Modeling the Annexin A1-S100A11 heterotetramer: a molecular dynamics investigation of structure and correlated motion

Annexin A1 (A1) has been shown to form a tetrameric complex (A1t) with S100A11 which is implicated in calcium homeostasis and EGFR pathways. In this work, a full-length model of the A1t was generated for the first time. Multiple molecular dynamics simulations were performed on the complete A1t model for several hundred nanoseconds each to assess the structure and dynamics of A1t. These simulations yielded three structures for the A1 N-terminus (ND) which were identified via principal component analysis. The orientations and interactions of the first 11 A1-ND residues for all three structures were conserved, and their binding modes were strikingly similar to those of the Annexin A2 N-terminus in the Annexin A2-p11 tetramer. In this study, we provided detailed atomistic information for the A1t. Strong interactions were identified within the A1t between the A1-ND and both S100A11 monomers. Residues M3, V4, S5, E6, L8, K9, W12, E15, and E18 of A1 were the strongest interactions between A1 and the S100A11 dimer. The different conformations of the A1t were attributed to the interaction between W12 of the A1-ND with M63 of S100A11 which caused a kink in the A1-ND. Cross-correlation analysis revealed strong correlated motion throughout the A1t. Strong positive correlation was observed between the ND and S100A11 in all simulations regardless of conformation. This work suggests that the stable binding of the first 11 residues of A1-ND to S100A11 is potentially a theme for Annexin-S100 complexes and that the flexibility of the A1-ND allows for multiple conformations of the A1t.Communicated by Ramaswamy H. Sarma.

Ca2+ ions facilitate the organization of the Annexin A2/S100A10 heterotetramer

Annexin A2 (A2) is a member of the Annexin family, which contains Ca2+ -regulated phospholipid-binding proteins. Annexins associate with S100 proteins to form heterotetramers. The A2/S100A10 heterotetramer (A2t) is the most extensively studied of these heterotetramers. It induces membrane microdomain formation, causes membrane budding, and facilitates proliferation of some cancers. In this work, the first molecular dynamics (MD) study on the complete A2t of 868 amino acids was performed. MD trajectories of more than 600 ns each were generated for complete A2t complexes with and without Ca2+ ions. The outward extension of membrane-binding residues A2-K279 and A2-K281 was shown to be inhibited in the absence of Ca2+ as they were captured by Ca2+ -binding residue D322. F-actin binding residue A2-D339 was observed to occupy either an exposed or buried state in the absence of Ca2+ , while it only occupied the buried state in the presence of Ca2+ . The observed motions of the A2t subunits are highly organized with a strongly correlated central region which is negatively correlated with the periphery of the complex. The central region contains the S100A10 (p11) dimer, A2-N, and A2-I, while the periphery contains A2-II, A2-III, and A2-IV. Novel interactions between A2 and p11 were identified. A2 residues outside of A2-N (K80, R77, E82, and R145) had strong interactions with p11. Residue R145 of A2 may have a significant effect on the dynamics of the system, with its interaction resulting in asymmetric motions of A2. The presented results provide novel insights to inform future experimental studies.

Heterotetrameric annexin A2/S100A10 (A2t) is essential for oncogenic human papillomavirus trafficking and capsid disassembly, and protects virions from lysosomal degradation

Human papillomavirus (HPV) entry into epithelial cells is independent of canonical endocytic pathways. Upon interaction with host cells, HPV establishes infection by traversing through an endocytic pathway that is clathrin- and caveolin-independent, but dependent on the annexin A2/S100A10 heterotetramer (A2t). We examined the contribution of monomeric annexin A2 (AnxA2) vs. A2t in HPV infection and endocytosis, and further characterized the role of these molecules in protein trafficking. We specifically show that cell surface A2t is not required for HPV attachment, and in the absence of A2t virion internalization remains clathrin-independent. Without A2t, viral progression from early endosomes to multivesicular endosomes is significantly inhibited, capsid uncoating is dramatically reduced, and lysosomal degradation of HPV is accelerated. Furthermore, we present evidence that AnxA2 forms a complex with CD63, a known mediator of HPV trafficking. Overall, the observed reduction in infection is less significant in the absence of S100A10 alone compared to full A2t, supporting an independent role for monomeric AnxA2. More broadly, we show that successful infection by multiple oncogenic HPV types is dependent on A2t. These findings suggest that A2t is a central mediator of high-risk HPV intracellular trafficking post-entry and pre-viral uncoating.

A new cis-acting motif is required for the axonal SMN-dependent Anxa2 mRNA localization

Spinal muscular atrophy (SMA) is caused by mutations and/or deletions of the survival motor neuron gene (SMN1). Besides its function in the biogenesis of spliceosomal snRNPs, SMN might possess a motor neuron specific role and could function in the transport of axonal mRNAs and in the modulation of local protein translation. Accordingly, SMN colocalizes with axonal mRNAs of differentiated NSC-34 motor neuron-like cells. We recently showed that SMN depletion gives rise to a decrease in the axonal transport of the mRNAs encoding Annexin A2 (Anxa2). In this work, we have characterized the structural features of the Anxa2 mRNA required for its axonal targeting by SMN. We found that a G-rich motif located near the 3'UTR is essential for axonal localization of the Anxa2 transcript. We also show that mutations in the motif sequence abolish targeting of Anxa2 reporter mRNAs in axon-like structures of differentiated NSC-34 cells. Finally, localization of both wild-type and mutated Anxa2 reporters is restricted to the cell body in SMN-depleted cells. Altogether, our studies show that this G-motif represents a novel and essential determinant for axonal localization of the Anxa2 mRNA mediated by the SMN complex.

Mechanism of Fibronectin Binding to Human Trabecular Meshwork Exosomes and Its Modulation by Dexamethasone

Exosomes are emerging as important mediators of cell-matrix interactions by means of specific adhesion proteins. Changes in the tissue-specific exosomal protein expression may underlie pathological conditions whereby extracellular matrix turnover and homeostasis is disrupted. Ocular hypertension due to extracellular matrix accumulation in the trabecular meshwork is a hallmark of glucocorticoid-induced glaucoma. In the trabecular meshwork, exosomal fibronectin mediates cell matrix interactions at cellular structures called “invadosomes”. Trabecular meshwork cells use invadosomes to turn over their surrounding matrix and maintain passageways for flow of aqueous humor. In this study, we observed that human trabecular meshwork explants treated with dexamethasone released exosomes with significantly reduced amounts of fibronectin bound per exosome. Further, we found that exosome-fibronectin binding is heparan sulfate-dependent, consistent with our observation that trabecular meshwork exosomes are enriched in the heparin/heparan sulfate binding annexins A2 and A6. In this way, dexamethasone-treated explants released exosomes with a significant reduction in annexin A2 and A6 per exosome. Interestingly, we did not detect exosomal matrix metalloproteinases, but we identified abundant dipeptidyl peptidase 4, a serine protease whose activity was reduced on exosomes isolated from dexamethasone-treated explants. Together, our findings demonstrate mechanistically how corticosteroid-induced alterations in exosomal adhesion cargo and properties can account for the pathological matrix accumulation seen in many glaucoma patients.

N-Terminal Polypeptide of Annexin A2 Decreases Infection of Mycoplasma hyorhinis to Gastric Cancer Cells

Mycoplasma infection in human and its contamination in cell cultures are worldwide problems. The drugs currently available for preventing or treating mycoplasma infection suffer from low sensitivity, strong resistance and high toxicity. Our previous work showed that Mycoplasma hyorhinis (M. hyorhinis) infection was mediated by the interaction between p37 of M. hyorhinis and Annexin A2 (ANXA2) of host cells, however the translational value of this mechanism was unknown. Herein, we synthesized the N-terminal of ANXA2 polypeptide (A2PP) and found that A2PP could decrease the infection of M. hyorhinis to gastric cancer cells and block M. hyorhinis infection-induced cell migration. Furthermore, we found that A2PP could reduce M. hyorhinis contamination of passage cells. Moreover, compared with the commercial antibiotics commonly used in cell culture to prevent M. hyorhinis infection, A2PP demonstrated a more effectiveness but a low toxicity on cell growth. Thus, our study for the first time revealed A2PP’s potential for the treatment and prevention of M. hyorhinis infection.

