Characterization of a Ca(2+)-binding site in human annexin II by site-directed mutagenesis

RNA packaging into extracellular vesicles: An orchestra of RNA-binding proteins?

Extracellular vesicles (EVs) are heterogeneous membranous particles released from the cells through different biogenetic and secretory mechanisms. We now conceive EVs as shuttles mediating cellular communication, carrying a variety of molecules resulting from intracellular homeostatic mechanisms. The RNA is a widely detected cargo and, impressively, a recognized functional intermediate that elects EVs as modulators of cancer cell phenotypes, determinants of disease spreading, cell surrogates in regenerative medicine, and a source for non-invasive molecular diagnostics. The mechanistic elucidation of the intracellular events responsible for the engagement of RNA into EVs will significantly improve the comprehension and possibly the prediction of EV "quality" in association with cell physiology. Interestingly, the application of multidisciplinary approaches, including biochemical as well as cell-based and computational strategies, is increasingly revealing an active RNA-packaging process implicating RNA-binding proteins (RBPs) in the sorting of coding and non-coding RNAs. In this review, we provide a comprehensive view of RBPs recently emerging as part of the EV biology, considering the scenarios where: (i) individual RBPs were detected in EVs along with their RNA substrates, (ii) RBPs were detected in EVs with inferred RNA targets, and (iii) EV-transcripts were found to harbour sequence motifs mirroring the activity of RBPs. Proteins so far identified are members of the hnRNP family (hnRNPA2B1, hnRNPC1, hnRNPG, hnRNPH1, hnRNPK, and hnRNPQ), as well as YBX1, HuR, AGO2, IGF2BP1, MEX3C, ANXA2, ALIX, NCL, FUS, TDP-43, MVP, LIN28, SRP9/14, QKI, and TERT. We describe the RBPs based on protein domain features, current knowledge on the association with human diseases, recognition of RNA consensus motifs, and the need to clarify the functional significance in different cellular contexts. We also summarize data on previously identified RBP inhibitor small molecules that could also be introduced in EV research as potential modulators of vesicular RNA sorting.

Bridging of membrane surfaces by annexin A2

The protein-mediated formation of membrane contacts is a crucial event in many cellular processes ranging from the establishment of organelle contacts to the docking of vesicles to a target membrane. Annexins are Ca²⁺ regulated membrane-binding proteins implicated in providing such membrane contacts; however, the molecular basis of membrane bridging by annexins is not fully understood. We addressed this central question using annexin A2 (AnxA2) that functions in secretory vesicle exocytosis possibly by providing membrane bridges. By quantitatively analyzing membrane contact formation using a novel assay based on quartz crystal microbalance recordings, we show that monomeric AnxA2 can bridge membrane surfaces Ca²⁺ dependently. However, this activity depends on an oxidative crosslink involving a cysteine residue in the N-terminal domain and thus formation of disulfide-linked dimers. Alkylated AnxA2 in which this cysteine residue has been modified and AnxA2 mutants lacking the N-terminal domain are not capable of bridging membrane surfaces. In contrast, a heterotetrameric complex comprising two membrane binding AnxA2 subunits linked by a S100A10 dimer can provide membrane contacts irrespective of oxidation status. Thus, monomeric AnxA2 only contains one lipid binding site and AnxA2-mediated linking of membrane surfaces under non-oxidative intracellular conditions most likely requires AnxA2-S100 complex formation.

The tumor suppressor annexin A10 is a novel component of nuclear paraspeckles

Annexin A10 is the latest identified member of the annexin family of Ca(2+)- and phospholipid-binding proteins. In previous studies, downregulation of annexin A10 was correlated with dedifferentiation, invasion, and tumor progression, pointing to a possible tumor suppressor role. However, the biochemical characteristics and functions of annexin A10 remain unknown. We show that annexin A10 displays biochemical characteristics atypical for an annexin, indicating a Ca(2+)- and membrane-binding-independent function. Annexin A10 co-localizes with the mRNA-binding proteins SFPQ and PSPC1 at paraspeckles, an only recently discovered nuclear body, and decreases paraspeckle numbers when overexpressed in HeLa cells. In addition, annexin A10 relocates to dark perinucleolar caps upon transcriptional inhibition of RNA polymerase II. We mapped the cap-binding function of annexin A10 to the proximal part of the core domain, which is missing in the short isoform of annexin A10, and show its independence from the remaining functional type II Ca(2+)-binding site. In contrast to this, paraspeckle recruitment required additional core regions and was negatively affected by the mutation of the last type II Ca(2+)-binding site. Additionally, we show that overexpression of annexin A10 in HeLa cells increases their sensitivity to apoptosis and reduces colony formation. The identification of unique nuclear and biochemical characteristics of annexin A10 points towards its membrane-independent role in paraspeckle-associated mRNA regulation or processing.

