Expression of bovine annexin A4 in E. coli rescues cytokinesis blocked by beta-lactam antibiotics

Treatment of bacteria with beta-lactam antibiotics can impair the process of cytokinesis, the final step in cell division, leading to the formation of a filamentous form of the bacteria. The expression of a mammalian calcium-dependent, membrane-binding protein, bovine annexin A4, in E. coli was found to reverse the inhibitory effects on cytokinesis of the beta-lactam antibiotics ampicillin, piperacillin, and cephalexin. This novel activity of the annexin was blocked by mutation of calcium binding sites in the annexin, indicating roles for calcium binding to the annexin and the binding of the annexin to membranes in restoring cytokinesis. The filamentous form of the bacteria has been reported to be more resistant to phagocytosis by cells of the immune system in eukaryotic hosts. Therefore, expression of annexins in pathogenic bacteria, by promoting the breakdown of the bacterial filaments, might serve as an adjuvant to enhance the efficacy of beta-lactam antibiotics.

Expression of Metazoan Annexins in Yeast Provides Protection Against Deleterious Effects of the Biofuel Isobutanol

The ability of microorganisms to produce biofuels by fermentation is adversely affected by the perturbing effects of the hydrophobic biofuel on plasma membrane structure. It is demonstrated here that heterologous expression of metazoan, calcium-dependent, membrane-binding proteins of the annexin class can reduce deleterious effects of isobutanol on Saccharomyces cerevisiae viability and complex membrane functions. Therefore, expression of annexins in industrial strains of yeast or bacteria may prove beneficial in biofuel production.

Quaternary structure of the small amino acid transporter OprG from Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen that causes nosocomial infections. The P. aeruginosa outer membrane contains specific porins that enable substrate uptake, with the outer membrane protein OprG facilitating transport of small, uncharged amino acids. However, the pore size of an eight-stranded β-barrel monomer of OprG is too narrow to accommodate even the smallest transported amino acid, glycine, raising the question of how OprG facilitates amino acid uptake. Pro-92 of OprG is critically important for amino acid transport, with a P92A substitution inhibiting transport and the NMR structure of this variant revealing that this substitution produces structural changes in the barrel rim and restricts loop motions. OprG may assemble into oligomers in the outer membrane (OM) whose subunit interfaces could form a transport channel. Here, we explored the contributions of the oligomeric state and the extracellular loops to OprG's function. Using chemical cross-linking to determine the oligomeric structures of both WT and P92A OprG in native outer membranes and atomic force microscopy, and single-molecule fluorescence of the purified proteins reconstituted into lipid bilayers, we found that both protein variants form oligomers, supporting the notion that subunit interfaces in the oligomer could provide a pathway for amino acid transport. Furthermore, performing transport assays with loop-deleted OprG variants, we found that these variants also can transport small amino acids, indicating that the loops are not solely responsible for substrate transport. We propose that OprG functions as an oligomer and that conformational changes in the barrel-loop region might be crucial for its activity.

Role of intracellular proteins in the regulation of calcium action and transmitter release during exocytosis

A central problem in cellular neurobiology is how the process of exocytosis of transmitters and hormones is regulated at the molecular level. Calcium is important in the process and may act by initiating the formation of fusion complexes of secretory granules to each other and to plasma membranes. We have discovered and isolated a new 47,000 MW protein (synexin) from adrenal medulla tissue that fuses chromaffin granule membranes only in the presence of calcium. Synexin activity was detected in a number of secretory tissues including human platelets and bovine brain, and the synexin molecule was found by immunofluorescent cytochemistry to be localized to the cytoplasm of chromaffin cells. Purified synexin molecules were found to self-associate in the presence of Ca++ to form paracrystalline arrays of 50 X 150 A rods, and the association was dependent on [Ca++] in an identical fashion to the Ca++ dependence of granule membrane fusion. On the basis of these data we suggest that synexin may be the intracellular receptor for calcium during exocytosis. However, the actual release event of 'fission' of the secretory vesicle-plasma membrane complex did not appear to be related to synexin action, and we have considered the hypothesis that the chemiosmotic mechanism for ATP, Cl--dependent chromaffin granule lysis might provide the necessary localized force. We have now shown that the granule model successfully predicts the secretory properties of human platelets, bovine parathyroid cells, and bovine chromaffin cells. Like the granule lysis system, secretion from these cells required specific anions in the medium was inhibited by anion transport blocking drugs and proton ionophores, and was suppressed by elevated osmotic strength. We suggest that secretory granules fused to plasma membranes in secreting cells, perhaps by synexin, may experience net solute uptake and subsequently undergo local, outwardly directed osmotic lysis, or exocytosis.

Assembly of high molecular weight complexes of lipin on a supported lipid bilayer observed by atomic force microscopy

Lipins are phosphatidic acid phosphatases involved in the biosynthesis of triacylglycerols and phospholipids. They are associated with the endoplasmic reticulum but can also travel into the nucleus and alter gene expression. Previous studies indicate lipins in solution form high molecular weight complexes, possibly tetramers. This study was undertaken to determine if lipins form complexes on membranes as well. Murine lipin 1b was applied to a supported bilayer of phosphatidylcholine, phosphatidylserine, and cholesterol and examined by atomic force microscopy (AFM) over time. Lipin on bare mica appeared as a symmetric particle with a volume consistent with the size of a monomer. On the bilayer, lipin initially bound as asymmetric, curved particles that sometimes assembled into circular structures with an open center. Subsequently, lipin assemblies grew into large, symmetric particles with an average volume 12 times that of the monomer. Over time, some of the lipin assemblies were removed from the bilayer by the AFM probe leaving behind "footprints" composed of complex patterns that may reflect the substructure of the lipin assemblies. The lipin complexes appeared very flat, with a diameter 20 times their height. The footprints had a similar diameter, providing confirmation of the extensive deformation of the protein under the AFM probe. The ability of lipin to form large complexes on membranes may have significant implications for the local concentrations of the product, diacylglycerol, formed during hydrolysis of phosphatidic acid and for cooperative hormonal regulation of lipin activity through phosphorylation of one or more monomers in the complexes.

Similarities and differences between tau protein and chromobindin A

Tau protein and Chromobindin A have several features in common but are not identical. Both consist of a small group of closely related proteins which can form aggregates. Both have a similar range of molecular weights (53-62 kDa) and isoelectric points (6.0-7.5). While Chromobindin A is known to be membrane associated, there is evidence that Tau protein also interacts with phospholipids. Both, not present in all tissues, can be found in the adrenal medulla. Despite these similarities both classes of proteins are unique and immunologically distinct. A rabbit antisera to Tau does not cross react with Chromobindin A. In addition, while protein kinase C and Ca/Calmodulin-dependent protein kinase II phosphorylate Tau protein, they do not phosphorylate Chromobindin A, demonstrating the specificity of these kinases for Tau protein phosphorylation.