Role of calcium in membrane interactions by PI(4,5)P₂-binding proteins

Ca²⁺ and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] are key agents in membrane-associated signalling events. Their temporal and spatial regulation is crucial for activation or recruitment of proteins in the plasma membrane. In fact, the interaction of several signalling proteins with PI(4,5)P₂ has been shown to be tightly regulated and dependent on the presence of Ca²⁺, with co-operative binding in some cases. In these proteins, PI(4,5)P₂ and Ca²⁺ binding typically occurs at different binding sites. In addition, several PI(4,5)P₂-binding proteins are known targets of calmodulin (CaM), which, depending on the presence of calcium, can compete with PI(4,5)P₂ for protein interaction, translating Ca²⁺ transient microdomains into variations of PI(4,5)P₂ lateral organization in time and space. The present review highlights different examples of calcium-dependent PI(4,5)P₂-binding proteins and discusses the possible impact of this dual regulation on fine-tuning of protein activity by triggering target membrane binding in the presence of subtle changes in the levels of calcium or PI(4,5)P₂.

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.

Structure of a C-terminal AHNAK peptide in a 1:2:2 complex with S100A10 and an acetylated N-terminal peptide of annexin A2

AHNAK, a large 629 kDa protein, has been implicated in membrane repair, and the annexin A2-S100A10 heterotetramer [(p11)(2)(AnxA2)(2))] has high affinity for several regions of its 1002-amino-acid C-terminal domain. (p11)(2)(AnxA2)(2) is often localized near the plasma membrane, and this C2-symmetric platform is proposed to be involved in the bridging of membrane vesicles and trafficking of proteins to the plasma membrane. All three proteins co-localize at the intracellular face of the plasma membrane in a Ca(2+)-dependent manner. The binding of AHNAK to (p11)(2)(AnxA2)(2) has been studied previously, and a minimal binding motif has been mapped to a 20-amino-acid peptide corresponding to residues 5654-5673 of the AHNAK C-terminal domain. Here, the 2.5 Å resolution crystal structure of this 20-amino-acid peptide of AHNAK bound to the AnxA2-S100A10 heterotetramer (1:2:2 symmetry) is presented, which confirms the asymmetric arrangement first described by Rezvanpour and coworkers and explains why the binding motif has high affinity for (p11)(2)(AnxA2)(2). Binding of AHNAK to the surface of (p11)(2)(AnxA2)(2) is governed by several hydrophobic interactions between side chains of AHNAK and pockets on S100A10. The pockets are large enough to accommodate a variety of hydrophobic side chains, allowing the consensus sequence to be more general. Additionally, the various hydrogen bonds formed between the AHNAK peptide and (p11)(2)(AnxA2)(2) most often involve backbone atoms of AHNAK; as a result, the side chains, particularly those that point away from S100A10/AnxA2 towards the solvent, are largely interchangeable. While the structure-based consensus sequence allows interactions with various stretches of the AHNAK C-terminal domain, comparison with other S100 structures reveals that the sequence has been optimized for binding to S100A10. This model adds new insight to the understanding of the specific interactions that occur in this membrane-repair scaffold.

Genotoxic agents promote the nuclear accumulation of annexin A2: role of annexin A2 in mitigating DNA damage

Annexin A2 is an abundant cellular protein that is mainly localized in the cytoplasm and plasma membrane, however a small population has been found in the nucleus, suggesting a nuclear function for the protein. Annexin A2 possesses a nuclear export sequence (NES) and inhibition of the NES is sufficient to cause nuclear accumulation. Here we show that annexin A2 accumulates in the nucleus in response to genotoxic agents including gamma-radiation, UV radiation, etoposide and chromium VI and that this event is mediated by the nuclear export sequence of annexin A2. Nuclear accumulation of annexin A2 is blocked by the antioxidant agent N-acetyl cysteine (NAC) and stimulated by hydrogen peroxide (H₂O₂), suggesting that this is a reactive oxygen species dependent event. In response to genotoxic agents, cells depleted of annexin A2 show enhanced phospho-histone H2AX and p53 levels, increased numbers of p53-binding protein 1 nuclear foci and increased levels of nuclear 8-oxo-2′-deoxyguanine, suggesting that annexin A2 plays a role in protecting DNA from damage. This is the first report showing the nuclear translocation of annexin A2 in response to genotoxic agents and its role in mitigating DNA damage.

The S100A10 subunit of the annexin A2 heterotetramer facilitates L2-mediated human papillomavirus infection

Mucosotropic, high-risk human papillomaviruses (HPV) are sexually transmitted viruses that are causally associated with the development of cervical cancer. The most common high-risk genotype, HPV16, is an obligatory intracellular virus that must gain entry into host epithelial cells and deliver its double stranded DNA to the nucleus. HPV capsid proteins play a vital role in these steps. Despite the critical nature of these capsid protein-host cell interactions, the precise cellular components necessary for HPV16 infection of epithelial cells remains unknown. Several neutralizing epitopes have been identified for the HPV16 L2 minor capsid protein that can inhibit infection after initial attachment of the virus to the cell surface, which suggests an L2-specific secondary receptor or cofactor is required for infection, but so far no specific L2-receptor has been identified. Here, we demonstrate that the annexin A2 heterotetramer (A2t) contributes to HPV16 infection and co-immunoprecipitates with HPV16 particles on the surface of epithelial cells in an L2-dependent manner. Inhibiting A2t with an endogenous annexin A2 ligand, secretory leukocyte protease inhibitor (SLPI), or with an annexin A2 antibody significantly reduces HPV16 infection. With electron paramagnetic resonance, we demonstrate that a previously identified neutralizing epitope of L2 (aa 108-120) specifically interacts with the S100A10 subunit of A2t. Additionally, mutation of this L2 region significantly reduces binding to A2t and HPV16 pseudovirus infection. Furthermore, downregulation of A2t with shRNA significantly decreases capsid internalization and infection by HPV16. Taken together, these findings indicate that A2t contributes to HPV16 internalization and infection of epithelial cells and this interaction is dependent on the presence of the L2 minor capsid protein.

Annexin A2 is a natural extrahepatic inhibitor of the PCSK9-induced LDL receptor degradation

Proprotein convertase subtilisin/kexin-9 (PCSK9) enhances the degradation of hepatic low-density lipoprotein receptor (LDLR). Deletion of PCSK9, and loss-of-function mutants in humans result in lower levels of circulating LDL-cholesterol and a strong protection against coronary heart disease. Accordingly, the quest for PCSK9 inhibitors has major clinical implications. We have previously identified annexin A2 (AnxA2) as an endogenous binding partner and functional inhibitor of PCSK9. Herein, we studied the relevance of AnxA2 in PCSK9 inhibition and lipid metabolism in vivo. Plasma analyses of AnxA2(-/-) mice revealed: i) a ∼1.4-fold increase in LDL-cholesterol without significant changes in VLDLs or HDLs, and ii) a ∼2-fold increase in circulating PCSK9 levels. Western blotting and immunohistochemistry of AnxA2(-/-) tissues revealed that the LDLR was decreased by ∼50% in extrahepatic tissues, such as adrenals and colon. We also show that AnxA2-derived synthetic peptides block the PCSK9≡LDLR interaction in vitro, and adenoviral overexpression of AnxA2 in mouse liver increases LDLR protein levels in vivo. These results suggest that AnxA2 acts as an endogenous regulator of LDLR degradation, mostly in extrahepatic tissues. Finally, we identified an AnxA2 coding polymorphism, V98L, that correlates with lower circulating levels of PCSK9 thereby extending our results on the physiological role of AnxA2 in humans.