Lipid segregation and membrane budding induced by the peripheral membrane binding protein annexin A2

The formation of dynamic membrane microdomains is an important phenomenon in many signal transduction and membrane trafficking events. It is driven by intrinsic properties of membrane lipids and integral as well as membrane-associated proteins. Here we analyzed the ability of one peripherally associated membrane protein, annexin A2 (AnxA2), to induce the formation of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-rich domains in giant unilamellar vesicles (GUVs) of complex lipid composition. AnxA2 is a cytosolic protein that can bind PI(4,5)P2 and other acidic phospholipids in a Ca(2+)-dependent manner and that has been implicated in cellular membrane dynamics in endocytosis and exocytosis. We show that AnxA2 binding to GUVs induces lipid phase separation and the recruitment of PI(4,5)P2, cholesterol and glycosphingolipids into larger clusters. This property is observed for the full-length monomeric protein, a mutant derivative comprising the C-terminal protein core domain and for AnxA2 residing in a heterotetrameric complex with its intracellular binding partner S100A10. All AnxA2 derivatives inducing PI(4,5)P2 clustering are also capable of forming interconnections between PI(4,5)P2-rich microdomains of adjacent GUVs. Furthermore, they can induce membrane indentations rich in PI(4,5)P2 and inward budding of these membrane domains into the lumen of GUVs. This inward vesiculation is specific for AnxA2 and not shared with other PI(4,5)P2-binding proteins such as the pleckstrin homology (PH) domain of phospholipase Cδ1. Together our results indicate that annexins such as AnxA2 can efficiently induce membrane deformations after lipid segregation, a mechanism possibly underlying annexin functions in membrane trafficking.

Annexin A1 is a new functional linker between actin filaments and phagosomes during phagocytosis

Remodelling of the actin cytoskeleton plays a key role in particle internalisation and the phagosome maturation processes. Actin-binding proteins (ABPs) are the main players in actin remodelling but the precise role of these proteins in phagocytosis needs to be clarified. Annexins, a group of ABPs, are known to be present on phagosomes. Here, we identified annexin A1 as a factor that binds to isolated latex bead phagosomes (LBPs) in the presence of Ca(2+) and facilitates the F-actin-LBP interaction in vitro. In macrophages the association of endogenous annexin A1 with LBP membranes was strongly correlated with the spatial and temporal accumulation of F-actin at the LBP. Annexin A1 was found on phagocytic cups and around early phagosomes, where the F-actin was prominently concentrated. After uptake was completed, annexin A1, along with F-actin, dissociated from the nascent LBP surface. At later stages of phagocytosis annexin A1 transiently concentrated only around those LBPs that showed transient F-actin accumulation ('actin flashing'). Downregulation of annexin A1 expression resulted in impaired phagocytosis and actin flashing. These data identify annexin A1 as an important component of phagocytosis that appears to link actin accumulation to different steps of phagosome formation.

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.

Annexin2 coating the surface of enlargeosomes is needed for their regulated exocytosis

Enlargeosomes are small cytoplasmic vesicles that undergo rapid, Ca2+-dependent exo/endocytosis. The role of the cytoskeleton in these processes was unknown. In PC12-27 cells, microtubule disassembly had little effect on enlargeosomes, whereas microfilament disassembly increased markedly both their resting and stimulated exocytosis, and inhibited their endocytosis. Even at rest enlargeosomes are coated at their cytosolic surface by an actin-associated protein, annexin2, bound by a dual, Ca2+-dependent and Ca2+-independent mechanism. In contrast, the other enlargeosome marker, desmoyokin/Ahnak, is transported across the organelle membrane, apparently by an ABC transporter, and binds to its lumenal face. Annexin2-GFP expression revealed that, upon stimulation, the slow and random enlargeosome movement increases markedly and becomes oriented toward the plasma membrane. After annexin2 downregulation enlargeosome exocytosis induced by both [Ca2+]i rise and cytoskeleton disruption is inhibited, and the NGF-induced differentiation is blocked. Binding of annexin2 to the enlargeosome membrane, the most extensive ever reported (>50% annexin2 bound to approximately 3% of total membrane area), seems therefore to participate in the regulation of their exocytosis.

Annexin A8 displays unique phospholipid and F-actin binding properties

Annexin A8 is a poorly characterized member of the annexin family of Ca2+-regulated membrane binding proteins. Initially only identified at the cDNA level it had been tentatively linked to acute promyelocytic leukaemia (APL) due to its high and regulated expression in APL-derived cells. Here we identify unique properties of the annexin A8 protein. We show that it binds Ca2+-dependently and with high specificity to phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P2) and is also capable of interacting with F-actin. In line with these characteristics annexin A8 is recruited to F-actin-associated PtdIns(4,5)P2-rich membrane domains formed in HeLa cells upon infection with non-invading enteropathogenic Escherichia coli. These properties suggest a role of annexin A8 in the organization of certain actin-associated membrane domains.

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.