Isolation of chromaffin granules

Adrenal medullary chromaffin granules (dense core secretory vesicles) have been a valuable model system for the study of the proteins and membrane components involved in the process of exocytosis. Because of the abundance of chromaffin granules in a readily available tissue source, bovine adrenal medullae, and their unique sedimentation properties, it is possible to obtain large quantities of highly purified granules and granule membranes in a short period of time. Two protocols are presented here for the isolation of chromaffin granules: a basic protocol based on differential centrifugation in an iso-osmotic medium that yields intact chromaffin granules, and an alternate protocol based on sedimentation through a density step gradient that provides a greater yield of more highly purified chromaffin granules. Since in the latter case the granules cannot be returned to a medium of physiological osmolarity without lysis after purification on the step gradient, the alternate protocol is more useful to obtain the granule membranes or contents for further study.

Novel protein ligands of the annexin A7 N-terminal region suggest pro-beta helices engage one another with high specificity

The N-terminal regions of annexins A7 (synexin) and A11 consist of an extended series of short sequence repeats rich in tyrosine, proline, and glycine that provide binding sites for other proteins that may be recruited to membranes by the annexins and that may modulate the calcium and membrane binding activities of the annexin core domains. In this study two new ligands for the annexin A7 N terminal region were identified by yeast two hybrid screening: the TNFalpha receptor regulatory protein SODD (Suppressor Of Death Domains) and KIAA0280, a protein of unknown function. Strikingly, the sites of interaction of these proteins with the annexin also contain sequence repeats similar to those present in the N-termini of annexins A7 and A11. It was also found that the annexin A7 N-terminal region interacts with itself in the two hybrid assay. These results suggest that sequence repeats of this nature form novel structures, called YP pro-beta helices, that are characterized by an ability to interact with one another. Specificity of interactions between the pro-beta helices in different proteins may be encoded by the variations of residues and lengths of the sequence repeats.

Copine-I represses NF-kappaB transcription by endoproteolysis of p65

Nuclear factor-kappaB (NF-kappaB) is a dynamic transcription factor that regulates important biological processes involved in cancer initiation and progression. Identifying regulators that control the half-life of NF-kappaB is important to understanding molecular processes that control the duration of transcriptional responses. In this study we identify copine-I, a calcium phospholipid-binding protein, as a novel repressor that physically interacts with p65 to inhibit NF-kappaB transcription. Knockdown of copine-I by siRNA increases tumor necrosis factor alpha-stimulated NF-kappaB transcription, while copine-I expression blocks endogenous transcription. Copine-I abolishes NF-kappaB transcription by inducing endoprotease processing of the N-terminus of p65, a process antagonized by IkappaB alpha. Copine-I stimulates endoproteolysis of p65 within a conserved region that is required for base-specific contact with DNA. p65 proteins lacking the N-terminus fail to bind to DNA and act as dominant-negative molecules that inhibit NF-kappaB transcription. Our work provides evidence that copine-I regulates the half-life of NF-kappaB transcriptional responses through a novel mechanism that involves endoproteolysis of the p65 protein.

Interactions of annexins with the mu subunits of the clathrin assembly proteins

A number of biochemical and genetic studies have suggested that certain annexins play important roles in the endocytic pathway, possibly involving the generation, localization, or fusion of endocytic compartments. In a yeast two-hybrid screen for proteins that interact with the N-terminal domain of annexin A2 we identified the mu2 subunit of the clathrin assembly protein complex AP-2. The interaction depended upon two copies of a Yxx phi amino acid sequence motif (Y = tyrosine, x = variable residue, phi = bulky, hydrophobic residue) in the annexin that is also characteristic of the binding site for mu2 on the cytoplasmic domains of transmembrane receptors. The interaction between mu2 and full-length annexin A2 was demonstrated in vitro to be direct, to require calcium, and to be functional in the sense that annexin A2 was able to recruit the mu2 to immobilized lipids. Examination of other annexins and mu subunits demonstrated that annexin A2 also binds the mu1 subunit of the AP-1 complex, that annexin A6 binds mu1 and mu2, and that annexin A1 binds only mu1. We propose that annexins can "masquerade" as transmembrane receptors when they are attached to membranes in the presence of calcium and that they might therefore function to initiate calcium-regulated coated pit formation at the cell surface or on intracellular organelles.

Annexins: linking Ca2+ signalling to membrane dynamics

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

The tricalbin C2 domains: lipid-binding properties of a novel, synaptotagmin-like yeast protein family

The tricalbins are a recently discovered family of Saccharomyces cerevisae proteins containing a predicted N-terminal transmembrane domain and at least three C2 domains. They are thought to be yeast homologues of synaptotagmin, a hypothesis supported by structural similarities and past studies that implicated tricalbins in processes of membrane trafficking and sorting. We expressed and purified constructs consisting of single tricalbin C2 domains, and assayed their ability to bind lipids in response to calcium. Protein-lipid overlay assays indicated that the C-terminal C2 domains (C2C) of tricalbins 1 and 3 bind numerous species of acidic phospholipid, including phosphatidylserine and several phosphoinositides, and the amount of protein bound was greatly enhanced in the presence of 1 mM calcium. Sedimentation assays using mixed phosphatidylserine/phosphatidylcholine (PS/PC) vesicles confirmed that the C2C domains of tricalbin 1 and 3 bind membranes in a calcium-responsive manner and showed that they are more sensitive to calcium than the C2A domain of synaptotagmin I. Both assays revealed that all of the C2 domains of tricalbin 2 are insensitive to calcium. Fluorimetric assays exploiting the position of naturally occurring tryptophans in tricalbin 1 C2C and tricalbin 3 C2C confirmed that these domains are capable of binding calcium and that this is coupled to the binding of acidic phospholipid. Combining this with past protein-protein interaction data, we theorize that the calcium-insensitive tricalbin 2 mediates the creation of hetero-oligomeric tricalbin complexes in which tricalbin 1 or 3 or both supply a calcium-dependent membrane binding activity.

