Change in the N-terminal domain conformation of annexin I that correlates with liposome aggregation is impaired by Ser-27 to Glu mutation that mimics phosphorylation

Annexin A1 accounts for an anti-inflammatory binding target of sesamin metabolites

Sesamin [(7α,7'α,8α,8'α)-3,4:3',4'-bis(methylenedioxy)-7,9':7',9-diepoxylignane] is a major lignan in sesame seeds. Sesamin is converted to the catechol metabolite, SC1 [(7α,7'α,8α,8'α)-3',4'-methylenedioxy-7,9':7',9-diepoxylignane-3,4-diol] with anti-inflammatory effects after oral administration. However, its molecular target remains unknown. Analysis using high-performance affinity nanobeads led to the identification of annexin A1 (ANX A1) as an SC1-binding protein. SC1 was found to bind to the annexin repeat 3 region of ANX A1 with a high-affinity constant (Kd = 2.77 μmol L^-1). In U937 cells, SC1 exhibited an anti-inflammatory effect dependent on ANX A1. Furthermore, administration of sesamin or SC1 attenuated carbon tetrachloride-induced liver damage in mice and concurrently suppressed inflammatory responses dependent on ANX A1. The mechanism involved SC1-induced ANX A1 phosphorylation at serine 27 that facilitates extracellular ANX A1 release. Consequently, the ANX A1 released into the extracellular space suppressed the production of tumor necrosis factor α. This study demonstrates that ANX A1 acts as a pivotal target of sesamin metabolites to attenuate inflammatory responses.

Nudged Elastic Band Study on the N-Terminal Domain Conformational Pathways of Annexin A1 from a Buried State to an Exposed State

Mapping the conformational pathway of biomolecules is a great challenge because of the large size and complexity of biomolecules. The nudged elastic band (NEB) method has been applied to study the reaction pathways for both small organic molecules and small peptides of a few amino acids. In this work, for the first time, the NEB method was employed to study the conformational pathways of Annexin A1, a membrane-binding protein of 334 amino acids. The N-terminal domain conformational change from the buried state within the core domain to the exposed state outside the core domain is a vital step for Annexin A1 to interact with membranes or target proteins. In this work, multiple molecular dynamics simulations using the NEB method were performed to simulate the N-terminal domain conformational pathway of Annexin A1. Our results suggested that the N-terminal domain of Annexin A1 is removed from the repeat III of the core domain in a sliding motion. The loop region of repeat III covering the N-terminal helix in the buried state does not lift up for the N-terminal to swing out of the pocket; instead, the N-terminal pulls out from the bottom of the core domain. The N-terminal domain linker region (S27-N42) flexibility is critical for the N-terminal domain conformational changes. Our results also suggested a two-step folding process for the helix D in repeat III, M247-V250 folds first followed by the folding of L251-E254. The results demonstrated that the NEB method could be an effective tool for theoretical studies on conformational pathways of biomolecules.

Effect of serine phosphorylation and Ser25 phospho-mimicking mutations on nuclear localisation and ligand interactions of annexin A2

Annexin A2 (AnxA2) interacts with numerous ligands, including calcium, lipids, mRNAs and intracellular and extracellular proteins. Different post-translational modifications participate in the discrimination of the functions of AnxA2 by modulating its ligand interactions. Here, phospho-mimicking mutants (AnxA2-S25E and AnxA2-S25D) were employed to investigate the effects of Ser25 phosphorylation on the structure and function of AnxA2 by using AnxA2-S25A as a control. The overall α-helical structure of AnxA2 is not affected by the mutations, since the thermal stabilities and aggregation tendencies of the mutants differ only slightly from the wild-type (wt) protein. Unlike wt AnxA2, all mutants bind the anxA2 3' untranslated region and β-γ-G-actin with high affinity in a Ca(2+)-independent manner. AnxA2-S25E is not targeted to the nucleus in transfected PC12 cells. In vitro phosphorylation of AnxA2 by protein kinase C increases its affinity to mRNA and inhibits its nuclear localisation, in accordance with the data obtained with the phospho-mimicking mutants. Ca(2+)-dependent binding of wt AnxA2 to phosphatidylinositol, phosphatidylinositol-3-phosphate, phosphatidylinositol-4-phosphate and phosphatidylinositol-5-phosphate, as well as weaker but still Ca(2+)-dependent binding to phosphatidylserine and phosphatidylinositol-3,5-bisphosphate, was demonstrated by a protein-lipid overlay assay, whereas binding of AnxA2 to these lipids, as well as its binding to liposomes, is inhibited by the Ser25 mutations. Thus, introduction of a modification (mutation or phosphorylation) at Ser25 appears to induce a conformational change leading to increased accessibility of the mRNA- and G-actin-binding sites in domain IV independent of Ca(2+) levels, while the Ca(2+)-dependent binding of AnxA2 to phospholipids is attenuated.