Atypical properties displayed by annexin A9, a novel member of the annexin family of Ca(2+) and lipid binding proteins

Annexin A9 is a novel member of the annexin family of Ca(2+) and phospholipid binding proteins which has so far only been identified in EST data bases and whose deduced protein sequence shows mutations in residues considered crucial for Ca(2+) coordination in other annexins. To elucidate whether the annexin A9 protein is expressed as such and to characterize its biochemical properties we probed cell extracts with specific anti-annexin A9 antibodies and developed a recombinant expression system. We show that the protein is found in HepG2 hepatoma cell lysates and that a green fluorescent protein-tagged form is abundantly expressed in the cytosol of HeLa cells. Recombinant expression in bacteria yields a soluble protein that can be enriched by conventional chromatographic procedures. The protein is capable of binding phosphatidylserine containing liposomes albeit only at Ca(2+) concentrations exceeding 2 mM. Moreover and in contrast to other annexins this binding appears to be irreversible as the liposome-bound annexin A9 cannot be released by Ca(2+) chelation. These results indicate that annexin A9 is a unique member of the annexin family whose intracellular activity is not subject to Ca(2+) regulation.

Membrane composition affects the reversibility of annexin A2t binding to solid supported membranes: a QCM study

By means of the quartz crystal microbalance (QCM) technique, we investigated the interaction of porcine heterotetrametric annexin A2t with solid supported lipid membranes. Dissociation and rate constants of annexin A2t binding to various lipid mixtures were determined as a function of Ca2+ concentrations in solution. In contrast to what has been observed for annexin A1, the binding affinity and kinetics of annexin A2t binding are not influenced by cholesterol. In the experimental setup chosen, the annexin A2t binding is strictly Ca2+-dependent and only affected by the amount of phosphatidylserine (PS) in the membrane and the Ca2+ concentration in solution. By Ca2+-titration experiments at constant annexin A2t concentration, we investigated the reversibility of annexin A2t adsorption and desorption. Surprisingly, Ca2+-titration curves display a significant hysteresis. Protein desorption curves starting from annexin A2t bound to the membrane at 1 mM CaCl2 exhibit high cooperativity with half-maximum Ca2+ concentrations in the submicromolar range. However, protein adsorption curves starting from an EGTA-containing solution with soluble annexin A2t always show two inflection points upon addition of Ca2+ ions. These two inflection points may be indicative of two protein populations differently bound to the solid-supported membrane. The ratio of these two annexin A2t populations depends on the amount of PS molecules and cholesterol in the membrane as well as on the Ca2+ concentration. We propose a model discussing the results obtained in terms of two binding sites differing in their affinity due to lipid rearrangement.

Folding energetics of ligand binding proteins II. Cooperative binding of Ca2+ to annexin I

The calcium binding properties of annexin I as observed by thermodynamic DSC studies have been compared to the structural information obtained from X-ray investigation. The calorimetric experiment permitted to evaluate both the reaction scheme - including binding of ligand and conformational changes - and the energetics of each reaction step. According to published X-ray data Annexin I has six calcium binding sites, three medium-affinity type II and three low-affinity type III sites. The present study shows that at 37 degrees C annexin I binds in a Hill type fashion simultaneously two calcium ions in a first step with medium affinity at a concentration of 0.6 mM and another three Ca(2+) ions again cooperatively at 30 mM with low affinity. Therefore it can be concluded that only two medium-affinity type II binding sites are available. The third site, that should be accessible in principle appears to be masked presumably due to the presence of the N terminus. In view of the large calcium concentration needed for saturation of the binding sites, annexin I may be expected to be Ca(2+) free in vivo unless other processes such as membrane interaction occur simultaneously. This assumption is consistent with the finding, that the affinity of annexins to calcium is usually markedly increased by the presence of lipids.

Modulation by Ca(2+) and by membrane binding of the dynamics of domain III of annexin 2 (p36) and the annexin 2-p11 complex (p90): implications for their biochemical properties

The modulation of the local structure and dynamics of domain III of annexin 2 (Anx2), in both the monomeric (p36) and heterotetrameric forms (p90), by calcium and by membrane binding was studied by time-resolved fluorescence intensity and anisotropy measurements of the single tryptophan residue (W212). The results yield the same dominant excited-state lifetime (1.4 ns) in both p36 and p90, suggesting that the conformation and environment of W212 are very similar. The fluorescence anisotropy decay data were analyzed by associative (two-dimensional) as well as nonassociative (one-dimensional) models. Although no statistical criterion is decisive for one model versus the other, only the associative model allows recovery of a physically relevant value of the Brownian rotational correlation of the protein. Using the associative model, a nanosecond flexibility is detectable in p90 but not in p36. When Ca(2+) binds in the millimolar concentration range to both forms of Anx2, a conformational change takes place leading to an increase of the major excited-state lifetime (2.6 ns) and to a suppression of the W212 local flexibility of p90. Binding to membranes of either p36 or p90 in the presence of Ca(2+) does not induce any conformational change other than that provoked by Ca(2+) binding alone. The W212 local flexibility in both proteins increases significantly, however, in their membrane-bound forms. In the presence of membranes, the conformation change of domain III in p90 displays a sensitivity to Ca(2+) 2 orders of magnitude higher than that of p36, reaching intracellular sub-micromolar concentration ranges. This higher Ca(2+) sensitivity correlates with the Ca(2+)-dependent membrane aggregation but not with their Ca(2+)-dependent binding to membranes. The significance of these structural and dynamical changes for the function of the protein is discussed.