Characterization of the yeast tricalbins: membrane-bound multi-C2-domain proteins that form complexes involved in membrane trafficking

In a survey of yeast genomic sequences encoding calcium- and phospholipid-binding C2 domains, three homologous genes were identified that encode proteins that each have three C2 domains and an apparent transmembrane domain near the N terminus. The name tricalbins is suggested for these proteins, corresponding to the open reading frames YOR086c (TCB1), YNL087w (TCB2), and YML072c (TCB3). An antiserum was raised against the C-terminal portion of tricalbin 2 and used on Western blots to demonstrate that the corresponding protein is expressed in yeast and appears as a high-molecular-weight band at 130 kDa with smaller fragments at 39 kDa and 46 kDa. A fusion protein consisting of full length tricalbin 2 fused to the green fluorescent protein was expressed in cells and found to traffic from the cell surface to intracellular vesicles near the vacuole. A two-hybrid interaction screen with the C-terminal portion of tricalbin 2 indicated that tricalbin 2 binds the C-terminal portions of tricalbins 1 and 3 suggesting that the tricalbins may form heterodimers in vivo. Tricalbin 2 also interacted with the activation domain of the pleiotropic drug resistance transcription factor Pdr1p. Combinatorial disruptions of the tricalbin genes revealed that tcb2 single mutants or tcb1, tcb3 double mutants have an altered vacuole morphology and are hypersensitive to cycloheximide. A screen for single-copy suppressors of the cycloheximide sensitivity of tricalbin mutants yielded RSP5, which encodes a C2-domain-containing, ubiquitin-conjugating ligase essential for receptor-mediated and fluid phase endocytosis. The results suggest that the tricalbins function as multimers in membrane-trafficking events and may provide insights into the roles of multi-C2-domain proteins, such as the synaptotagmins, in other organisms.

Calcium-dependent regulation of tumour necrosis factor-alpha receptor signalling by copine

The role of copines in regulating signalling from the TNF-alpha (tumour necrosis factor-alpha) receptor was probed by the expression of a copine dominant-negative construct in HEK293 (human embryonic kidney 293) cells. The construct was found to reduce activation of the transcription factor NF-kappaB (nuclear factor-kappaB) by TNF-alpha. The introduction of calcium into HEK293 cells either through the activation of muscarinic cholinergic receptors or through the application of the ionophore A23187 was found to enhance TNF-alpha-dependent activation of NF-kappaB. This effect of calcium was completely blocked by the copine dominant-negative construct. TNF-alpha was found to greatly enhance the expression of endogenous copine I, and the responsiveness of the TNF-alpha signalling pathway to muscarinic stimulation increased in parallel with the increased copine I expression. The copine dominant-negative construct also inhibited the TNF-alpha-dependent degradation of IkappaB, a regulator of NF-kappaB. All of the effects of the dominant-negative construct could be reversed by overexpression of full-length copine I, suggesting that the construct acts specifically through competitive inhibition of copine. One of the identified targets of copine I is the NEDD8-conjugating enzyme UBC12 (ubiquitin C12), that promotes the degradation of IkappaB through the ubiquitin ligase enzyme complex SCF(betaTrCP). Therefore the copine dominant-negative construct might inhibit TNF-alpha signalling by dysregulation or mislocalization of UBC12. Based on these results, a hypothesis is presented for possible roles of copines in regulating other signalling pathways in animals, plants and protozoa.

Identification of targets for calcium signaling through the copine family of proteins. Characterization of a coiled-coil copine-binding motif

We provide evidence that copines, members of a ubiquitous family of calcium-dependent, membrane-binding proteins, may represent a universal transduction pathway for calcium signaling because we find copines are capable of interacting with a wide variety of "target" proteins including MEK1, protein phosphatase 5, and the CDC42-regulated kinase, that are themselves components of intracellular signaling pathways. The copine target proteins were identified by yeast two-hybrid screening and the interactions were verified in vitro using purified proteins. In the majority of cases the copine binds to a domain of the target protein that is predicted to form a characteristic coiled-coil. A consensus sequence for the coiled-coil copine-binding site was derived and found to have predictive value for identifying new copine targets. We also show that interaction with copines may result in recruitment of target proteins to membrane surfaces and regulation of the enzymatic activities of target proteins.

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

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

Copines: a ubiquitous family of Ca(2+)-dependent phospholipid-binding proteins

The copines are a novel family of ubiquitous Ca(2+)-dependent, phospholipid-binding proteins. They contain two Ca(2+)- and phospholipid-binding domains known as 'C2 domains' present in proteins such as protein kinase C, phospholipase C and synaptotagmin. Copines are thought to be involved in membrane-trafficking phenomena because of their phospholipid-binding properties. They may also be involved in protein-protein interactions since they contain a domain similar to the protein-binding 'A domain' of integrins. The biochemistry, gene structure, tissue distribution and possible biological roles of copines are discussed, including recent observations with Arabidopsis that indicate that copines may be involved in cell division and growth.

Purine composition of the crystalline cytoplasmic inclusions of Paramecium tetraurelia

Crystalline cytoplasmic inclusions were isolated by differential centrifugation from mass cultures of Paramecium tetraurelia feeding on Klebsiella pneumonia. Physical and chemical measurements of intact and solubilized crystals determined that they consist primarily of guanine and hypoxanthine with traces of xanthine. Crystals from the mutant sombre consist primarily of xanthine, suggesting there is a disorder of purine metabolism in this mutant.

Control of the nuclear-cytoplasmic partitioning of annexin II by a nuclear export signal and by p11 binding

This study investigated mechanisms controlling the nuclear-cytoplasmic partitioning of annexin II (AnxII). AnxII and its ligand, p11, were localized by immunofluorescence to the cytoplasmic compartment of U1242MG cells, with minimal AnxII or p11 detected within nuclei. Similarly, GFP-AnxII and GFP-p11 chimeras localized to the endogenous proteins. Likewise, GFP-AnxII(1-22) was excluded from nuclei, whereas GFP-AnxII(23-338) and GFP alone were distributed throughout the cells. Immunoprecipitation and biochemical studies showed that GFP-AnxII did not form heteromeric complexes with endogenous p11 and AnxII. Thus, the AnxII N-tail is necessary and sufficient to cause nuclear exclusion of the GFP fusion protein but this does not involve p11 binding. A nuclear export signal consensus sequence was found in the AnxII 3-12 region. The consensus mutant GFP-AnxII(L10A/L12A) confirmed that these residues are necessary for nuclear exclusion. The nuclear exclusion of GFP-AnxII(1-22) was temperature-dependent and reversible, and the nuclear export inhibitor leptomycin B (LmB) caused GFP-AnxII or overexpressed AnxII monomer to accumulate in nuclei. Therefore, AnxII monomer can enter the nucleus and is actively exported. However, LmB had little effect on the localization of AnxII/p11 complex in U1242MG cells, indicating that the complex is sequestered in the cytoplasm. By contrast, LmB treatment of v-src-transformed fibroblasts caused endogenous AnxII to accumulate in nuclei. The LmB-induced nuclear accumulation of AnxII was accelerated by pervanadate and inhibited by genistein, suggesting that phosphorylation promotes nuclear entry of AnxII. Thus, nuclear exclusion of AnxII results from nuclear export of the monomer and sequestration of AnxII/p11 complex, and may be modulated by phosphorylation.