The complex understanding of Annexin A1 phosphorylation

Annexin A1 (ANXA1) is the first characterized member of the annexins superfamily. It binds the cellular membrane phospholipids in Ca(2+) regulated manner. Annexin A1 has been found in several tissues and many physiological roles as hormones secretion, vesiculation, inflammatory response, apoptosis and differentiation have been shown. Its subcellular localization and binding with many partner proteins are altered accordingly with its physiological role. The Annexin A1 membrane localization is crucial for binding to receptors, suggesting a paracrine and juxtacrine extracellular action. Annexin A1 is subjected to several post-translational modifications. In particular the protein is phosphorylated on several residues both on the N-terminal functional domain and on the C-terminus core. Different kinases have been identified as responsible for the phosphorylation status of selective residues. The specific change in the phosphorylation status on the different sites alters ANXA1 localization, binding properties and functions. This review shows the physiological relevance of the ANXA1 phosphorylation leading to the conclusion that numerous and different roles of Annexin A1 could be associated with different phosphorylations to alter not only intracellular localization and bindings to its partners but also the extracellular receptor interactions.

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-A1: a pivotal regulator of the innate and adaptive immune systems

The glucocorticoids are the most potent anti-inflammatory drugs that we possess and are effective in a wide variety of diseases. Although their action is known to involve receptor mediated changes in gene transcription, the exact mechanisms whereby these bring about their pleiotropic action in inflammation are yet to be totally understood. Whilst many different genes are regulated by the glucocorticoids, we have identified one particular protein-annexin A1 (Anx-A1)-whose synthesis and release is strongly regulated by the glucocorticoids in many cell types. The biology of this protein, as revealed by studies using transgenic animals, peptide mimetics and neutralizing antibodies, speaks to its role as a key modulator of both of the innate and adaptive immune systems. The mechanism whereby this protein exerts its effects is likely to be through the FPR receptor family-a hitherto rather enigmatic family of G protein coupled receptors, which are increasingly implicated in the regulation of many inflammatory processes. Here we review some of the key findings that have led up to the elucidation of this key pathway in inflammatory resolution.

Insight into the location and dynamics of the annexin A2 N-terminal domain during Ca(2+)-induced membrane bridging

Annexin A2 (AnxA2) is a Ca(2+)- and phospholipid-binding protein involved in many cellular regulatory processes. Like other annexins, it is constituted by two domains: a conserved core, containing the Ca(2+) binding sites, and a variable N-terminal segment, containing sites for interactions with other protein partners like S100A10 (p11). A wealth of data exists on the structure and dynamics of the core, but little is known about the N-terminal domain especially in the Ca(2+)-induced membrane-bridging process. To investigate this protein region in the monomeric AnxA2 and in the heterotetramer (AnxA2-p11)(2), the reactive Cys8 residue was specifically labelled with the fluorescent probe acrylodan and the interactions with membranes were studied by steady-state and time-resolved fluorescence. In membrane junctions formed by the (AnxA2-p11)(2) heterotetramer, the flexibility of the N-terminal domain increased as compared to the protein in solution. In "homotypic" membrane junctions formed by monomeric AnxA2, acrylodan moved to a more hydrophobic environment than in the protein in solution and the flexibility of the N-terminal domain also increased. In these junctions, this domain is probably not in close contact with the membrane surface, as suggested by the weak quenching of acrylodan observed with doxyl-PCs, but pairs of N-termini likely interact, as revealed by the excimer-forming probe pyrene-maleimide bound to Cys8. We present a model of monomeric AnxA2 N-terminal domain organization in "homotypic" bridged membranes in the presence of Ca(2+).