Characterization of the Ca2+-binding sites of annexin II tetramer

Annexin II heterotetramer (AIIt) is a multifunctional Ca(2+)-binding protein composed of two 11-kDa subunits and two annexin II subunits. The annexin II subunit contains three type II and two type III Ca(2+)-binding sites which are thought to regulate the interaction of AIIt with anionic phospholipid, F-actin, and heparin. In the present study we utilized site-directed mutagenesis to create AIIt mutants with inactive type III (TM AIIt), type II (CM AIIt), and both type II and III Ca(2+)-binding sites (TCM AIIt). Surprisingly, we found that in the presence of Ca(2+), the TM, CM, and TCM AIIt bound phospholipid and F-actin with similar affinity to the wild type AIIt (WT AIIt). Furthermore, the TCM mutant, and to a lesser extent the TM and CM AIIt displayed dose-dependent Ca(2+)-independent phospholipid aggregation and binding. While the TM and CM AIIt demonstrated Ca(2+)-dependent binding to F-actin, the binding of the TCM AIIt was Ca(2+)-independent. These results suggest that the type II or type III Ca(2+)-binding sites do not directly participate in anionic phospholipid or F-actin binding. We therefore propose that in the absence of Ca(2+), the type II and type III Ca(2+)-binding sites of AIIt stabilize a conformation of AIIt that is unfavorable for binding phospholipid and F-actin. Ca(2+) binding to these sites, or the inactivation of these Ca(2+)-binding sites by site-directed mutagenesis, results in a conformational change that promotes binding to anionic phospholipid and F-actin. Since the TM, CM, and TCM AIIt require Ca(2+) for binding to heparin, we also propose that novel Ca(2+)-binding sites regulate this binding event.

Development and dissipation of Ca(2+) gradients in adrenal chromaffin cells

We used pulsed laser imaging to measure the development and dissipation of Ca(2+) gradients evoked by the activation of voltage-sensitive Ca(2+) channels in adrenal chromaffin cells. Ca(2+) gradients appeared rapidly (<5 ms) upon membrane depolarization and dissipated over several hundred milliseconds after membrane repolarization. Dissipation occurred with an initial fast phase, as the steep gradient near the membrane collapsed, and a slower phase as the remaining shallow gradient dispersed. Inhibition of active Ca(2+) uptake by the endoplasmic reticulum (thapsigargin) and mitochondria (carbonylcyanide p-trifluoro-methoxyphenylhydrazone/oligomycin) had no effect on the size of Ca(2+) changes or the rate of gradient dissipation, suggesting that passive endogenous Ca(2+) buffers are responsible for the slow Ca(2+) redistribution. We used a radial diffusion model incorporating Ca(2+) diffusion and binding to intracellular Ca(2+) buffers to simulate Ca(2+) gradients. We included a 3D optical sectioning model, simulating the effects of out-of-focus light, to allow comparison with the measured gradients. Introduction of a high-capacity immobile Ca(2+) buffer, with a buffer capacity on the order of 1000 and appropriate affinity and kinetics, approximated the size of the Ca(2+) increases and rate of dissipation of the measured gradients. Finally, simulations without exogenous buffer suggest that the Ca(2+) signal due to Ca(2+) channel activation is restricted by the endogenous buffer to a space less than 1 microm from the cell membrane.

Phospholipid vesicle binding and aggregation by four novel fish annexins are differently regulated by Ca2+

Four members of the annexin family, herein referred to as max (for medaka annexin) 1-4, have recently been identified through hybridization cloning in the killifish Oryzias latipes (D. Osterloh, J. Wittbrodt and V. Gerke, Characterization and developmentally regulated expression of four annexins in the killifish medaka. DNA and Cell Biol., in press). These annexins which are expressed in a developmentally regulated manner are present as a maternal pool in unfertilized eggs of another fish species, Misgurnus fossilis, and it has been proposed that they play a role in the Ca2+-regulated exocytosis of cortical granules occurring after fertilization. To characterize biochemical properties of the medaka proteins possibly relevant to their function in early development, we analyzed the ability of recombinantly expressed max 1-4 to interact with the principal structures of the egg cortex, phospholipid membranes and actin filaments. We show that all medaka annexins bind to acidic phospholipids in a Ca2+-regulated manner, although exhibiting different Ca2+ sensitivities. All medaka annexins, but max 1, are also capable of inducing, in a Ca2+-dependent manner, phospholipid vesicle aggregation, albeit only max 3 displays this activity at Ca2+ concentrations met in stimulated (i.e. fertilized) eggs. Max 3 is also the only medaka annexin able to interact with F-actin in the presence of Ca2+. These data identify by biochemical criteria max 3 as a close relative of the mammalian annexins I and II, thus supporting previous sequence-based comparisons. Max 3 is therefore the prime annexin candidate for being involved in cortical granule exocytosis, possibly by providing granule granule, granule plasma membrane and/or granule cytoskeleton contacts.