Domain formation in a fluid mixed lipid bilayer modulated through binding of the C2 protein motif

The role and mechanism of formation of lipid domains in a functional membrane have generally received limited attention. Our approach, based on the hypothesis that thermodynamic coupling between lipid-lipid and protein-lipid interactions can lead to domain formation, uses a combination of an experimental lipid bilayer model system and Monte Carlo computer simulations of a simple model of that system. The experimental system is a fluid bilayer composed of a binary mixture of phosphatidylcholine (PC) and phosphatidylserine (PS), containing 4% of a pyrene-labeled anionic phospholipid. Addition of the C2 protein motif (a structural domain found in proteins implicated in eukaryotic signal transduction and cellular trafficking processes) to the bilayer first increases and then decreases the excimer/monomer ratio of the pyrene fluorescence. We interpret this to mean that protein binding induces anionic lipid domain formation until the anionic lipid becomes saturated with protein. Monte Carlo simulations were performed on a lattice representing the lipid bilayer to which proteins were added. The important parameters are an unlike lipid-lipid interaction term and an experimentally derived preferential protein-lipid interaction term. The simulations support the experimental conclusion and indicate the existence of a maximum in PS domain size as a function of protein concentration. Thus, lipid-protein coupling is a possible mechanism for both lipid and protein clustering on a fluid bilayer. Such domains could be precursors of larger lipid-protein clusters ('rafts'), which could be important in various biological processes such as signal transduction at the level of the cell membrane.

Differential lipid specificities of the repeated domains of annexin IV

The roles of the four domains of annexin IV in binding to phospholipids and glycolipids were assessed by analyzing the binding of a group of mutant annexins IV in which one or more of the four domains was inactivated by replacing a critical amino residue(s) (Asp or Glu) with the neutral residue Ala. The data reveal that individual annexin domains may have characteristic affinities for different lipids. In particular, inactivation of the fourth domain inhibits the binding to phosphatidylserine (PS) and phosphatidylinositol (PI) but not to phosphatidylglycerol (PG), suggesting that this domain specifically can accommodate the larger head groups of PS and PI whereas the other three domains may form more restricted binding pockets. In order to block binding to PG, domain 1, or both domains 2 and 3 must be inactivated in addition to domain 4, suggesting that all four domains may be able to accommodate the headgroup of PG to some extent. Binding to acidic glycolipids (sulfatides) was also sensitive to inactivation of domain 4. However, in the case of sulfatides the nature of the binding reaction is fundamentally different compared with the binding to phospholipids since the interaction with sulfatides was highly sensitive to an increase in ionic strength. The binding to sulfatides may depend therefore on charge-charge interactions whereas the binding to phospholipid may involve a more specific interaction between the lipid headgroup and the protein surface, and/or interaction of the protein with the hydrophobic portion of a lipid bilayer.

Biochemical characterization of copine: a ubiquitous Ca2+-dependent, phospholipid-binding protein

The copines are a novel group of Ca(2+)-dependent, phospholipid-binding proteins first isolated from Paramecium tetraurelia [Creutz, C. E., et al. (1998) J. Biol. Chem. 273, 1393-1402] and found in a wide range of organisms, from plants to humans. They have a Ca(2+) and phospholipid-binding domain consisting of two C2 domains and a core domain in the C-terminal portion that is homologous to the A domain found in certain integrins. We provide here the first description of the properties and distribution of a native mammalian copine, copine I. This protein is expressed in all major adult rat organs as demonstrated by probing Western blots of rat organ homogenates with anticopine antibodies. The highest levels of copine are found in the spleen. A protocol for purifying copine to homogeneity from bovine spleen is described. Purified native copine is a 58 kDa monomer that exhibits Ca(2+) self-association to form higher-order multimers, and Ca(2+)-dependent, phospholipid binding activity with preference for negatively charged phospholipids over neutral phospholipids and selectivity for Ca(2+) over Mg(2+). Half-maximal association with vesicles enriched in phosphatidylserine occurs at Ca(2+) concentrations between 1 and 10 microM. Copine I exhibits Mn(2+) binding activity that is strongly competed by Mg(2+) and partially competed by Ca(2+), suggesting that the copine I A domain may be a functional MIDAS metal binding site similar to that found in integrins [Lee, J. O., et al. (1995) Cell 80, 631-638]. Roles for copine in binding membranes and target proteins or small molecules are discussed.

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

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

Characterization of the Ca2+-dependent binding of annexin IV to surfactant protein A

We have shown previously that surfactant protein A (SP-A) binds to annexin IV in a Ca2+-dependent manner [Sohma, Matsushima, Watanabe, Hattori, Kuroki and Akino (1995) Biochem. J. 312, 175-181]. Annexin IV is a member of the annexin family having four consensus repeats of about 70 amino acids and a unique N-terminal tail. In the present study, the functional site of both annexin IV and SP-A for the Ca2+-dependent binding was investigated using mutant proteins. SP-A bound in a Ca2+-dependent manner to an annexin-IV truncation mutant consisting of the N-terminal domain and the first three domains (T(N-1-2-3)). SP-A also bound to T3-4, but this interaction was not Ca2+-dependent. SP-A bound weakly to the other truncation mutants (T(N-1-2), T(2-3) and T(2-3-4)). Each consensus repeat of annexin IV possesses a conserved acidic amino acid residue (Glu70, Asp142, Glu226 and Asp301) that putatively ligates Ca2+. Using annexin-IV DE mutants in which one, two or three residues out of the four Asp/Glu were altered to Ala by site-directed mutagenesis [Nelson and Creutz (1995) Biochemistry 34, 3121-3132], it was revealed that Ca2+ binding in the third domain is more important than in the other Ca2+-binding sites. SP-A is a member of the animal lectin group homologous with mannose-binding protein A. The substitution of Arg197 of rat SP-A with Asp or Asn eliminated binding to annexin IV, whereas the substitution of Glu195 with Gln was silent. These results suggest that the Ca2+ binding to domain 3 of annexin IV is required for the Ca2+-dependent binding by SP-A and that Arg197 of SP-A is important in this binding.