Schistosoma mansoni Annexin 2: molecular characterization and immunolocalization

We here describe the cloning and characterization of the Schistosoma mansoni Annexin 2, previously identified in the tegument by proteomic studies, and as an up-regulated gene in schistosomulum stage by microarray data. In silico analysis predicts a conserved core containing four repeat domains of Annexin (ANX) and a variable N-terminal region similar to that described for mammalian isoforms. Real-time RT-PCR and Western blot analysis determined that S. mansoni Annexin 2 is significantly up-regulated in the transition from free-living cercaria to schistosomulum and adult worm parasitic stages. Immunolocalization experiments and tegument membrane preparations confirmed Annexin 2 as a protein mainly localized in the tegument of schistosomula and adult worms. Furthermore, it binds to the tegument surface membranes in a calcium-dependent manner. These results suggest that S. mansoni Annexin 2 is closely associated to the tegument arrangement, being a potential target for immune intervention.

Interferon-gamma-dependent tyrosine phosphorylation of MEKK4 via Pyk2 is regulated by annexin II and SHP2 in keratinocytes

IFNgamma (interferon-gamma) binding to its cognate receptor results, through JAK (Janus kinase), in direct activation of receptor-bound STAT1 (signal transducer and activator of transcription 1), although there is evidence for additional activation of a MAPK (mitogen-activated protein kinase) pathway. In the present paper, we report IFNgamma-dependent activation of the MEKK4 (MAPK/extracellular-signal-regulated kinase kinase kinase 4) pathway in HaCaT human keratinocytes. MEKK4 is tyrosine-phosphorylated and the IFNgamma-dependent phosphorylation requires intracellular calcium. Calcium-dependent phosphorylation of MEKK4 is mediated by Pyk2. Moreover, MEKK4 and Pyk2 co-localize in an IFNgamma-dependent manner in the perinuclear region. Furthermore, the calcium-binding protein, annexin II, and the calcium-regulated kinase, Pyk2, co-immunoprecipitate with MEKK4 after treatment with IFNgamma. Immunofluorescence imaging of HaCaT cells shows an IFNgamma-dependent co-localization of annexin II with Pyk2 in the perinuclear region, suggesting that annexin II mediates the calcium-dependent regulation of Pyk2. Tyrosine phosphorylation of MEKK4 correlates with its activity to phosphorylate MKK6 (MAPK kinase 6) in vitro and subsequent p38 MAPK activation in an IFNgamma-dependent manner. Additional studies demonstrate that the SH2 (Src homology 2)-domain-containing tyrosine phosphatase SHP2 co-immunoprecipitates with MEKK4 in an IFNgamma-dependent manner and co-localizes with MEKK4 after IFNgamma stimulation in the perinuclear region in HaCaT cells. Furthermore, we provide evidence that SHP2 dephosphorylates MEKK4 and Pyk2, terminating the MEKK4-dependent branch of the IFNgamma signalling pathway.

Annexin A2/P11 interaction: new insights into annexin A2 tetramer structure by chemical crosslinking, high-resolution mass spectrometry, and computational modeling

During the past few years, the structural analysis of proteins and protein complexes by chemical crosslinking and mass spectrometry has enjoyed increasing popularity. With this approach we have investigated the quaternary structure of the complex between annexin A2 and p11, which is involved in numerous cellular processes. Although high-resolution data are available for both interaction partners as well as for the complex between two p11 subunits and two annexin A2 N-terminal peptides, the structure of the complete annexin A2/p11 heterotetramer has not yet been solved at high resolution. Thus, the quaternary structure of the biologically relevant, membrane-bound annexin A2/p11 complex is still under discussion, while the existence of a heterotetramer or a heterooctamer is the prevailing opinion. We gained further insight into the spatial organization of the annexin A2/p11 heterotetramer by employing chemical crosslinking combined with high-resolution mass spectrometry. Furthermore, tandem mass spectrometry served as a tool for an exact localization of crosslinked amino acid residues and for a confirmation of crosslinked product assignment. On the basis of distance constraints from the crosslinking data we derived structural models of the annexin A2/p11 heterotetramer by computational docking with Rosetta. We propose an octameric model for the annexin A2/p11 complex, which exerts annexin A2 function. The proposed structure of the annexin A2/p11 octamer differs from so far suggested models and sheds new light into annexin A2/p11 interaction.

Annexins sense changes in intracellular pH during hypoxia

The pH(i) (intracellular pH) is an important physiological parameter which is altered during hypoxia and ischaemia, pathological conditions accompanied by a dramatic decrease in pH(i). Sensors of pH(i) include ion transport systems which control intracellular Ca2+ gradients and link changes in pH(i) to functions as diverse as proliferation and apoptosis. The annexins are a protein family characterized by Ca2+-dependent interactions with cellular membranes. Additionally, in vitro evidence points to the existence of pH-dependent, Ca(2+)-independent membrane association of several annexins. We show that hypoxia promotes the interaction of the recombinant annexin A2-S100A10 (p11) and annexin A6 with the plasma membrane. We have investigated in vivo the influence of the pH(i) on the membrane association of human annexins A1, A2, A4, A5 and A6 tagged with fluorescent proteins, and characterized this interaction for endogenous annexins present in smooth muscle and HEK (human embryonic kidney)-293 cells biochemically and by immunofluorescence microscopy. Our results show that annexin A6 and the heterotetramer A2-S100A10 (but not annexins A1, A4 and A5) interact independently of Ca2+ with the plasma membrane at pH 6.2 and 6.6. The dimerization of annexin A2 within the annexin A2-S100A10 complex is essential for the pH-dependent membrane interaction at this pH range. The pH-induced membrane binding of annexins A6 and A2-S100A10 might have consequences for their functions as membrane organizers and channel modulators.

Comprehensive interaction of dicalcin with annexins in frog olfactory and respiratory cilia

Dicalcin (renamed from p26olf) is a dimer form of S100 proteins found in frog olfactory epithelium. S100 proteins form a group of EF-hand Ca(2+)-binding proteins, and are known to interact with many kinds of target protein to modify their activities. To determine the role of dicalcin in the olfactory epithelium, we identified its binding proteins. Several proteins in frog olfactory epithelium were found to bind to dicalcin in a Ca(2+)-dependent manner. Among them, 38 kDa and 35 kDa proteins were most abundant. Our analysis showed that these were a mixture of annexin A1, annexin A2 and annexin A5. Immunohistochemical analysis showed that dicalcin and all of these three subtypes of annexin colocalize in the olfactory cilia. Dicalcin was found to be present in a quantity almost sufficient to bind all of these annexins. Colocalization of dicalcin and the three subtypes of annexin was also observed in the frog respiratory cilia. Dicalcin facilitated Ca(2+)-dependent liposome aggregation caused by annexin A1 or annexin A2, and this facilitation was additive when both annexin A1 and annexin A2 were present. In this facilitation effect, the effective Ca(2+) concentrations were different between annexin A1 and annexin A2, and therefore the dicalcin-annexin system in frog olfactory and respiratory cilia can cover a wide range of Ca(2+) concentrations. These results suggested that this system is associated with abnormal increases in the Ca(2+) concentration in the olfactory and other motile cilia.

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.