Mutational and crystallographic analyses of interfacial residues in annexin V suggest direct interactions with phospholipid membrane components

Annexin V belongs to a family of eukaryotic calcium-dependent membrane-binding proteins. The calcium-binding sites at the annexin-membrane interface have been investigated in some detail; however, little is known about the functional roles of highly conserved interfacial residues that do not coordinate calcium themselves. In the present study, the importance of tryptophan 185, and threonine or serine at positions 72, 144, 228, and 303, in rat annexin V is investigated by site-directed mutagenesis, X-ray crystallography, and functional assays. The high-resolution crystal structures of the mutants show that the mutations do not cause structural perturbations of the annexin molecule itself or disappearance of bound calcium ions from calcium-binding sites. The assays indicate that relative to wild-type annexin V, loss of the methyl substituent at position 72 (Thr72-->Ser) has no effect while loss of the hydroxyl group (Thr72-->Ala or Thr72-->Lys) causes reduction of membrane binding. Multiple lysine substitutions (e.g., Thr72,Ser144,Ser228,Ser303-->Lys) have a greater adverse effect than the single lysine mutation, suggesting that in annexin V the introduction of potentially favorable electrostatic interactions between the lysine side chains and the net negatively charged membrane surface is not sufficient to overcome the loss of the hydroxyl side chains. Replacement of the unique tryptophan, Trp185, by alanine similarly decreases membrane binding affinity. Taken together, the data suggest that the side chains mutated in this study contribute to phospholipid binding and participate directly in intermolecular contacts with phospholipid membrane components.

Interactions of benzodiazepine derivatives with annexins

Human annexins III and V, members of the annexin family of calcium- and membrane-binding proteins, were complexed within the crystals with BDA452, a new 1,4-benzodiazepine derivative by soaking and co-crystallization methods. The crystal structures of the complexes were analyzed by x-ray crystallography and refined to 2.3- and 3.0-A resolution. BDA452 binds to a cleft which is located close to the N-terminus opposite to the membrane binding side of the proteins. Biophysical studies of the interactions of various benzodiazepine derivatives with annexins were performed to analyze the binding of benzodiazepines to annexins and their effects on the annexin-induced calcium influx into phosphatidylserine/phosphatidylethanolamine liposomes. Different effects were observed with a variety of benzodiazepines and different annexins depending on both the ligand and the protein. Almost opposite effects on annexin function are elicited by BDA250 and diazepam, its 7-chloro-derivative. We conclude that benzodiazepines modulate the calcium influx activity of annexins allosterically by stabilizing or destabilizing the conducting state of peripherally bound annexins in agreement with suggestions by Kaneko (Kaneko, N., Ago, H., Matsuda, R., Inagaki, E., and Miyano, M. (1997) J. Mol. Biol., in press).

Inhibition of cytosolic phospholipase A2 by annexin V in differentiated permeabilized HL-60 cells. Evidence of crucial importance of domain I type II Ca2+-binding site in the mechanism of inhibition

Annexin V belongs to a family of proteins that interact with phospholipids in a Ca2+-dependent manner. This protein has been demonstrated to have anti-phospholipase A2 activity. However, this effect has never yet been reported with the 85-kDa cytosolic PLA2 (cPLA2). We studied, in a model of differentiated and streptolysin O-permeabilized HL-60 cells, the effect of annexin V on cPLA2 activity after stimulation by calcium, GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), formyl-Met-Leu-Phe, or phorbol 12-myristate 13-acetate. Both recombinant and human placental purified annexin V inhibit cPLA2 activity whatever the stimulus used. The decrease of arachidonic acid release is of 40 and 50%, respectively, at [Ca2+] of 3 and 10 microM. The mechanism of inhibition was also analyzed. cPLA2 requires calcium and protein kinase C (PKC) or mitogen-activated protein kinase phosphorylation for its activation. As annexin V was shown to be an endogenous inhibitor of PKC, PKC-stimulated cPLA2 activity was analyzed. Using GF109203x, a specific PKC inhibitor, we demonstrated that this pathway is of minor importance in our model. cPLA2 inhibition by annexin V is not linked to PKC inhibition. To test the hypothesis of phospholipid depletion, mutants of annexin V were constructed using mutagenesis directed to Ca2+ site. We demonstrate that the Ca2+ site located in domain I is necessary for the inhibitory effect of annexin V on cPLA2 activity. The site in domain IV is also involved but with less efficiency. In contrast, mutations in site II and III do not modify this effect. Moreover, annexin V mutated on all sites does not inhibit cPLA2. Thus, we propose a predominant role of module (I/IV) in the biological action of annexin V, which, in physiological conditions, may control cPLA2 activity by depletion of the phospholipid substrate.