Transcription, biochemistry and localization of nematode annexins

The transcription of three annexin genes in the nematode, Caenorhabditis elegans, was detected by reverse transcriptase/polymerase chain reaction amplification of messenger RNAs. The highest level of expression was from the nex-1 gene, with lower levels detected for the nex-2 and nex-3 genes. The expression of nex-1 was reduced in the Dauer larval stage relative to the other annexins, correlating with the absence of the spermathecal valves, a major site of nex-1 protein localization. Recombinant nex-1 protein was expressed in yeast, isolated by calcium-dependent binding to acidic phospholipids, and its membrane binding and aggregating activities characterized using bovine chromaffin granules as a representative intracellular substrate. Binding to granule membranes was promoted by calcium with half-maximal binding seen at 630 microM calcium. Chromaffin granule aggregation was similarly promoted by the nex-1 protein at 630 microM calcium. This low sensitivity to calcium suggests the annexin can only be activated in vivo near the plasma membrane or other sources of calcium. Sequences including the nex-1 promoter were fused to the gene for green fluorescent protein and this construct was introduced into nematodes by microinjection. Examination of transgenic offspring revealed specific nex-1 promoter activity in the pharynx, the hypodermal cells, the vulva, and the spermathecal valve, locations in which the annexin may function in collagen secretion/deposition and membrane-membrane interactions. A sensitive anti-nex-1 antibody labelled with rhodamine was injected into body cavities of the nematode but did not detect extracellular nex-1 protein. Therefore, this annexin is apparently cytosolic and may function on the cytoplasmic side of the plasma membrane of the spermathecal valve to chaperon the folding of this membrane during the opening and closing of the valve.

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

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

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

The crystal structure of a calcium-bound form of bovine annexin VI has been determined with X-ray diffraction data to 2.9 A by molecular replacement. Six Ca2+ ions were found, five in AB loops, one in a DE loop. Two loops (II-AB, which binds calcium, and V-AB, which does not) have conformations that differ significantly from those in calcium-free, human recombinant annexin VI. There are only small differences between the calci- and the apo-annexin VI in the rest of the molecule. Calcium by itself does not promote a major conformational change.

The copines, a novel class of C2 domain-containing, calcium-dependent, phospholipid-binding proteins conserved from Paramecium to humans

In an attempt to identify proteins that might underlie membrane trafficking processes in ciliates, calcium-dependent, phospholipid-binding proteins were isolated from extracts of Paramecium tetraurelia. The major protein obtained, named copine, had a mass of 55 kDa, bound phosphatidylserine but not phosphatidylcholine at micromolar levels of calcium but not magnesium, and promoted lipid vesicle aggregation. The sequence of a 920-base pair partial cDNA revealed that copine is a novel protein that contains a C2 domain likely to be responsible for its membrane active properties. Paramecium was found to have two closely related copine genes, CPN1 and CPN2. Current sequence data bases indicate the presence of multiple copine homologs in green plants, nematodes, and humans. The full-length sequences reveal that copines consist of two C2 domains at the N terminus followed by a domain similar to the A domain that mediates interactions between integrins and extracellular ligands. A human homolog, copine I, was expressed in bacteria as a fusion protein with glutathione S-transferase. This recombinant protein exhibited calcium-dependent phospholipid binding properties similar to those of Paramecium copine. An antiserum raised against a fragment of human copine I was used to identify chromobindin 17, a secretory vesicle-binding protein, as a copine. This association with secretory vesicles, as well the general ability of copines to bind phospholipid bilayers in a calcium-dependent manner, suggests that these proteins may function in membrane trafficking.

Calcium-dependent binding of sorcin to the N-terminal domain of synexin (annexin VII)

The annexins are characterized by their ability to bind phospholipid membranes in a Ca2+-dependent manner. Sequence variability between the N-terminal domains of the family members may contribute to the specific cellular function of each annexin. To identify proteins that interact with the N-terminal domain of synexin (annexin VII), a fusion protein was constructed composed of glutathione S-transferase fused to amino acids 1-145 of human synexin. Affinity chromatography using this construct identified sorcin as a Ca2+-dependent synexin-binding protein. Overlay assays confirmed the interaction. The glutathione S-transferase construct associates with recombinant sorcin over the range of pCa2+ = 4.7-3.1 with no binding observed at pCa2+ = 5.4. Overlay assays using deletion constructs of the synexin N-terminal domain mapped the sorcin binding site to the N-terminal 31 amino acids of the synexin protein. Additionally, synexin forms a complex with sorcin and recruits this protein to chromaffin granule membranes in a Ca2+-dependent manner. Sorcin is able to inhibit synexin-mediated chromaffin granule aggregation in a manner saturable with increasing sorcin concentrations, but does not influence the Ca2+ sensitivity of synexin-mediated granule aggregation. Therefore, the interaction between sorcin and synexin may serve to regulate the functions of these proteins on membrane surfaces in a Ca2+-dependent manner.

The crystal structure of annexin VI indicates relative rotation of the two lobes upon membrane binding

The crystal structure of bovine liver annexin VI has been determined to low resolution by molecular replacement. The first lobe (domains 1-4) is rotated about 90 degrees relative to the second lobe (domains 5-8). Since the same crystal form (P4(3), 68 X 68 X 205 A) grew from (NH4)2SO4, polyethylene glycol, and sodium acetate with and without added calcium, this probably reflects the structure in solution. When bound to a lipid monolayer both lobes of annexin VI are coplanar. This implies a significant change in conformation upon binding to membranes.

Calcium-dependent self-association of synaptotagmin I

Synaptotagmin I, an integral membrane protein of secretory vesicles, appears to have an essential role in calcium-triggered hormone and neurotransmitter release. The large cytoplasmic domain of synaptotagmin I has two C2 domains that are thought to mediate calcium and phospholipid binding. A recombinant protein (p65 1-5) comprised of the cytoplasmic domain was previously shown to aggregate purified chromaffin granules and artificial phospholipid vesicles in a calcium-dependent manner. p65 1-5 may be able to aggregate membrane vesicles by a self-association reaction. This hypothesis led us to investigate the ability of synaptotagmin I protein fragments to multimerize in vitro. We found that p65 1-5, in the absence of membranes, was able to self-associate to form large aggregates in a calcium-dependent manner as shown by light-scattering assays and electron microscopy. In addition, a recombinant protein comprised of only the second half of the cytoplasmic domain, including the second C2 domain, was also able to self-associate and aggregate phospholipid vesicles in a calcium-dependent manner. A recombinant protein comprised of only the first C2 domain was not able to self-associate or aggregate vesicles. These results suggest that synaptotagmin I is able to bind calcium in the absence of membranes and that the second half of the cytoplasmic domain is able to bind calcium and mediate its multimerization in a calcium-dependent manner. The ability of synaptotagmin I protein fragments to multimerize in a calcium-dependent manner in vitro suggests that multimerization may have an important function in vivo.