The mRNA-binding site of annexin A2 resides in helices C-D of its domain IV

Annexin A2 (AnxA2) is a Ca(2+)-binding and phospholipid-binding protein involved in different intracellular processes including exocytosis, endocytosis and membrane-cytoskeleton movements. We have previously identified AnxA2 as an mRNA-binding protein present in cytoskeleton-bound polysomes, that binds to a specific approximately 100 nucleotide region in the 3'-untranslated region of c-myc and its cognate mRNA. In the present study, we show by UV cross-linking assays and surface plasmon resonance analyses that the mRNA-binding site of AnxA2 resides in its domain IV. Furthermore, the interaction of full-length AnxA2 with the 3'-untranslated region of anxA2 mRNA is Ca(2+)-dependent. By contrast, the interaction is Ca(2+)-independent for the isolated domain IV of AnxA2, suggesting that the mRNA-binding site is masked in Apo-AnxA2 and gains exposure through a Ca(2+)-induced conformational change of AnxA2 generating a favourable mRNA-binding site. The AnxA2-mRNA interaction is specific and involves helices C and D in domain IV of AnxA2, since point mutagenesis of several charged and polar exposed residues of these helices in the full-length protein strongly reduce RNA binding. The interaction appears to be sequential involving an initial phase of recognition dominated by electrostatic interactions, most likely between lysine residues and the phosphate backbone of RNA, followed by a second phase contributing to the specificity of the interaction.

Insights into S100 target specificity examined by a new interaction between S100A11 and annexin A2

S100 proteins are a group of EF-hand calcium-signaling proteins, many of which interact with members of the calcium- and phospholipid-binding annexin family of proteins. This calcium-sensitive interaction enables two neighboring membrane surfaces, complexed to different annexin proteins, to be brought into close proximity for membrane reorganization, using the S100 protein as a bridging molecule. S100A11 and S100A10 are two members of the S100 family found to interact with the N-termini of annexins A1 and A2, respectively. Despite the high degree of structural similarity between these two complexes and the sequences of the peptides, earlier studies have shown that there is little or no cross-reactivity between these two S100s and the annexin peptides. In the current work the specificity and the affinity of the interaction of the N-terminal sequences of annexins A1 and A2 with Ca2+-S100A11 were investigated. Through the use of alanine-scanning peptide array experiments and NMR spectroscopy, an approximate 5-fold tighter interaction was identified between Ca2+-S100A11 and annexin A2 (approximately 3 microM) compared to annexin A1 (approximately 15 microM). Chemical shift mapping revealed that the binding site for annexin A2 on S100A11 was similar to that observed for the annexin A1 but with distinct differences involving the C-terminus of the annexin A2 peptide. In addition, kinetic measurements based on NMR titration data showed that annexin A2 binding to Ca2+-S100A11 occurs at a comparable rate (approximately 120 s(-1)) to that observed for membrane fusion processes such as endo- and exocytosis.

Purification of recombinant annexins without the use of phospholipids

Due to their involvement in a variety of physiological and pathological processes, different isoforms of annexins are being utilized as markers of some human diseases and bio-imaging of tissue injury (due to apoptosis), and have been proposed as drug delivery vehicles. These, in addition to extensive biophysical studies on the role of annexins in organizing lipid domains in biological membranes, have necessitated development of an efficient protocol for producing annexins in bulk quantities. In this paper, we report a one-step purification protocol for annexin a5 without using lipid vesicles or involving any column chromatographic step. Depending on the growth and expression condition, a fraction of recombinant annexin a5 (cloned in pET3d vector) was sequestered into inclusion bodies. When these inclusion bodies were dissolved in 6 M urea, subjected to a 10-fold snap dilution in the presence of 5 mM Ca(2+) and stored overnight at 4 degrees C, annexin a5 was precipitated as a homogenous protein as judged by SDS-PAGE. This one-step purification protocol produced about 35 mg of highly purified annexin a5 per liter of bacterial culture. The annexin a5 purified from inclusion bodies exhibited similar properties to that obtained from the soluble fraction using the conventional lipid-partitioning approach. Our purification protocol for annexin a5 elaborated herein is equally effective for purification of annexin A2, and we believe, will serve as general protocol for purifying other annexins in bulk quantities for diagnostic as well as detailed biophysical studies.

Crystallographic analysis of calcium-dependent heparin binding to annexin A2

Annexin A2 and heparin bind to one another with high affinity and in a calcium-dependent manner, an interaction that may play a role in mediating fibrinolysis. In this study, three heparin-derived oligosaccharides of different lengths were co-crystallized with annexin A2 to elucidate the structural basis of the interaction. Crystal structures were obtained at high resolution for uncomplexed annexin A2 and three complexes of heparin oligosaccharides bound to annexin A2. The common heparin-binding site is situated at the convex face of domain IV of annexin A2. At this site, annexin A2 binds up to five sugar residues from the nonreducing end of the oligosaccharide. Unlike most heparin-binding consensus patterns, heparin binding at this site does not rely on arrays of basic residues; instead, main-chain and side-chain nitrogen atoms and two calcium ions play important roles in the binding. Especially significant is a novel calcium-binding site that forms upon heparin binding. Two sugar residues of the heparin derivatives provide oxygen ligands for this calcium ion. Comparison of all four structures shows that heparin binding does not elicit a significant conformational change in annexin A2. Finally, surface plasmon resonance measurements were made for binding interactions between annexin A2 and heparin polysaccharide in solution at pH 7.4 or 5.0. The combined data provide a clear basis for the calcium dependence of heparin binding to annexin A2.

Cloning and Characterization of the Annexin II Receptor on Human Marrow Stromal Cells

Annexin II is a heterotetramer, consisting of two 11-kDa (p11) and two 36-kDa (p36) subunits, that is produced by osteoclasts and stimulates osteoclast formation. However, its receptor is unknown. We showed that annexin II binds to normal primary human marrow stromal cells and the Paget's marrow-derived PSV10 stromal cell line to induce osteoclast formation. 125I-Labeled annexin II binding assays with PSV10 cells demonstrated that there was a single class of annexin II receptors with a Kd of 5.79 nm and Bmax of 2.13 x 10(5) receptors/cell. Annexin III or annexin V did not bind this receptor. Using 125I-labeled annexin II binding to screen NIH3T3 transfected with a human marrow cDNA expression library, we identified a putative annexin II receptor clone, which encoded a novel 26-kDa type I membrane receptor protein when expressed in HEK 293 cells. HEK 293 cells transformed with the cloned annexin II receptor cDNA showed a similar binding affinity to annexin II as that observed in PSV10 cells. Chemical cross-linking experiments with biotinylated annexin II and intact PSV10 cells identified a 55-kDa band on Western blot analysis that reacted with both an anti-p11 antibody and streptavidin but not anti-p36 antibody. A rabbit polyclonal antibody raised against the putative recombinant annexin II receptor also recognized the same 26-kDa protein band detected in PSV10 cells. Importantly, the annexin II receptor antibody dose-dependently blocked the stimulatory effects of annexin II on human osteoclast formation, demonstrating that the receptor mediates the effects of annexin II on osteoclast formation.

Engineering, Biophysical Characterisation and Binding Properties of a Soluble Mutant form of Annexin A2 Domain IV that Adopts a Partially Folded Conformation

The four approximately 75-residue domains (repeats) that constitute the annexin core structure all possess an identical five-alpha-helix bundle topology, but the physico-chemical properties of the isolated domains are different. Domain IV of the annexins has previously been expressed only as inclusion bodies, resistant to solubilisation. Analysis of the conserved, exposed hydrophobic residues of the four annexin domains reveals that domain IV contains the largest number of hydrophobic residues involved in interfacial contacts with the other domains. We designed five constructs of domain IV of annexin A2 in which several interfacial hydrophobic residues were substituted by hydrophilic residues. The mutant domain, in which all fully exposed hydrophobic interfacial residues were substituted, was isolated as a soluble protein. Circular dichroism measurements indicate that it harbours a high content of alpha-helical secondary structure and some tertiary structure. The CD-monitored (lambda=222 nm) thermal melting profile suggests a weak cooperative transition. Nuclear magnetic resonance (1H-15N) correlation spectroscopy reveals heterogeneous line broadening and an intermediate spectral dispersion. These properties are indicative of a partially folded protein in which some residues are in a fairly structured conformation, whereas others are in an unfolded state. This conclusion is corroborated by 1-anilinonaphthalene-8-sulfonate fluorescence (ANS) analyses. Surface plasmon resonance measurements also indicate that this domain binds heparin, a known ligand of domain IV in the full-length annexin A2, although with lower affinity.