Characterization of human recombinant annexin II tetramer purified from bacteria: role of N-terminal acetylation

Annexin II tetramer (AIIt) is a Ca2+-dependent, phosphatidylserine-binding, and F-actin-bundling phosphoprotein which is localized to both the extracellular and cytoplasmic surfaces of the plasma membrane. The tetramer is composed of two p36 heavy chains and two p11 light chains. We have produced prokaryotic cDNA expression constructs for both p36 and p11. Both proteins were expressed in large amounts in Escherichia coli upon induction with IPTG. Electrospray ionization mass spectrometry and amino acid sequence analysis of purified recombinant p36 (rp36) and recombinant p11 (rp11) suggested that the recombinant proteins were identical to their native counterparts except for the lack of N-terminal acetylation of rp36. Furthermore, the non-acetylated rp36 bound rp11 and formed AIIt. The circular dichroism spectra and urea denaturation profiles of acetylated AIIt and non-acetylated rAIIt were identical. In addition, both the acetylated AIIt and non-acetylated rAIIt were similar in their Ca2+ dependence and concentration dependence of phospholipid liposome aggregation, chromaffin granule aggregation, and F-actin bundling. These results suggest that N-terminal acetylation of p36 is not in fact necessary for binding of the protein to p11 and that N-terminal acetylation does not affect the conformational stability of AIIt or the in vitro activities of AIIt. The availability of large amounts of rAIIt will facilitate further characterization of the structure-function relationships of the protein.

Tyrosine phosphorylation of annexin II tetramer is stimulated by membrane binding

In the present article we have examined if the interaction of the Ca2+-binding protein, annexin II tetramer (AIIt) with the plasma membrane phospholipids or with the submembranous cytoskeleton, effects the accessibility of the tyrosine phosphorylation site of AIIt. In the presence of Ca2+, pp60(c-src) catalyzed the incorporation of 0.22 +/- 0.05 mol of phosphate/mol of AIIt (mean +/- S.D., n = 5). The Ca2+-dependent binding of AIIt to purified adrenal medulla plasma membrane or phosphatidylserine vesicles stimulated the pp60(c-src)-dependent phosphorylation of AIIt to 0.62 +/- 0.04 mol of phosphate/mol of AIIt (mean +/- S.D., n = 5) or 0.93 +/- 0.07 mol of phosphate/mol of AIIt (mean +/- S.D., n = 5), respectively. Phosphatidylserine- or phosphatidylinositol-containing vesicles but not vesicles composed of phosphatidylcholine or phosphatidylethanolamine, stimulated the phosphorylation of AIIt. In contrast, the binding of AIIt to F-actin resulted in the incorporation of only 0.04 +/- 0.04 mol of phosphate/mol of AIIt (mean +/- S.D., n = 5). These results suggest that the interaction of AIIt with plasma membrane and not the submembranous cytoskeleton, activates the tyrosine phosphorylation of AIIt by inducing a conformational change in the protein resulting in the enhanced exposure or accessibility of the tyrosine-phosphorylation site.

Mapping of a regulatory important site for protein kinase C phosphorylation in the N-terminal domain of annexin II

Annexin II is a Ca(2+)-regulated membrane- and cytoskeleton-binding protein implicated in membrane transport events along the Ca(2+)-regulated secretory and the early endocytic pathway. Biochemical properties of this annexin and its intracellular distribution are regulated by complex formation with p11 (S100A10), a member of the S100 protein family. The annexin II-p11 interaction is mediated through the unique N-terminal domain of annexin II and is inhibited by protein kinase C phosphorylation of a serine residue in annexin II. To map this regulatory serine phosphorylation site we developed a baculovirus-mediated expression system for wild-type annexin II and for a series of annexin II mutants which contained substitutions in one or more serine residues present in the N-terminal domain. The different mutant derivatives were purified and shown to display the same biochemical properties as recombinant wild-type annexin II and the authentic protein purified from porcine intestine. However, significant differences in phosphate incorporation were observed when the individual serine mutants were subjected to phosphorylation by protein kinase C. A comparison of the phosphorylation patterns obtained identified Ser-II as the protein kinase C site responsible for regulating the annexin II-p11 interaction. Ser-II lies within the sequence mediating p11 binding, i.e. amino-acid residues 1 to 14 of annexin II, and phosphorylation at this site most likely leads to a direct spatial interference with p11 binding.

Collagen binding activity of recombinant and N-terminally modified annexin V (anchorin CII)

We have cloned the full coding cDNA sequence of chicken annexin V and of a mutant lacking 8 amino acid residues of the N-terminal tail for prokaryotic expression. Both proteins were synthesized in Escherichia coli upon induction with isopropyl thio-beta-D-galactoside, and were purified following two different protocols: one based on the ability of these proteins to interact reversibly with liposomes in the presence of calcium, and the other based on two sequential ion-exchange chromatographic steps. Spectroscopical analysis of recombinant annexin V revealed that binding of calcium did not change the circular dichroism spectra indicating no significant changes on the secondary structure; however, a conformational change affecting the exposition to the solvent of the tryptophan residue 187 was detected by analysis of fluorescence emission spectra. Recombinant annexin V binds with high affinity to collagen types II and X, and with lower affinity to collagen type I in a calcium-independent manner. Heat denaturing of collagen decreases this interaction while pepsin-treatment of collagen almost completely abolishes annexin V binding. Mutated annexin V interacts with collagen in a similar way as the nonmutated recombinant protein, indicating that the N-terminal tail of annexin V is not essential for collagen binding.

