Conserved charged residues in the leucine-rich repeat domain of the Ran GTPase activating protein are required for Ran binding and GTPase activation

GTPase activating proteins (GAPs) for Ran, a Ras-related GTPase participating in nucleocytoplasmic transport, have been identified in different species ranging from yeast to man. All RanGAPs are characterized by a conserved domain consisting of eight leucine-rich repeats (LRRs) interrupted at two positions by so-called separating regions, the latter being unique for RanGAPs within the family of LRR proteins. The cytosolic RanGAP activity is essential for the Ran GTPase cycle which in turn provides directionality in nucleocytoplasmic transport, but the structural basis for the interaction between Ran and its GAP has not been elucidated. In order to gain a better understanding of this interaction we generated a number of mutant RanGAPs carrying amino acid substitutions in the LRR domain and analysed their complex formation with Ran as well as their ability to stimulate the intrinsic GTPase activity of the G protein. We show that conserved charged residues present in the separating regions of the LRR domain are indispensable for efficient Ran binding and GAP activity. These separating regions contain three conserved arginines which could possibly serve as catalytic residues similar to the arginine fingers identified in GAPs for other small GTPases. However, mutations in two of these arginines do not affect the GAP activity and replacement of the third conserved arginine (Arg91 in human RanGAP) severely interferes not only with GAP activity but also with Ran binding. This indicates that RanGAP-stimulated GTP hydrolysis on Ran does not involve a catalytic arginine residue but requires certain charged residues of the LRR domain of the GAP for mediating the protein-protein interaction.

A comparison of the energetics of annexin I and annexin V

The annexins comprise a family of soluble Ca2+- and phospholipid-binding proteins. Although highly similar in three-dimensional structure, different annexins are likely to exhibit different biochemical and functional properties and to play different roles in various membrane related events. Since it must be expected that these functional differences arise from differences in the characteristic thermodynamic parameters of these proteins, we performed high-sensitivity differential scanning microcalorimetry (DSC) and isothermal guanidinium hydrochloride (GdnHCl)-induced unfolding studies on annexin I and compared its thermodynamic parameters with those of annexin V published previously. The DSC data were analyzed using a model that permits quantitative treatment of the irreversible reaction. It turned out, however, that provided a heating rate of 2 K min-1 is used, unfolding of annexin I can be described satisfactorily in terms of a simple two-state reaction. At pH 6.0 annexin I is characterized by the following thermodynamic parameters: t1/2=61.8 degrees C, DeltaHcal=824 kJ mol-1 and DeltaCp=19 kJ mol-1 K-1. These parameters result in a stability value of DeltaG0D (20 degrees C)=51 kJ mol-1. The GdnHCl induced isothermal unfolding of annexin I in Mes buffer (pH 6.0), yielded DeltaG0D (buffer) values of 48, 60 and 36 kJ mol-1 at 20, 12 and 5 degrees C, respectively. These DeltaG0D values are in reasonable agreement with the values obtained from the DSC studies. The comparison of annexin I and annexin V under identical conditions (pH 8.0 or pH 6.0) shows that despite the pronounced structural homology of these two members of the annexin familiy, the stability parameters are remarkably different. This difference in stability is consistent with and provides a thermodynamic basis for the potential different in vivo functions proposed for these two annexins.

Role of the C-terminal extension in the interaction of S100A1 with GFAP, tubulin, the S100A1- and S100B-inhibitory peptide, TRTK-12, and a peptide derived from p53, and the S100A1 inhibitory effect on GFAP polymerization

Whereas native and recombinant S100A1 inhibited GFAP assembly, a truncated S100A1 lacking the last six C-terminal residues (Phe88-Ser93) (S100A1Delta88-93) proved unable to do so. The inhibitory effects of native and recombinant S100A1 on GFAP assembly were blocked by both TRTK-12, a synthetic peptide derived from the alpha-subunit of the actin capping protein, CapZ, and a synthetic peptide derived from the tumor-suppressor protein, p53, in a dose-dependent manner. By fluorescent spectroscopy, TRTK-12 and the p53 peptide, like GFAP and tubulin, caused a dose- and Ca2+-dependent blue-shift of the fluorescence maximum of acrylodan-S100A1. In contrast, GFAP, tubulin, TRTK-12, or the p53 peptide caused no significant changes in the fluorescence spectrum of acrylodan-S100A1Delta88-93. By chemical crosslinking, both TRTK-12 and the p53 peptide strongly reduced or blocked the formation of GFAP-S100A1 or tubulin-S100A1 complexes, respectively, and S100A1Delta88-93 was unable to complex with tubulin, whereas a remarkably reduced complexation of GFAP with the truncated protein was observed. All the above observations show that the C-terminal extension of S100A1 is an essential part of the S100A1 site implicated in the recognition of GFAP, tubulin, p53, and the alpha-subunit of CapZ.

