAP C2 domain

Annexin VI is a Ca(2+)-dependent membrane and phospholipid binding protein. It mediates a protein-protein interaction with the Ras p21 regulatory protein p120GAP. In this study we have mapped the binding site of GAP within the annexin VI protein. Using Far Western overlay binding assays and cell lysate competition studies we have mapped the site of interaction to the inter-lobe linker region; amino acids 325-363. Finally, using a GST fusion protein corresponding to this linker region we have demonstrated that cellular loading of the fusion protein into Rat-1 fibroblasts by electroporation blocks the interaction and co-immunoprecipitation of annexin VI and GAP.

ATP-Binding site of annexin VI characterized by photochemical release of nucleotide and infrared difference spectroscopy

Structural changes induced by nucleotide binding to porcine liver annexin VI (AnxVI) were probed by reaction-induced difference spectroscopy (RIDS). Photorelease of the nucleotide from ATP[Et(PhNO2)] produced RIDS of AnxVI characterized by reproducible changes in the amide I region. The magnitude of the infrared change was comparable to RIDS of other ATP-binding proteins, such as Ca(2+)-ATPase and creatine and arginine kinases. Analysis of RIDS revealed the existence of ATP-binding site(s) (K(d) < 1 microM) within the AnxVI molecule, comprising five to six amino acid residues located in the C-terminal portion of the protein molecule. The binding stoichiometry of ATP:AnxVI was determined as 1:1 (mol/mol). ATP, in the presence of Ca2+, induced changes in protein secondary structure reflected by a 5% decrease in alpha-helix content of the protein in favor of unordered structure. Such changes may influence the affinity of AnxVI for Ca2+ and modulate its interaction with membranes.

Annexin VI interacts with adenine nucleotides and their analogs

Annexin VI (AnxVI), a member of the annexin family of Ca2+- and membrane-binding proteins, has been shown to interact in vitro with adenine nucleotides. Furthermore, it has been proposed that within the AnxVI molecule a nucleotidde-binding domain exists, which is located in the C-terminal half of the protein, in the vicinity of Trp343. By comparison of exposure of tryptophan and multiple tyrosine residues upon nucleotide binding, as revealed by quenching of intrinsic fluorescence of AnxVI by ATP, ADP or cAMP, it can be concluded that the binding of nucleotides evokes changes in the protein tertiary structure. Moreover, in the course of present study we have found that AnxVI binds to a non-hydrolysable analog of ATP, the triazine dye Cibacron blue 3GA (CB3GA), immobilized on agarose. Binding reveals negative cooperativity with respect to protein concentration and is Ca2+-dependent. Binding is prevented by ATP. CB3GA binds to AnxVI also in solution, evoking the formation of annexin multimers. On the basis of this observation it can be suggested that interaction of CB3GA with AnxVI is useful to examine, with some limitations, the self-association of annexin molecules implying to play a role in interacting of AnxVI with biological membranes.

Membrane domain formation by calcium-dependent, lipid-binding proteins: insights from the C2 motif

We propose a novel role in cellular function for some membrane-binding proteins and, specifically, the C2 motif. The C2 motif binds phospholipid in a manner that is modulated by Ca2+ and is thought to confer membrane-binding ability on a wide variety of proteins, primarily proteins involved in signal transduction and membrane trafficking events. We hypothesize that in the absence of Ca2+ the C2 motif couples the free energy of binding to a bilayer membrane comprised of zwitterionic and negatively charged lipids to the formation of a domain enriched in the negative lipids. This in turn leads to the dynamic clustering of bound homologous or heterologous proteins incorporating the C2 motif, or other acidic lipid-binding motifs. In the presence of Ca2+, the protein clusters may be further stabilized. In support of this hypothesis we present evidence for membrane domain formation by the first C2 domain of synaptotagmin in the absence of Ca2+. Fluid state phospholipid mixtures incorporating a pyrene-labeled phospholipid probe exhibited a change in pyrene excimer/monomer fluorescence ratio consistent with domain formation upon binding the C2 domain. In addition, we present the results of simulations of the interaction of the C2 domain with the membrane that indicate that protein clusters and lipid domains form in concert.