Identification of the trapped calcium in the gelsolin segment 1-actin complex: implications for the role of calcium in the control of gelsolin activity

The X-ray structure of the complex of actin with gelsolin segment 1 revealed the presence of two calcium ions, one bound at an intramolecular site within segment 1 and the other bridging the segment directly to actin. Although earlier calcium binding studies at pH 8.0 revealed only a single calcium trapped in the complex (and also in the binary gelsolin-actin complex), it is here shown that two calcium ions are bound under the conditions of crystallization at physiological pH. Mutation of acidic residues in either actin or segment 1 involved in ligation of the intermolecular calcium ion resulted in loss of one of the bound calcium ions at pH < 7, but not at pH 8. Thus the calcium ion trapped in the segment 1-actin complex is that located at the intramolecular site. The implications of this for gelsolin function are discussed.

The effects of PMA and TFP and alterations in intracellular pH and calcium concentration on the membrane associations of phospholipid-binding proteins fodrin, protein kinase C and annexin II in cultured MDCK cells

Annexin II, alpha-fodrin and protein kinase C (PKC) are associated with the cytoplasmic surface of the plasma membranes. When assayed with liposomes, they show affinity for acidic phospholipids and bind calcium ions. They also respond to or participate in cell signal transduction by altered membrane binding properties. In the present work we have studied the properties of these proteins in epithelial MDCK cells in response to elevated intracellular calcium ion concentration, lowered pH, treatment with tumor promoter phorbol myristoyl acetate (PMA) and calmodulin inhibitor trifluoperazine (TFP). In untreated polarized MDCK cells annexin II was seen both along the lateral walls and membranes of intracellular vesicles, fodrin was located along the lateral walls, whereas PKC was seen in the cytoplasm. There was no observable translocation of these proteins upon elevation of the intracellular calcium concentration using a calcium ionophore A23187. On the other hand, treatment with TFP led to a release of annexin II from the plasma membranes which was accompanied by a transient peak in the intracellular calcium. Treatment with PMA led to a loss of the cubic form of the cells, a slight elevation in the intracellular calcium concentration and a drop in the intracellular pH. Simultaneously fodrin was released from the lateral walls, but still remained insoluble in Triton X-100, PKC became associated with the intracellular membranes and fibers, whereas annexin II remained along the lateral walls. These changes could be prevented by clamping the intracellular pH neutral during PMA treatment. On the other hand, lowering of intracellular pH below 6.5 with the nigericin treatment led to a similar translocation of fodrin and PKC as PMA. This suggests that the protein redistribution is caused by cytoplasmic acidification and is due to an increased hydrophobicity and enhanced protonation of lipids and proteins. In contrast, no changes were seen in the annexin II distribution in response to altered pH. Hence, its release by TFP is presumably due to changes in the cationic properties of the inner phase of the plasma membrane. Thus, proteins which show similar binding properties with liposomes show different characteristics in their association with the intracellular membranes.

Organisation of the chicken annexin V gene and its correlation with the tertiary structure of the protein

Chicken annexin V (anchorin CII) is a collagen binding, membrane-associated molecule with Ca2+ channel activity. Here we report on the coding sequences, promoter region, size and distribution of exons, and exon-intron junctions of the chicken annexin V gene. It is about 25 kb long and codes for 13 short exons between 50 and 581 bp length. Exon sizes and locations of splice sites are almost completely homologous to those of the human and mouse annexin II or pigeon annexin I genes, although there is only 50-60% homology in the sequence of the corresponding proteins. The four repeat structure and symmetry of the annexin V as evident from sequence and X-ray analysis studies is only partially reflected in this highly conserved exon distribution. In the first two repeats of chicken annexin V the exons correlate with protein domains containing one, two, or three alpha-helices, while in the repeats 3 and 4 exon junctions and alpha-helical domains do not correlate. The analysis of the promoter structure revealed the absence of a typical TATA-box, but a GC-rich region which may possibly promote transcription from several start sites.

A rapid and efficient purification method for recombinant annexin V for biophysical studies

Annexin V binds in a calcium-dependent manner to acidic phospholipids and exhibits ion channel activity in vitro. We are investigating mutants of annexin V by single channel measurements, X-ray crystallography and electron microscopy in order to understand the structure-function relationships of the ion channel activity. We describe here a method to obtain very pure recombinant annexin V required for such studies. The initial step is the mild opening of the bacterial cells by an osmotic shock. In the purification procedure, use is made of the reversible calcium-mediated binding of annexin V to liposomes. In the last purification step the protein is subjected to ion-exchange chromatography and elutes as a single peak free of any detectable contaminants.