Identification of an AHNAK binding motif specific for the Annexin2/S100A10 tetramer

The Annexin2 tetramer (A2t), which consists of two Annexin2 molecules bound to a S100A10 dimer, is implicated in membrane-trafficking events. Here, we showed using a yeast triple-hybrid experiment and in vitro binding assay that Annexin2 is required for strong binding of S100A10 to the C-terminal domain of the protein Ahnak. We also revealed that this effect involves only the Annexin2 N-terminal tail, which is implicated in S100A10/Annexin2 tetramerization. The minimal A2t binding motif (A2tBP1) in Ahnak was mapped to a 20-amino acid peptide, and this peptide is highly specific for A2t. We also identified a second A2t binding motif (A2tBP2) present in the N-terminal domain of Ahnak, which binds to A2t, albeit with less affinity. When overexpressed as an EGFP fusion protein in MDCK cells, A2tBPs cofractionate in a calcium-dependent manner and co-immunoprecipitate with S100A10 and Annexin2. In living cells, A2tBPs target EGFP to the cytoplasm as does Annexin2. In response to oxidative and mechanical stress, EGFP-A2tBPs relocalize within minutes to the plasma membrane; a behavior shared with Annexin2-GFP. These results suggest that the A2t complex exists within the cytoplasm of resting living cells and that its localization at the plasma membrane relies on cellular signaling. Together, our data demonstrate that A2tBP1 is a specific A2t complex binding domain and may be a powerful tool to help elucidate A2t structure and cellular functions.

Regulation of ecto-5'-nucleotidase activity via Ca2+-dependent, annexin 2-mediated membrane rearrangement?

The spatial segregation of the plasma membrane plays a prominent role in distinguishing and sorting a large number of signals a cell receives simultaneously. The plasma membrane comprises regions known as lipid rafts, which serve as signal-transduction hubs and platforms for sorting membrane-associated proteins. Ca(2+)-binding proteins of the annexin family have been ascribed a role in the regulation of raft dynamics. Glycosylphosphatidylinositol-anchored 5'-nucleotidase is an extracellular, raft-associated enzyme responsible for conversion of extracellular ATP into adenosine. Our results point to a regulation of ecto-5'-nucleotidase activity by Ca(2+)-dependent, annexin-mediated stabilization of membrane rafts.

Phosphatidylserine membrane domain clustering induced by annexin A2/S100A10 heterotetramer

By means of scanning force and fluorescence microscopy of artificial membranes immobilized on mica surfaces, the lateral organization of the annexin A2/S100A10 heterotetramer (annexin A2t) and its influence on the lateral organization of the lipids within the membrane have been elucidated. Planar lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) were prepared on atomically flat mica surfaces by the spreading of unilamellar vesicles. Fluorescence images of fluorescently labeled annexin A2t and scanning force microscopy images of nonlabeled protein bound to POPC/POPS bilayers show the formation of micrometer-sized lateral protein domains in the presence of 1 mM CaCl2. By means of scanning force microscopy, not only protein domains became discernible but also small membrane domains, which were attributed to POPS-enriched areas. A depletion of these POPS domains was observed in the vicinity of annexin A2t protein domains. These results indicate that annexin A2t is a peripheral membrane-binding complex capable of inducing lipid segregation.

The Retention Factor p11 Confers an Endoplasmic Reticulum-Localization Signal to the Potassium Channel TASK-1

The interaction of the adaptor protein p11, also denoted S100A10, with the C-terminus of the two-pore-domain K+ channel TASK-1 was studied using yeast two-hybrid analysis, glutathione S-transferase pull-down, and co-immunoprecipitation. We found that p11 interacts with a 40 amino-acid region in the proximal C-terminus of the channel. In heterologous expression systems, deletion of the p11-interacting domain enhanced surface expression of TASK-1. Attachment of the p11-interacting domain to the cytosolic tail of the reporter protein CD8 caused retention/retrieval of the construct in the endoplasmic reticulum (ER). Attachment of the last 36 amino acids of p11 to CD8 also caused ER localization, which was abolished by removal or mutation of a putative retention motif (H/K)xKxxx, at the C-terminal end of p11. Imaging of EGFP-tagged TASK-1 channels in COS cells suggested that wild-type TASK-1 was largely retained in the ER. Knockdown of p11 with siRNA enhanced trafficking of TASK-1 to the surface membrane. Our results suggest that binding of p11 to TASK-1 retards the surface expression of the channel, most likely by virtue of a di-lysine retention signal at the C-terminus of p11. Thus, the cytosolic protein p11 may represent a 'retention factor' that causes localization of the channel to the ER.

Phosphoinositide specificity of and mechanism of lipid domain formation by annexin A2-p11 heterotetramer

Annexin A2 is a phospholipid-binding protein that forms a heterotetramer (annexin II-p11 heterotetramer; A2t) with p11 (S100A10). It has been reported that annexin A2 is involved in binding to phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and in inducing membrane microdomain formation. To understand the mechanisms underlying these findings, we determined the membrane binding properties of annexin A2 wild type and mutants both as monomer and as A2t. Our results from surface plasmon resonance analysis showed that A2t and annexin A2 has modest selectivity for PtdIns(4,5)P2 over other phosphoinositides, which is conferred by conserved basic residues, including Lys279 and Lys281, on the convex surface of annexin A2. Fluorescence microscopy measurements using giant unilamellar vesicles showed that A2t of wild type, but not (K279A)2-(p11)2 or (K281A)2-(p11)2, specifically induced the formation of 1-microm-sized PtdIns(4,5)P2 clusters, which were stabilized by cholesterol. Collectively, these studies elucidate the structural determinant of the PtdIns(4,5)P2 selectivity of A2t and suggest that A2t may be involved in the regulation of PtdIns(4,5)P2 clustering in the cell.

Detection and Identification of Protein Interactions of S100 Proteins by ProteinChip Technology

The aim of this work was to establish an approach for identification of protein interactions. This assay used an anti-S100A8 antibody coupled on beads and incubated with cell extract. The bead eluates were analyzed using ProteinChip technology and subsequently subjected to an appropriate digestion. Molecular masses of digestion fragments were determined by SELDI-MS, and database analysis revealed S100A10 as interacting protein. This result was confirmed by co-immunoprecipitation and immunocapturing. Using S100A10 as new bait, a specific interaction with S100A7 was detectable.