A ten-residue domain (Y11–A20) in the NH2-terminus modulates membrane association of annexin A7

Annexin A7 (synexin, annexin VII) is postulated to promote membrane fusion during surfactant secretion in alveolar type II cells and catecholamine secretion in adrenal chromaffin cells. Recently, we demonstrated that the 1-29 residues in the NH(2)-terminus could, possibly by interaction with the COOH-terminus, influence the Ca(2+)-dependent membrane binding, aggregation, and fusion properties of annexin A7 (A7). In this study, we further investigated this 29-residue domain by evaluating several deletion and point mutations for membrane-associated functions of A7. In comparison to A7, the mutants lacking 1-29 residues (A7Delta(1-29)) or 1-21 residues (A7Delta(1-21)), but not those lacking 1-10 residues (A7Delta(1-10)) or 21-29 residues (A7Delta(21-29)), showed diminished membrane binding. Segmental deletion of 10-20 residues (A7Delta(10-20)) also decreased the protein binding to membranes. The Ca(2+)-dependent membrane aggregation of PLV with A7Delta(1-29) was maximally diminished but less so with A7Delta(10-20) or A7Delta(1-21) in comparison to that with A7. However, phospholipid vesicle (PVL) aggregation was unaffected with A7Delta(1-10) or A7Delta(21-29). The Ca(2+)-dependent membrane fusion of PLV was also diminished with A7Delta(10-20) and A7Delta(1-29), but not with A7Delta(1-10). Since the mode of annexin A7 association and function with biological membranes could be different, we also evaluated these proteins for functional changes with isolated lung lamellar bodies. In comparison to A7, the binding to lamellar bodies was diminished for A7Delta(1-29) and A7Delta(1-21) but not for A7Delta(1-10). The Ca(2+)-dependent fusion of isolated lamellar bodies with PLV was also diminished with A7Delta(1-29), but not with A7Delta(10-20) or A7Delta(1-21). Taken together, our studies suggest that the 10-residue domain (Y(11)-A(20)) in the NH(2)-terminus modifies the phospholipid binding and aggregation properties of annexin A7. For binding and fusion of biological membranes, the 10-29-residue domain may be required although the annexin A7 properties are primarily modulated through the Y(11)-A(20) domain.

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.

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.

Mycobacteria-containing phagosomes associate less annexins I, VI, VII and XI, but not II, concomitantly with a diminished phagolysosomal fusion

We have studied the intracellular localization of annexins I,II, VI, VII, and XI in cells containing latex beads or Mycobacterium avium at different times after ingestion in order to establish whether a correlation existed between the association of annexins to phagosomes and phagolysosomal fusion, since the intracellular survival of mycobacteria is linked to an impairment of phagosome maturation. We demonstrate an important decrease in the levels of association of annexins I, VI, VII and XI, but not II to phagosomes containing either live or killed mycobacteria compared with phagosomes containing inert latex particles. The reduced association of annexins observed was detected only on M. avium-containing phagosomes and not in other cell membrane nor in cytosolic fractions from infected cells, and was apparent from 8 hours through to 4 days after phagocytosis. These findings add elements to the present knowledge of the phagosomal modifications that accompany the survival of intracellular pathogens, suggesting that annexins I, VI, VII, and XI play a secondary role in phagosomal fusion events while annexin II does not seem to be related to the mechanism of regulation of endolysosomal fusion.

Phosphorylation mutants elucidate the mechanism of annexin IV-mediated membrane aggregation