Calcium-dependent binding of the plasma protein apolipoprotein A-I to two members of the annexin family

Affinity chromatography with purified annexins coupled to CNBr-activated Sepharose 4B was used to determine the capacity of proteins found in cytosolic fractions of the bovine adrenal medulla to bind to an immobilized annexin in a Ca2+-dependent manner. Several proteins were eluted from a recombinant annexin I column in the presence of 2 mM EGTA, including protein kinase C (PKC), members of the annexin family, and a 26 kDa protein that appeared as the most prominent band on SDS-PAGE. The identities of PKC, annexin I, annexin IV, annexin VI, and annexin VII were confirmed by Western blotting. The 26 kDa protein was purified by anion exchange chromatography on a Poros Q column and determined to be apolipoprotein A-I (apoA-I) by peptide sequencing. Comigration of apoA-I and chromobindin 2 on two-dimensional gels identified apoA-I as chromobindin 2. Overlay assays were performed to verify the apoA-I-annexin I interaction using apoA-I immobilized on nitrocellulose and annexin I in solution with binding detected using anti-annexin I antiserum. Additionally, the ability of biotin-labeled apoA-I in solution to bind to several purified annexins immobilized on nitrocellulose was determined by detection with horseradish peroxidase-conjugated avidin. Using these methods, it was shown that both annexin I and annexin VII bind to bovine apoA-I in a Ca2+-dependent manner. Other annexins, such as annexin IV and annexin VI, do not exhibit this binding. The results suggest that certain annexins may function as extracellular binding sites for plasma proteins.

Identification, localization, and functional implications of an abundant nematode annexin

Cultures of the nematode C. elegans were examined for the presence of calcium-dependent, phospholipid-binding proteins of the annexin class. A single protein of apparent mass on SDS-polyacrylamide gels of 32 kD was isolated from soluble extracts of nematode cultures on the basis of its ability to bind to phospholipids in a calcium-dependent manner. After verification of the protein as an annexin by peptide sequencing, an antiserum to the protein was prepared and used to isolate a corresponding cDNA from an expression library in phage lambda gt11. The encoded protein, herein referred to as the nex-1 annexin, has a mass of 35 kD and is 36-42% identical in sequence to 10 known mammalian annexins. Several unique modifications were found in the portions of the sequence corresponding to calcium-binding sites. Possible phosphorylation sites in the NH2-terminal domain of the nematode annexin correspond to those of mammalian annexins. The gene for this annexin (nex-1) was physically mapped to chromosome III in the vicinity of the dpy-17 genetic marker. Two other annexin genes (nex-2 and nex-3) were also identified in chromosome III sequences reported by the nematode genomic sequencing project (Sulston, J., Z. Du, K. Thomas, R. Wilson, L. Hillier, R. Staden, N. Halloran, P. Green, J. Thierry-Mieg, L. Qiu, et al. 1992. Nature (Lond.). 356:37-41). The nex-1 annexin was localized in the nematode by immunofluorescence and by electron microscopy using immunogold labeling. The protein is associated with membrane systems of the secretory gland cells of the pharynx, with sites of cuticle formation in the grinder in the pharynx, with yolk granules in oocytes, with the uterine wall and vulva, and with membrane systems in the spermathecal valve. The presence of the annexin in association with the membranes of the spermathecal valve suggests a novel function of the protein in the folding and unfolding of these membranes as eggs pass through the valve. The localizations also indicate roles for the annexin corresponding to those proposed in mammalian systems in membrane trafficking, collagen deposition, and extracellular matrix formation.

Synaptotagmin II expression partially rescues the growth defect of the yeast sec15 secretory mutant

Synaptotagmins are a family of calcium- and phospholipid-binding proteins implicated in the function of cell exocytosis. Synaptotagmins I and II are neurally expressed proteins thought to be involved in neurotransmitter release from neurons. We have expressed rat synaptotagmin II in several Saccharomyces cerevisiae temperature-sensitive secretory mutants that are defective in Golgi to plasma membrane vesicular transport. Synaptotagmin II expression was able to partially rescue the growth defect in one particular mutant, sec15. No suppression was observed when synaptotagmin II was expressed in sec1, sec2, sec4, sec5, sec6, sec8, sec9, sec14, sec17, or sec18. Two synaptotagmin II deletion mutants were also expressed in sec15 and screened for suppression. The expression of the cytoplasmic domain of synaptotagmin alone was not able to suppress the sec15 growth defect. In addition, the expression of a synaptotagmin II fragment lacking the second half of the cytoplasmic domain including the second C2 domain did not suppress sec15. We have isolated a membrane fraction enriched in post-Golgi vesicles from a sec15 strain expressing synaptotagmin II and found that synaptotagmin II co-purifies with this fraction, suggesting that the rat synaptotagmin II is targeted to membranes in yeast. Sec15p forms a large multisubunit protein complex that includes Sec6p and Sec8p. This protein complex is thought to function in a late stage of exocytosis in yeast. Sec6p and Sec8p homologs have been identified in mammalian cells. Our studies suggest that synaptotagmin may be a part of this complex or regulate its function in mammalian cells.

Yeast cysteine proteinase gene ycp1 induces resistance to bleomycin in mammalian cells

Tumor resistance to the glycopeptide anticancer drug bleomycin (BLM) has been suggested to involve metabolic inactivation by BLM hydrolase. Direct evidence for this hypothesis is lacking due to difficulties in obtaining full-length BLM hydrolase cDNA from mammalian cells. In the present investigation, we used the yeast cysteine proteinase gene ycp1, a homologue of the mammalian BLM hydrolase gene, to provide direct evidence of the importance of BLM metabolism in BLM resistance. Transfection of ycp1 into NIH 3T3 cells induced resistance of these cells to BLM. The ycp1-transfected cells also metabolized BLM A2 to its inactive metabolite deamido-BLM A2 to a much greater extent. The ycp1-induced BLM resistance was completely reversed by the cysteine proteinase inhibitor E-64, a known inhibitor of BLM hydrolase. Transfection of NIH 3T3 cells with the plasmid pUT533-Sh ble, a bacterial BLM resistance gene that encodes a 14-kDa protein that does not metabolize BLM, also induced BLM resistance, but this resistance was not overcome by E-64. The results demonstrate that increased BLM hydrolase activity in NIH 3T3 cells causes BLM resistance and that inhibition of BLM metabolism sensitizes these cells to BLM. Thus, the molecular approach described in the present study directly implicates BLM hydrolase in BLM resistance.