Molecular composition of drusen and possible involvement of anti-retinal autoimmunity in two different forms of macular degeneration in cynomolgus monkey (Macaca fascicularis)

We have previously reported a cynomolgus monkey (Macaca fascicularis) pedigree with early onset macular degeneration that develops drusen at 2 yr after birth. In this study, the molecular composition of drusen in monkeys affected with late onset and early onset macular degeneration was both characterized. Involvement of anti-retinalautoimmunity in the deposition of drusen and the pathogenesis of the disease was also evaluated. Funduscopic and histological examinations were performed on 278 adult monkeys (mean age=16.94 yr) for late onset macular degeneration. The molecular composition of drusen was analyzed by immunohistochemistry and/or direct proteome analysis using liquid chromatography tandem mass spectroscopy (LC-MS/MS). Anti-retinal autoantibodies in sera were screened in 20 affected and 10 age-matched control monkeys by Western blot techniques. Immunogenic molecules were identified by 2D electrophoresis and LC-MS/MS. Relative antibody titer against each antigen was determined by ELISA in sera from 42 affected (late onset) and 41 normal monkeys. Yellowish-white spots in the macular region were observed in 90 (32%) of the late onset monkeys that were examined. Histological examination demonstrated that drusen or degenerative retinal pigment epithelium (RPE) cells were associated with the pigmentary abnormalities. Drusen in both late and early onset monkeys showed immunoreactivities for apolipoprotein E, amyloid P component, complement component C5, the terminal C5b-9 complement complex, vitronectin, and membrane cofactor protein. LC-MS/MS analyses identified 60 proteins as constituents of drusen, including a number of common components in drusen of human age-related macular degeneration (AMD), such as annexins, crystallins, immunoglobulins, and complement components. Half of the affected monkeys had single or multiple autoantibodies against 38, 40, 50, and 60 kDa retinal proteins. The reacting antigens of 38 and 40 kDa were identified as annexin II and mu-crystallin, respectively. Relative antibody titer against annexin II in affected monkeys was significantly higher than control animals (P<0.01). Significant difference was not observed in antibody titer against mu-crystallin; however, several affected monkeys showed considerably elevated titer (360-610%) compared with the mean for unaffected animals. Monkey drusen both in late and early onset forms of macular degeneration had common components with drusen in human AMD patients, indicating that chronic inflammation mediated by complement activation might also be involved in the formation of drusen in these affected monkeys. The high prevalence of anti-retinalautoantibodies in sera from affected monkeys demonstrated an autoimmune aspect of the pathogenesis of the disease. Although further analyses are required to determine whether and how autoantibodies against annexin II or mu-crystallin relate to the pathogenesis of the disease, it could be hypothesized that immune responses directed against these antigens might trigger chronic activation of the complement cascade at the site of drusen formation.

The conserved core domains of annexins A1, A2, A5, and B12 can be divided into two groups with different Ca2+-dependent membrane-binding properties

The hallmark of the annexin super family of proteins is Ca(2+)-dependent binding to phospholipid bilayers, a property that resides in the conserved core domain of these proteins. Despite the structural similarity between the core domains, studies reported herein showed that annexins A1, A2, A5, and B12 could be divided into two groups with distinctively different Ca(2+)-dependent membrane-binding properties. The division correlates with the ability of the annexins to form Ca(2+)-dependent membrane-bound trimers. Site-directed spin-labeling and Forster resonance energy transfer experimental approaches confirmed the well-known ability of annexins A5 and B12 to form trimers, but neither method detected self-association of annexin A1 or A2 on bilayers. Studies of chimeras in which the N-terminal and core domains of annexins A2 and A5 were swapped showed that trimer formation was mediated by the core domain. The trimer-forming annexin A5 and B12 group had the following Ca(2+)-dependent membrane-binding properties: (1) high Ca(2+) stoichiometry for membrane binding ( approximately 12 mol of Ca(2+)/mol of protein); (2) binding to membranes was very exothermic (> -60 kcal/ mol of protein); and (3) binding to bilayers that were in the liquid-crystal phase but not to bilayers in the gel phase. In contrast, the nontrimer-forming annexin A1 and A2 group had the following Ca(2+)-dependent membrane-binding properties: (1) lower Ca(2+) stoichiometry for membrane binding (<or=4 mol of Ca(2+)/mol of protein); (2) binding to membranes was relatively less exothermic (< -33 kcal/ mol of protein); and (3) binding to bilayers that were in either the liquid-crystal phase or gel phase. The biological implications of this subdivision are discussed.

Annexin II is a novel receptor for Pseudomonas aeruginosa

Infections with Pseudomonas aeruginosa (P. aeruginosa) are critical in ventilated and poly-traumatized patients. Most important, these bacteria cause frequent and chronic pulmonary infections in patients with cystic fibrosis. Therefore, identification of molecular mechanisms that mediate the infection of mammalian cells with P. aeruginosa is urgently required. Here, we aimed to identify novel receptors that are involved in internalization of P. aeruginosa into mammalian epithelial cells. Employing SDS-PAGE purification and mass spectrometry we demonstrate that annexin II specifically binds to P. aeruginosa. The significance of the interaction of annexin II with P. aeruginosa for the infection of mammalian cells is indicated by the finding that neutralization of the ligands on P. aeruginosa by incubation of the bacteria with recombinant, soluble annexin II prevents internalization of P. aeruginosa into human epithelial cells.

S100A10, annexin A2, and annexin a2 heterotetramer as candidate plasminogen receptors

The defining characteristic of a tumor cell is its ability to escape the constraints imposed by neighboring cells, invade the surrounding tissue and metastasize to distant sites. This invasive property of tumor cells is dependent on activation of proteinases at the cell surface. The serine proteinase plasmin is one of the key proteinases that participate in the pericellular proteolysis associated with the invasive program of tumor cells. The assembly of plasminogen and tissue plasminogen activator at the endothelial cell surface or on the fibrin clot provides a focal point for plasmin generation and therefore plays an important role in maintaining blood fluidity and promoting fibrinolysis. S100A10, a member of the S100 family of Ca2+-binding proteins, is a dimeric protein composed of two 11 kDa subunits. Typically, S100A10 is found in most cells bound to its annexin A2 ligand as the heterotetrameric (S100A10)2(annexin A2)2 complex, AIIt. In addition to an intracellular distribution, S100A10 is present on the extracellular surface of many cells. The carboxyl-terminal lysines of S100A10 bind tPA and plasminogen resulting in the stimulation of tPA-dependent plasmin production. Carboxypeptidases cleave the carboxyl-terminal lysines of S100A10, resulting in a loss of binding and activity. Plasmin binds to S100A10 at a distinct site and the formation of the S100A10-plasmin complex stimulates plasmin autoproteolysis thereby providing a highly localized transient pulse of plasmin activity at the cell surface. The binding of tPA and plasmin to S100A10 also protects against inhibition by physiological inhibitors, PAI-1 and alpha2-antiplasmin, respectively. S100A10 also colocalizes plasminogen with the uPA-uPAR complex thereby localizing and stimulating uPA-dependent plasmin formation to the surface of cancer cells. The loss of S100A10 from the extracellular surface of cancer cells results in a significant loss in plasmin generation. In addition, S100A10 knock-down cells demonstrate a dramatic loss in extracellular matrix degradation and invasiveness as well as reduced metastasis. Annexin A2 plays an important role in plasminogen regulation by controlling the levels of extracellular S100A10 and by acting as a plasmin reductase. The mechanism by which annexin A2 regulates the extracellular levels of S100A10 is unknown. This review highlights the important part that S100A10 plays in plasmin regulation and the role this protein plays in cancer cell invasiveness and metastasis.

Novel role for Na,K-ATPase in phosphatidylinositol 3-kinase signaling and suppression of cell motility

The Na,K-ATPase, consisting of alpha- and beta-subunits, regulates intracellular ion homeostasis. Recent studies have demonstrated that Na,K-ATPase also regulates epithelial cell tight junction structure and functions. Consistent with an important role in the regulation of epithelial cell structure, both Na,K-ATPase enzyme activity and subunit levels are altered in carcinoma. Previously, we have shown that repletion of Na,K-ATPase beta1-subunit (Na,K-beta) in highly motile Moloney sarcoma virus-transformed Madin-Darby canine kidney (MSV-MDCK) cells suppressed their motility. However, until now, the mechanism by which Na,K-beta reduces cell motility remained elusive. Here, we demonstrate that Na,K-beta localizes to lamellipodia and suppresses cell motility by a novel signaling mechanism involving a cross-talk between Na,K-ATPase alpha1-subunit (Na,K-alpha) and Na,K-beta with proteins involved in phosphatidylinositol 3-kinase (PI3-kinase) signaling pathway. We show that Na,K-alpha associates with the regulatory subunit of PI3-kinase and Na,K-beta binds to annexin II. These molecular interactions locally activate PI3-kinase at the lamellipodia and suppress cell motility in MSV-MDCK cells, independent of Na,K-ATPase ion transport activity. Thus, these results demonstrate a new role for Na,K-ATPase in regulating carcinoma cell motility.