Site-directed mutagenesis, electron microscopy, and X-ray crystallography were used to probe the structural basis of annexin IV-induced membrane aggregation and the inhibition of this property by protein kinase C phosphorylation. Site-directed mutants that either mimic (Thr6Asp, T6D) or prevent (Thr6Ala, T6A) phosphorylation of threonine 6 were produced for these studies and compared with wild-type annexin IV. In vitro assays showed that unmodified wild-type annexin IV and the T6A mutant, but not PKC-phosphorylated wild-type or the T6D mutant, promote vesicle aggregation. Electron crystallographic data of wild-type and T6D annexin IV revealed that, similar to annexin V, the annexin IV proteins form 2D trimer-based ordered arrays on phospholipid monolayers. Cryo-electron microscopic images of junctions formed between lipid vesicles in the presence of wild-type annexin IV indicated a separation distance corresponding to the thickness of two layers of membrane-bound annexin IV. In this orientation, a single layer of WT annexin IV, attached to the outer leaflet of one vesicle, would undergo face-to-face self-association with the annexin layer of a second vesicle. The 2.0-A resolution crystal structure of the T6D mutant showed that the mutation causes release of the N-terminal tail from the protein core. This change would preclude the face-to-face annexin self-association required to aggregate vesicles. The data suggest that reversible complex formation through phosphorylation and dephosphorylation could occur in vivo and play a role in the regulation of vesicle trafficking following changes in physiological states.

N-Terminal domain of annexin 2 regulates Ca(2+)-dependent membrane aggregation by the core domain: a site directed mutagenesis study

Annexin 2 binds and aggregates biological membranes in a Ca(2+)-dependent manner. This protein exists as a monomer (p36) or as a heterotetramer (p90) in which two p36 chains are associated with a dimer of p11, a member of the S100 protein family. Protein kinase C phosphorylates the protein at the level of the N-terminal tail on serines 11 and 25, thereby modifying its oligomeric structure and its properties of membrane aggregation. To analyze these effects, the properties of a series of mutants in which serines 11 and 25 were replaced by alanine and/or glutamic acid were investigated. The affinity for p11 light chain was decreased in the S11E mutants. Glutamic acid residues in positions 11 or 25 did not change membrane binding, either in the tetrameric or in the monomeric form. On the other hand, these mutations affected the aggregation properties of the two forms. For the tetramer, the aggregation efficiency was decreased but not the Ca(2+) sensitivity, whereas the latter was affected in the case of the monomer. The effects were stronger in the S11E mutants, and they were cumulative in the double mutant. They suggest a different conformation of the N-terminal domain in the mutants (and in the phosphorylated protein), a hypothesis which is supported by proteolysis experiments. This conformational change would affect aggregation by the monomer through a dimerization step.

Mechanism of annexin I-mediated membrane aggregation

It has been proposed that annexin I has two separate interaction sites that are involved in membrane binding and aggregation, respectively. To better understand the mechanism of annexin I-mediated membrane aggregation, we investigated the properties of the inducible secondary interaction site implicated in membrane aggregation. X-ray specular reflectivity measurements showed that the thickness of annexin I layer bound to the phospholipid monolayer was 31 +/- 2 A, indicating that annexin I binds membranes as a protein monomer or monolayer. Surface plasmon resonance measurements of annexin I, V, and mutants, which allowed evaluation of membrane aggregation activity of annexin I separately from its membrane binding, revealed direct correlation between the relative membrane aggregation activity and the relative affinity of the secondary interaction site for the secondary membrane. The secondary binding was driven primarily by hydrophobic interactions, unlike calcium-mediated electrostatic primary membrane binding. Chemical cross-linking of membrane-bound annexin I showed that a significant degree of lateral association of annexin I molecules precedes its membrane aggregation. Taken together, these results support a hypothetical model of annexin I-mediated membrane aggregation, in which a laterally aggregated monolayer of membrane-bound annexin I directly interacts with a secondary membrane via its induced hydrophobic interaction site.

Cleavage of annexin I in human neutrophils is mediated by a membrane-localized metalloprotease

A truncated form of annexin I, formed during Ca2+-induced translocation to neutrophil specific granules and secretory vesicles/plasma membranes, is generated through the action of an endogenous membrane protease. The cleavage of annexin I is inhibited by the metalloprotease inhibitor 1,10-phenanthroline as well as by Triton X-100 and dithiothreitol, classifying the protease as a membrane-bound, thiol-dependent metalloprotease. The cleavage site is located close to the N-terminal of annexin I, leaving a truncated form of the molecule, des1-8 annexin I, that contains the Ca2+-binding sites, as well as a number of phosphorylation sites of importance for the function of the protein. When assessing binding capacity to different neutrophil organelles, full-length annexin I bound to azurophil granules, specific granules, and secretory vesicles/plasma membranes, while des1-8 annexin I only bound to specific granules and secretory vesicles/plasma membranes, but not to azurophil granules (C. Sjölin, C. Dahlgren, Biochim. Biophys. Acta 1281 (1996) 227-234). This implies that there are different mechanisms of binding to neutrophil organelles of full-length annexin I and the truncated form, and that cleavage of annexin I might be of regulatory importance for the degranulation process.