Formation of the annexin II2p112 complex upon differentiation of F9 teratocarcinoma cells

Murine teratocarcinoma F9 cells, which remain undifferentiated under standard cell culture conditions, can form cellular layers resembling early embryonic tissues upon induction of differentiation by retinoic acid and cyclic AMP. We have employed a combination of Northern and Western blot analyses to elucidate the regulation of expression of the tyrosine kinase substrate annexin II and its cellular ligand p11 during this differentiation process. Interestingly, the synthesis of the two subunits of the annexin II2p112 complex is not coregulated during F9 differentiation. Annexin II, which is only very weakly expressed in undifferentiated F9 cells, shows a strong increase in the amount of transcript and protein once the differentiated phenotype is established. The level of this induction does not depend on the type of F9 differentiation. In contrast to the regulated synthesis of annexin II, a significant amount of p11 mRNA and protein is already present in the undifferentiated cells and remains constant during the differentiation of F9 cells. Immunofluorescence analysis reveals that annexin II and p11 are concentrated in the submembranous region of the differentiated F9 cells. In contrast, p11 is uniformly distributed throughout the cytoplasm of undifferentiated cells. p11 is translocated to the submembranous region of the undifferentiated F9 cells upon coexpression of an exogenous annexin II introduced by transient transfection. Thus the localization of annexin II and p11 to the submembranous cytoskeleton depends on the formation of the tight annexin II2p112 complex.

The tight association of the tyrosine kinase substrate annexin II with the submembranous cytoskeleton depends on intact p11- and Ca(2+)-binding sites

Annexin II, a member of the annexin family of Ca(2+)- and lipid-binding proteins, is a major substrate of the pp60src kinase. It is unique within the annexin protein family, since it can form a tight heterotetrameric complex with the cellular protein ligand p11, a member of the S100 protein family. Within the cell, the annexin II2p11(2) complex is localized at the cytoplasmic surface of the plasma membrane in the submembranous cytoskeleton. This intracellular localization is thought to be the consequence of a typical annexin II property observed in vitro, its Ca(2+)-dependent binding to phospholipids and cytoskeletal elements (F-actin, non-erythroid spectrin). We employed site-directed mutagenesis to create mutant annexin II molecules with defects either in the p11-binding site or in the Ca(2+)-binding sites present in repeats 2, 3 and 4. The mutated annexin II derivatives were expressed in HeLa and RMCD cells by transfection of the appropriate DNA constructs in order to analyze the importance of p11- and Ca(2+)-binding for the intracellular localization of annexin II. Immunofluorescence microscopy with a monoclonal antibody that specifically detected the transfected annexin II derivatives indicated that the Ca(2+)-dependent incorporation of annexin II into the submembranous network depended on its ability to form the annexin II/p11 complex and on the presence of intact Ca(2+)-binding sites. Neither monomeric annexin II lacking an intact p11-binding site, nor the annexin II mutant with defects in the Ca(2+)-binding sites in repeats 2, 3 and 4 were associated with the Triton X-100-resistant network of the submembranous cytoskeleton.

Mapping of three unique Ca(2+)-binding sites in human annexin II

Site-directed mutagenesis was employed to map and characterize Ca(2+)-binding sites in annexin II, a member of the annexin family of Ca(2+)- and phospholipid-binding proteins which serves as a major cellular substrate for the tyrosine kinase encoded by the src oncogene. Several single amino acid substitutions were introduced in the human annexin II and the various mutant proteins were scored for their affinity towards Ca2+ in different assays. The data support our previous finding [Thiel, C., Weber, K. and Gerke V. (1991) J. Biol. Chem. 266, 14,732-14,739] that a Ca(2+)-binding site is present in the third of the four repeat segments which comprise the 33-kDa protein core of annexin II. In addition to Gly206 and Thr207, which are localized in the highly conserved endonexin fold of the third repeat, Glu246 is involved in the formation of this site. Thus the architecture of this Ca(2+)-binding site in solution is very similar, if not identical, to that of Ca2+ sites identified recently in annexin V crystals [Huber, R., Schneider, M., Mayr, I., Römisch, J. and Paques, E.-P. (1990) FEBS Lett. 275, 15-21]. In addition to the site in repeat 3, we have mapped sites of presumably similar architecture in repeats 2 and 4 of annexin II. Again, an acidic amino acid which is located 40 residues C-terminal to the conserved glycine at position 4 of the endonexin fold is indispensable for high-affinity Ca2+ binding: Asp161 in the second and Asp321 in the fourth repeat. In contrast, repeat 1 does not contain an acidic amino acid at a corresponding position and also shows deviations from the other repeats in the sequence surrounding the conserved glycine. These results on annexin II together with the crystallographic information on annexin V reveal that annexins can differ in the position of the Ca2+ sites. Ca(2+)-binding sites of similar structure are present in repeats 2, 3, and 4 of annexin II while in annexin V they occur in repeats 1, 2, and 4. We also synthesized an annexin II derivative with mutations in all three Ca2+ sites. This molecule shows a greatly reduced affinity for the divalent cation. However, it is still able to bind Ca2+, indicating the presence of (an) additional Ca2+ site(s) of presumably different architecture.