Structural elucidation of the protein- and membrane-binding properties of the N-terminal tail domain of human annexin II

The conformational preferences and the solution structure of AnxII(N31), a peptide corresponding to the full-length sequence (residues 1-31) of the human annexin II N-terminal tail domain, were investigated by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy. CD results showed that AnxII(N31) adopts a mainly alpha-helical conformation in hydrophobic or membrane-mimetic environments, while a predominantly random structure is adopted in aqueous buffer. In contrast to previous results of the annexin I N-terminal domain peptide [Yoon et al. (2000) FEBS Lett. 484, 241-245], calcium ions showed no effect on the structure of AnxII(N31). The NMR-derived structure of AnxII(N31) in 50% TFE/water mixture showed a horseshoe-like fold comprising the N-terminal amphipathic alpha-helix, the following loop, and the C-terminal helical region. Together, the results establish the first detailed structural data on the N-terminal tail domain of annexin II, and suggest the possibility of the domain to undergo Ca(2+)-independent membrane-binding.

Annexin A2 heterotetramer: role in tight junction assembly

The tight junction has been characterized as a domain of focal fusions of the exoplasmic leaflets of the lipid bilayers from adjacent epithelial cells. Approximating membranes to within fusion distance is a thermodynamically unfavorable process and requires the participation of membrane-bridging or -fusion proteins. No known tight junction protein exhibits such activities. Annexin A2 (A2), in particular its heterotetramer (A2t), is known to form junctions between lipid bilayer structures through molecular bridging of their external leaflets. We demonstrate abundant A2 expression in Madin-Darby canine kidney II monolayers by two-dimensional gel electrophoresis. Confocal immunofluorescence microscopic analysis suggests the bulk of A2 is located along the apical and lateral plasma membrane in its tetrameric configuration, consisting of two A2 and two p11 (an 11-kDa calmodulin-related protein, S100A10) subunits. Immunocytochemistry and ultrastructural immunogold labeling demonstrate colocalization of the A2 subunit with bona fide tight junction proteins, zonula occludens-1, occludin, and claudin-1, at cell-cell contacts. The extracellular addition of a synthetic peptide, targeted to disrupt the binding between A2 and p11, completely aborts tight junction assembly in calcium chelation studies. We propose A2t as a member of a new class of tight junction proteins responsible for the long-observed convergence of adjacent exoplasmic lipid leaflets in tight junction assembly.

Characterization of a noncovalent lipocalin complex by liquid chromatography/electrospray ionization mass spectrometry

Nanoscale liquid chromatography coupled to electrospray ionization mass spectrometry was used to identify the nature of the ligand that binds noncovalently to siderocalin (lipocalin 2). The folded state siderocalin-ligand complex was separated from free, unfolded siderocalin using reversed phase chromatography, and the molecular weight of the siderocalin ligand was then determined from the deconvoluted molecular weights of the complex and of the free protein. The ligand was identified as dihydroxybenzoyl-serine, a breakdown product of enterobactin, an iron-chelating compound ("siderophore") synthesized in bacteria. These results demonstrate that, in some cases, electrostatic noncovalent protein complexes can survive the denaturing conditions of reversed phase liquid chromatography and the gas phase transfer occurring during electrospray ionization.

Specific localization of the annexin II heterotetramer in brain lipid raft fractions and its changes in spatial learning

Annexin-II (AII) is a Ca(2+)-dependent phospholipid-binding protein that is present in both intracellular and extracellular compartments. In the present study AII immunoreactivity was found in a subpopulation of neurons in specific brain regions, including the cerebral cortex and the surface of hippocampal pyramidal neurons from adult rats. AII from synaptic membranes was detected by immunoblotting as multiple species containing the monomer (AII36) and heterotetramer (AIIt). AIIt was resistant to beta-mercaptoethanol and dithiothreitol in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but was completely reduced to monomers (36 kDa) by two-dimensional electrophoresis. AIIt resided exclusively in the detergent-resistant lipid rafts concentrated in neuronal dendrites, and its recruitment to those structures was enhanced by antibody cross-link. AII abundantly distributed on the outer leaflet of neuronal membranes and between spaces of neurons appeared to be neuronal adhesive. The formation of AIIt required synthesis of sphingolipids and cholesterol, and its stability depended on Ca2+. Increases in neuronal activities such as depolarization and learning were shown to promote formation of AIIt. Our results suggest that, via a dynamic association with dendritic lipid rafts, AII may play a role in synaptic signal transduction and remodeling. This probably involves focal adhesion and interactions with actin that are associated with brain development and memory consolidation.

Identification of binding domains in the sodium channel Na(V)1.8 intracellular N-terminal region and annexin II light chain p11

The interaction of p11 (annexin II light chain) with the N-terminal domain of Na(V)1.8, a tetrodotoxin-resistant sodium channel, is essential for the functional expression of the channel. Here we show that p11 binds to Na(V)1.8 but not to sodium channel isoforms Na(V)1.2, 1.5, 1.7 or Na(V)1.9. The binding of amino acids 74-103 of Na(V)1.8 to p11 residues 33-78 occurs in a random coiled region flanked by two EF hand motifs whose crystal structure has been established. As Na(V)1.8 channel expression is associated with pain pathways, drugs that disrupt the Na(V)1.8-p11 interaction and down-regulate channel expression may have analgesic activity.

Heat shock protein 90alpha-dependent translocation of annexin II to the surface of endothelial cells modulates plasmin activity in the diabetic rat aorta

The goals of this article were (1) to identify cell surface proteins whose expression was regulated by diabetes and (2) to assess their contribution to diabetic complications. We purified heat shock protein 90alpha (Hsp90alpha) from the membrane fraction of high glucose-treated endothelial cells (ECs) as a binding partner for a diabetes-specific phage. Further investigation revealed that high glucose elevated cell surface Hsp90alpha in cultured cells, and that diabetes increased the amount of Hsp90alpha on the luminal surface of the aorta. We also found that high glucose or diabetes promoted the association of Hsp90alpha with annexin II and increased the expression of annexin II on the surface of aortic ECs. Finally, plasmin activity was increased by high glucose or diabetes, and this change was partially reversed with an annexin II antibody. These findings reveal a novel glucose-regulated interaction between Hsp90alpha and annexin II, and raise the possibility that increased expression of annexin II, which promotes the generation of plasmin, is linked to clotting abnormalities associated with the diabetic state.

Annexin II Is Required for Apical Transport in Polarized Epithelial Cells

The sorting of apical proteins comprises an initial recognition step in the trans Golgi network and a final partitioning of the apical pool of proteins into at least two different types of vesicular carriers. One criteria of these carriers is the association or non-association of the protein content with lipid rafts. We have previously characterized a population containing the raft-associated sucrase-isomaltase-carrying vesicles (SAVs) and another one, the non-raft-associated lactase-phlorizin hydrolase-carrrying vesicles (LAVs) that are targeted separately to the apical membrane. Here, we demonstrate biochemically and by employing confocal laser microscopy that the annexin II-S100A10 complex is a component of SAVs and is absent from LAVs. The unequivocal role of annexin II in the apical targeting of SI is clearly demonstrated when down-regulation of this protein by annexin II-specific small interfering RNA drastically decreases the apical delivery of SI in the epithelial cell line Madin-Darby canine kidney. The annexin II-S100A10 complex plays therefore a crucial role in routing SAVs to the apical membrane of epithelial cells.