Comparison of the expression of native and mutant bovine annexin IV in Escherichia coli using four different expression systems

Bovine annexin IV, a Ca(2+)-dependent, membrane-binding protein, was expressed in E. coli using four different prokaryotic expression vector systems. An annexin IV cDNA was mutated in the 5' noncoding region to introduce an NcoI restriction site at the translation initiation site. The coding sequence was then excised and ligated into the expression vectors: pKK233-2 (which uses a hybrid trc promoter), pFOG405 (which uses the alkaline phosphatase promoter and generates a fusion protein with the alkaline phosphatase signal sequence that targets the protein for secretion), pOTSNco12 (which provides temperature-sensitive expression from the lambda phage promoter), and pET11d (which uses the T7lac promoter and a protease-deficient host). Expression of wild type and mutant annexin IV in the various systems was compared. Differences in level of expression, formation of inclusion bodies, and yield of purified protein were observed. The pET11d system was found to be the most effective expression system for annexin IV and various annexin IV mutant constructs, providing the highest yield of functional protein from the soluble fraction of cell lysates. Bovine chromaffin granule binding and aggregating activities of recombinant annexin IV were found to be virtually indistinguishable from those of bovine annexin IV isolated from liver tissue. Truncation constructs containing one, two, or three of the four conserved 70-amino-acid domains of native annexin IV were successfully created and expressed in E. coli, but the recombinant proteins were generally insoluble. pET11d annexin constructs containing point mutations in residues involved in binding calcium produced soluble protein at levels comparable to those of constructs expressing wild type protein.

Combinatorial mutagenesis of the four domains of annexin IV: effects on chromaffin granule binding and aggregating activities

This study addresses the roles of individual annexin IV domains in calcium-dependent membrane binding and aggregation through an analysis of the activities of mutant annexin IV proteins in which critical residues in one or more domains have been altered. The consensus sequence for high-affinity Ca(2+)-binding pockets obtained from the annexin V crystal structure is GXGT-38 residues-D/E [Huber, R., et al. (1992) J. Mol. Biol. 223, 683-704]. Site-directed mutagenesis was used to change the conserved acidic residues (D/E) in these sequences to alanine residues in each of the four domains of bovine annexin IV, singly or in combinations. Fourteen mutants with one, two, three, or four mutated domains were constructed and expressed in Escherichia coli. Purified recombinant product was evaluated for Ca(2+)-dependent binding to and aggregation of bovine chromaffin granules. Increases in the number of mutated domains resulted in increased Ca2+ requirements for both granule binding and aggregation. Further analysis revealed that mutations in individual domains had preferential effects on the binding or aggregating activities of the protein. For example, mutation of the first or fourth domains had a greater effect on membrane binding than aggregation, while conversely, mutation of the second domain had a more dramatic effect on membrane aggregation. Mutation of the third domain was largely silent in these assays. An additional mutation was made in the fourth domain to substitute a serine for a highly conserved arginine residue (Arg274) present at the C-terminus of the fourth endonexin fold. This mutation increased the calcium requirement for membrane binding 2-fold and for membrane aggregation 3-fold. This mutant protein was found to be an effective inhibitor of membrane aggregation by native annexin IV at intermediate levels of calcium.

Identification of the major chromaffin granule-binding protein, chromobindin A, as the cytosolic chaperonin CCT (chaperonin containing TCP-1)

Chromobindin A is a multisubunit complex ATPase that binds to chromaffin granule membranes in a calcium-dependent manner and requires ATP for release from the membrane (Martin, W. H., and Creutz, C. E. (1987) J. Biol. Chem. 262, 2803-2810). Here we report that the seven previously characterized subunits of chromobindin A cross react with antisera specific to subunits of CCT, the chaperonin containing TCP-1 (Kubota, H., Hynes, G., Carne, A., Ashworth, A., and Willison, K. (1994) Curr. Biol. 4, 89-99). The chromobindin A subunits previously called chromobindins 12, 13, 14, 15, 16, 18, and 19 cross-react specifically with subunits beta, delta, theta, alpha, zeta, xi, and gamma, respectively, of CCT. Additional similarities in subunit molecular weights, isoelectric points, and the morphologies of the two protein complexes as determined by electron microscopy support identification of chromobindin A as an adrenal medullary form of CCT. The chromobindin A/CCT complex was found to bind at least 7-fold more efficiently to affinity columns of chromaffin granule membranes than of adrenal medullary cytosol proteins, suggesting a specific interaction occurs between the complex and membrane components. The results indicate that the previously described characteristics of chromobindin A are likely to be relevant to the functions of CCT and suggest that the adrenal medullary form of CCT may play a role in the activities of secretory vesicle membranes.

Synergistic membrane interactions of the two C2 domains of synaptotagmin

Synaptotagmin, an integral membrane protein localized to secretory vesicles, has been implicated in the docking and fusion steps in calcium-regulated exocytosis. The large cytoplasmic domain contains two C2 motifs, each similar to the Ca2+ and phospholipid binding domain of protein kinase C. To study the membrane binding and aggregating properties of these C2 domains, three recombinant fragments of rat synaptotagmin I were expressed in Escherichia coli and purified. A recombinant protein containing both C2 domains (p65 1-5) was found to bind to and aggregate bovine chromaffin granules in a calcium-dependent manner, with half-maximal binding and aggregation occurring at approximately p Ca2+ = 4.2. However, recombinant proteins containing either the first (p65 1-3) or second (p65 3-5) C2 domain alone were not able to bind to the granules, indicating that both C2 domains are required for binding to chromaffin granules. p65 1-5 also bound to and aggregated liposomes made from chromaffin granule lipid extracts, as well as granules treated extensively with trypsin, suggesting that p65 1-5 binding to granules is mediated by the lipids in the granule membrane and not the granule membrane proteins. Although p65 1-3 and p65 3-5 did not bind to granules or lipids extracted from granules, both did bind to phosphatidylserine (PS)/phosphatidylcholine (PC) vesicles (10%-40%PS). Half-maximal binding of p65 1-3 to vesicles occurred at approximately p Ca2+ = 5.2, while p65 3-5 appeared to bind independently of calcium over the range of pCa2+ = 5.5-2.8. p65 1-5 exhibited binding to PS/PC vesicles with characteristics of both the smaller proteins, displaying some binding in EGTA and increased binding in calcium. Larger amounts of p65 1-5 bound to PS/PC vesicles than of either of the smaller fragments. These results suggest that the two C2 domains of synaptotagmin act synergistically to promote binding to biological membranes and to affect calcium sensitivity and membrane binding capacity.