Structure of the trigonal crystal form of bovine annexin IV

The structure of a trigonal crystal form of N-terminally truncated [des-(1-9)] bovine annexin IV, an annexin variant that exhibits the distinctive property of binding both phospholipids and carbohydrates in a Ca2+-dependent manner, has been determined at 3 A (0.3 nm) resolution -space group: R3; cell parameters: a=b=118.560 (8) A and c=82.233 (6) A-. The overall structure of annexin IV, crystallized in the absence of Ca2+ ions, is highly homologous to that of the other known members of the annexin family. The trimeric assembly in the trigonal crystals of annexin IV is quite similar to that found previously in non-isomorphous crystals of human, chicken and rat annexin V and to the subunit arrangement in half of the hexamer of hydra annexin XII. Moreover, it resembles that found in two-dimensional crystals of human annexin V bound to phospholipid monolayers. The propensity of several annexins to generate similar trimeric arrays supports the hypothesis that trimeric complexes of such annexins, including annexin IV, may represent the functional units that interact with membranes.

Structural analysis of junctions formed between lipid membranes and several annexins by cryo-electron microscopy

The (annexin II-p11)2 tetramer has been proposed to participate in exocytosis and several other members of the annexin superfamily have been reported to aggregate liposomes in vitro. In this context, the Ca2+-dependent binding of several annexins to chromaffin granules and liposomes was investigated by cryo-electron microscopy. The Ca2+-dependent aggregation of lipid membranes by (annexin II-p11)2 results from the spontaneous self-organization of the protein into two-dimensional plaques, which are visualized in projection as characteristic junctions. The junctions have a constant thickness of 210(+/-10) A and present a symmetrical distribution of electron-dense material arranged into seven stripes. They were observed over a wide range of Ca2+ concentrations, down to 2 microM. The molecular components corresponding to the seven electron-dense stripes were assigned as follows: the two associated membranes give rise to two outer stripes each and the three central stripes correspond to the (annexin II-p11)2 tetramer. Each annexin II molecule interacts with the outer lipid leaflet of one membrane, giving rise to one stripe, while the central stripe is due to the (p11)2 dimer with which both annexin II molecules interact. Both annexin II and annexin I also induced the Ca2+-dependent aggregation of liposomes via junctions that lack the central (p11)2 moiety and present only six high-density stripes. As expected, both annexin V and annexin III bind to liposomes without inducing their aggregation.

Effects of profilin-annexin I association on some properties of both profilin and annexin I: modification of the inhibitory activity of profilin on actin polymerization and inhibition of the self-association of annexin I and its interactions with liposomes

We have previously shown that annexin I, a member of a family of calcium-dependent phospholipid and membrane binding proteins, interacts with profilin with high specificity and affinity. This finding further suggests that annexin I is involved through profilin in the regulation of membrane-cytoskeleton organization. We have investigated the consequences of a complex formed by these two proteins on the functions of both profilin and annexin I. Annexin I is able to modify the inhibitory effect of profilin on actin polymerization. This action is partial and the mechanism involved appears to be complex. On the other hand, the association between annexin I and profilin is sufficiently strong to inhibit the self-association of annexin I. The binding capacity of annexin I to liposomes containing phosphatidylserine, which mimics annexin I binding to membranes, is also decreased by profilin. This binding is nevertheless restored when phosphatidylinositol 4,5-biphosphate (PtdInsP2) is included in the liposomes. Finally, the capacity of annexin I to aggregate liposomes is also modified. It is worthwhile mentioning that the liposomes-binding and liposomes-aggregating activities of annexin I are independently regulated. The cell localization and functions of annexin I and profilin suggest that interaction between these two proteins may be directly implicated in the regulation of membrane-cytoskeleton. The phospholipid composition of membranes may be one of the modulating factors.