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.

Regulation of annexin A2 by reversible glutathionylation

The annexin A2-S100A10 heterotetramer (AIIt) is a multifunctional Ca(2+)-dependent, phospholipid-binding, and F-actin-binding phosphoprotein composed of two annexin A2 subunits and two S100A10 subunits. It was reported previously that oxidative stress from exogenous hydrogen peroxide or generated in response to tumor necrosis factor-alpha results in the glutathionylation of Cys(8) of annexin A2. In this study, we demonstrate that AIIt is an oxidatively labile protein whose level of activity is regulated by the redox status of its sulfhydryl groups. Oxidation of AIIt by diamide resulted in a time- and concentration-dependent loss of the ability of AIIt to interact with phospholipid liposomes and F-actin. The inhibitory effect of diamide on the activity of AIIt was partially reversed by dithiothreitol. In addition, incubation of AIIt with diamide and GSH resulted in the glutathionylation of AIIt in vitro. Mass spectrometry established the incorporation of 2 mol of GSH/mol of annexin A2 subunit at Cys(8) and Cys(132). Glutathionylation potentiated the inhibitory effects of diamide on the activity of AIIt. Furthermore, AIIt could be deglutathionylated by glutaredoxin (thiol transferase). Thus, we show for the first time that AIIt can undergo functional reactivation by glutaredoxin, therefore establishing that AIIt is regulated by reversible glutathionylation.

Dysferlin Interacts with Annexins A1 and A2 and Mediates Sarcolemmal Wound-healing

Mutations in the dysferlin gene cause limb girdle muscular dystrophy type 2B and Miyoshi myopathy. We report here the results of expression profile analyses and in vitro investigations that point to an interaction between dysferlin and the Ca2+ and lipid-binding proteins, annexins A1 and A2, and define a role for dysferlin in Ca2+-dependent repair of sarcolemmal injury through a process of vesicle fusion. Expression profiling identified a network of genes that are co-regulated in dysferlinopathic mice. Co-immunofluorescence, co-immunoprecipitation, and fluorescence lifetime imaging microscopy revealed that dysferlin normally associates with both annexins A1 and A2 in a Ca2+ and membrane injury-dependent manner. The distribution of the annexins and the efficiency of sarcolemmal wound-healing are significantly disrupted in dysferlin-deficient muscle. We propose a model of muscle membrane healing mediated by dysferlin that is relevant to both normal and dystrophic muscle and defines the annexins as potential muscular dystrophy genes.

CEACAM1, a Cell-Cell Adhesion Molecule, Directly Associates with Annexin II in a Three-dimensional Model of Mammary Morphogenesis

The epithelial cell adhesion molecule CEACAM1 (carcinoembryonic antigen cell adhesion molecule-1) is down-regulated in colon, prostate, breast, and liver cancer. Here we show that CEACAM1-4S, a splice form with four Ig-like ectodomains and a short cytoplasmic domain (14 amino acids), directly associates with annexin II, a lipid raft-associated molecule, which is also down-regulated in many cancers. Annexin II was identified using a glutathione S-transferase pull-down assay in which the cytoplasmic domain of CEACAM-4S was fused to glutathione S-transferase, the fusion protein was incubated with cell lysates, and isolated proteins were sequenced by mass spectrometry. The interaction was confirmed first by reciprocal immunoprecipitations using anti-CEACAM1 and anti-annexin II antibodies and second by confocal laser microscopy showing co-localization of CEACAM1 with annexin II in mammary epithelial cells grown in Matrigel. In addition, CEACAM1 co-localized with p11, a component of the tetrameric AIIt complex at the plasma membrane, and with annexin II in secretory vesicles. Immobilized, oriented peptides from the cytoplasmic domain of CEACAM1-4S were shown to directly associate with bovine AIIt, which is 98% homologous to human AIIt, with average KD values of about 30 nM using surface plasmon resonance, demonstrating direct binding of functionally relevant AIIt to the cytoplasmic domain of CEACAM1-4S.

Fusion of lamellar body with plasma membrane is driven by the dual action of annexin II tetramer and arachidonic acid

Annexin II has been implicated in membrane fusion during the exocytosis of lamellar bodies from alveolar epithelial type II cells. Most previous studies were based on the fusion assays by using model membranes. In the present study, we investigated annexin II-mediated membrane fusion by using isolated lamellar bodies and plasma membrane as determined by the relief of octadecyl rhodamine B (R18) self-quenching. Immunodepletion of annexin II from type II cell cytosol reduced its fusion activity. Purified annexin II tetramer (AIIt) induced the fusion of lamellar bodies with the plasma membrane in a dose-dependent manner. This fusion is Ca2+-dependent and is highly specific to AIIt because other annexins (I and II monomer, III, IV, V, and VI) were unable to induce the fusion. Modification of the different functional residues of AIIt by N-ethylmaleimide, nitric oxide, or peroxynitrite abolished AIIt-mediated fusion. Arachidonic acid enhanced AIIt-mediated fusion and reduced its Ca2+ requirement to an intracellularly achievable level. This effect is due to membrane-bound arachidonic acid, not free arachidonic acid. Other fatty acids including linolenic acid, palmitoleic acid, myristoleic acid, stearic acid, palmitic acid, and myristic acid had little effect. AIIt-mediated fusion was suppressed by the removal of arachidonic acid from lamellar body and plasma membrane using bovine serum albumin. The addition of arachidonic acid back to the arachidonic acid-depleted membranes restored its fusion activity. Our results suggest that the fusion between lamellar bodies with the plasma membrane is driven by the synergistic action of AIIt and arachidonic acid.

Nuclear annexin II negatively regulates growth of LNCaP cells and substitution of ser 11 and 25 to glu prevents nucleo-cytoplasmic shuttling of annexin II

BACKGROUND

Annexin II heavy chain (also called p36, calpactin I) is lost in prostate cancers and in a majority of prostate intraepithelial neoplasia (PIN). Loss of annexin II heavy chain appears to be specific for prostate cancer since overexpression of annexin II is observed in a majority of human cancers, including pancreatic cancer, breast cancer and brain tumors. Annexin II exists as a heterotetramer in complex with a protein ligand p11 (S100A10), and as a monomer. Diverse cellular functions are proposed for the two forms of annexin II. The monomer is involved in DNA synthesis. A leucine-rich nuclear export signal (NES) in the N-terminus of annexin II regulates its nuclear export by the CRM1-mediated nuclear export pathway. Mutation of the NES sequence results in nuclear retention of annexin II.

RESULTS

Annexin II localized in the nucleus is phosphorylated, and the appearance of nuclear phosphorylated annexin II is cell cycle dependent, indicating that phosphorylation may play a role in nuclear entry, retention or export of annexin II. By exogenous expression of annexin II in the annexin II-null LNCaP cells, we show that wild-type annexin II is excluded from the nucleus, whereas the NES mutant annexin II localizes in both the nucleus and cytoplasm. Nuclear retention of annexin II results in reduced cell proliferation and increased doubling time of cells. Expression of annexin II, both wild type and NES mutant, causes morphological changes of the cells. By site-specific substitution of glutamic acid in the place of serines 11 and 25 in the N-terminus, we show that simultaneous phosphorylation of both serines 11 and 25, but not either one alone, prevents nuclear localization of annexin II.

CONCLUSION

Our data show that nuclear annexin II is phosphorylated in a cell cycle-dependent manner and that substitution of serines 11 and 25 inhibit nuclear entry of annexin II. Aberrant accumulation of nuclear annexin II retards proliferation of LNCaP cells.