Ca(2+)-dependent binding of endonexin (annexin IV) to membranes: analysis of the effects of membrane lipid composition and development of a predictive model for the binding interaction

Endonexin (annexin IV) is a member of the annexin family of homologous proteins that bind membranes and aggregate vesicles in a calcium-dependent fashion. This study examines the lipid modulation and mechanism of the binding of endonexin to membranes using a fluorescence energy transfer assay to measure bovine endonexin binding to well-defined large unilamellar vesicles. The calcium sensitivity for endonexin-membrane binding is observed to be highly dependent on the types of membrane lipids present. As with most annexins, negatively charged lipids best promote endonexin binding to phosphatidylcholine (PC) containing membranes. However, a comparison of 11 different types of lipids reveals that other factors such as the type of ion contributing the charge and head-group size are also important. The concentrations of calcium required for half-maximal binding of endonexin to PC vesicles containing 30% phosphatidylserine (PS) or 30% phosphatidylinositol (PI), both lipids with net charge-1, are 48 +/- 6 and 114 +/- 19 microM, respectively, while half-maximal binding to 30% phosphatidylinositol bisphosphate (PIP2), with a greater net charge of -3 to -5, occurs at 65 microM calcium, similar to the calcium requirement for binding to PS. The apparent affinities of endonexin for seven different types of lipids parallel those reported for annexin V [Andree, H. A. M., Reutelingsperger, C. P. M., Hauptmann, R., Hemker, H. C., Hermans, W. T., & Willems, G. M. (1990) J. Biol. Chem. 265, 4923-4928], except for a greater preference of endonexin for membranes containing phosphatidic acid. Mixing PS and phosphatidylethanolamine (PE) or PS and PI in the same PC vesicle synergistically enhances endonexin-membrane binding, indicating that even lipids with no net charge such as PE may dramatically affect endonexin binding to mixed-lipid membranes. The maximum amount of endonexin able to bind to PS/PC vesicles at 1 mM calcium increases with mole % PS. A simple and general model that treats protein-membrane binding as a two-step process, with adsorption to a membrane surface followed by interaction with specific lipid molecules [Lentz, B. R., & Hermans, J. (1989) Biochemistry 28, 7459-7461], is extended to include the coupled binding of calcium with binding of specific lipid molecules. This extended model accurately predicts trends observed when protein and calcium titrations of endonexin binding to PS/PC vesicles are performed under a wide variety of conditions and suggests that 3-5 calcium ions and 9-18 PS molecules participate in each endonexin-membrane complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Annexin IV reduces the rate of lateral lipid diffusion and changes the fluid phase structure of the lipid bilayer when it binds to negatively charged membranes in the presence of calcium

Bovine annexin IV (endonexin) was bound to supported planar bilayers composed of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) in the first monolayer facing the substrate, and varying mole fractions of POPC, 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG) and small amounts of the fluorescent lipid analogs NBD-PC or NBD-PG in the second monolayer facing the large aqueous compartment. Lateral diffusion coefficients and mobile fractions of these phospholipids were measured by fluorescence recovery after photobleaching (FRAP) as a function of protein concentration and lipid composition in the presence of 2 mM CaCl2 or 1 mM EDTA. In the absence of annexin IV, the lateral diffusion coefficients depended only little on the POPC:POPG ratios and were approximately 3.0 microns2/s for NBD-PG (no Ca2+), 2.5 microns2/s for NBD-PG (2 mM Ca2+), and 1.6 microns2/s for NBD-PC (with or without 2 mM Ca2+). In the presence of 2 mM Ca2+ these diffusion coefficients decreased as a function of the added annexin concentration. A transition from a state with "rapid" lipid diffusion to a state with "slow" lipid diffusion occurred at about 80 nM annexin IV and was independent of the POPC:POPG ratio. In addition to reducing the lipid lateral diffusion coefficients, annexin IV also gave rise to two-component lateral diffusion of the lipids in these mixed bilayers. The split of the single diffusion coefficient of NBD-PG into two components occurred at most POPC:POPG ratios upon binding of annexin IV, but required higher annexin concentrations at mole fractions of POPC between 66 and 82 mol % than at high mole fractions of POPG or 90 mol % POPC.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the amino-terminal domain in regulating interactions of annexin I with membranes: effects of amino-terminal truncation and mutagenesis of the phosphorylation sites

Phosphorylation of the N-terminal tail by protein kinase C strongly inhibits the ability of bovine or human annexin I to aggregate chromaffin granules by increasing the calcium requirement 4-fold (Wang, W., & Creutz, C. E. (1992) Biochemistry 31, 9934-9936). In the present study three forms of human annexin I truncated in the amino terminus at residue Trp-12, Lys-26, or Lys-29 exhibit dramatic differences in their sensitivities to calcium in a chromaffin granule aggregation assay, while the [Ca2+](1/2)max values for binding of the truncated proteins to granule membranes are similar. Cleavage at Trp-12 causes a 3-fold decrease in calcium sensitivity in the membrane aggregation assay, while cleavage at Lys-26 causes a 4-fold enhancement of calcium sensitivity. In contrast, cleavage at Lys-29 results in virtually no change in calcium sensitivity. Mutagenic substitution with negatively charged amino acids of Ser-27, a site for phosphorylation by protein kinase C, or Tyr-21, a site for phosphorylation by the epidermal growth factor receptor kinase, mimics the inhibition of granule-aggregating activity seen with phosphorylation by protein kinase C. When bovine chromaffin cells are stimulated to secrete by nicotine, annexin I is phosphorylated in the amino terminus. Thr-24 and Ser-28, which are sites for phosphorylation by protein kinase C in vitro, are two of the sites phosphorylated in vivo in stimulated chromaffin cells. These data demonstrate that the ability of annexin I to promote membrane aggregation is highly sensitive to changes in the structure of the N-terminal domain of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)