The crystal structure of a complex of p11 with the annexin II N-terminal peptide

The aggregation and membrane fusion properties of annexin II are modulated by the association with a regulatory light chain called p11.p11 is a member of the S100 EF-hand protein family, which is unique in having lost its calcium-binding properties. We report the first structure of a complex between p11 and its cognate peptide, the N-terminus of annexin II, as well as that of p11 alone. The basic unit for p11 is a tight, non-covalent dimer. In the complex, each annexin II peptide forms hydrophobic interactions with both p11 monomers, thus providing a structural basis for high affinity interactions between an S100 protein and its target sequence. Finally, p11 forms a disulfide-linked tetramer in both types of crystals thus suggesting a model for an oxidized form of other S100 proteins that have been found in the extracellular milieu.

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.

Ca2+-independent interaction of annexin I with phospholipid monolayers

At pH 6.0, the interaction of annexin I, a proteolytic fragment of annexin I and annexin V, was studied with monolayers composed of dipalmitoylphosphatidylserine (DPPS), dipalmitoylphosphatidylcholine (DPPC) or DPPS/DPPC mixtures (molar ratio 1:4). The measurements reveal that only annexin I shows a significant increase in the surface pressure at constant surface area in the absence of Ca2+ ions. We interpret these pressure changes as reflecting penetration of the protein. Kinetic analyses of the annexin I/monolayer interaction at pH 6.0 in the presence and absence of Ca2+ ions show differences between the interaction mechanisms that support the occurrence of a pH-regulated process. At pH 7.4, Ca2+ ions are required for the interaction.

Characterization and developmentally regulated expression of four annexins in the killifish medaka

Annexins are Ca2+-regulated membrane binding proteins implicated in a wide range of membrane-related and signal transduction events, including the endocytosis of membrane receptors and Ca2+-regulated as well as constitutive secretion. To date, 10 unique members of this multigene family have been identified in a variety of cell types and tissues of higher vertebrates, with different members showing distinct tissue distributions in the adult organisms. To establish whether annexins also function in embryonic development, we analyzed the expression pattern during vertebrate morphogenesis using the medaka fish Oryzias latipes as a model system. From a larval medaka cDNA library, we isolated four types of clones, which were shown by sequence analysis to encode four different annexins (herein referred to as max 1-4). A comparison with known annexin sequences in the databases revealed that two medaka annexins (max 1 and 2) are highly similar in sequence to mammalian annexins V and IV, respectively, whereas the other two medaka annexins (max 3 and 4) are probably novel members of the family most closely related to mammalian annexins I and XI. Using whole-mount RNA in situ hybridization, we showed that the expression of the different medaka annexins during embryogenesis was strictly regulated at both the spatial and the temporal level. High levels of max 1, 2, and 3 transcripts were present in the developing stomach, gut, liver, air-bladder, and rectum during somitogenesis, thus identifying the digestive tract as the prime region of annexin expression. Interestingly, two structures playing crucial roles in neuronal patterning showed a distinct expression of annexins. The mesendoderm of the anterior prechordal plate of neurula-stage embryos was a site of max 4 transcription, and the floor plate of somitogenesis-stage embryos showed expression of max 2 and 3 to differing rostrocaudal extends along the brain and spinal cord. These results suggest specific functions of different annexins during vertebrate morphogenesis.

Disruption of endothelial microfilaments selectively reduces the transendothelial migration of monocytes

The transendothelial migration of leukocytes (diapedesis) is a central event in inflammatory and immunological processes. Although leukocyte-endothelium interactions occurring during diapedesis have been investigated intensively, little is known about the actual transmigration and the molecular mechanisms involved. Toward this end we analyzed whether the endothelial cytoskeleton plays a direct role during the transendothelial migration of monocytes. Filter-grown monolayers of human microvascular endothelial cells (HMEC-1) were treated with cytoskeleton stabilizing or destabilizing drugs and the effect of this treatment on the transmigration of peripheral blood monocytes was analyzed in a two-chamber assay. Our results show that taxol-induced stabilization of microtubules causes a reduction of leukocyte transmigration through HMEC-1, while the opposite effect is induced by the destabilization of microtubules with colchicine or nocodazol. Disruption of microfilaments with cytochalasin B or latrunculin A, on the other hand, significantly reduces the transendothelial migration although monocyte adhesion and endothelial permeability for macromolecules are slightly increased. An active participation of the endothelial microfilament system with a direct role of unconventional, calmodulin-regulated myosins is suggested by the finding that monocyte transmigration is decreased upon treatment of the endothelial cells with the Ca2+/CaM antagonist triflouperazine.

Hydrophobic residues in the C-terminal region of S100A1 are essential for target protein binding but not for dimerization

S100 proteins are a family of small dimeric proteins characterized by two EF hand type Ca2+ binding motifs which are flanked by unique N- and C-terminal regions. Although shown unequivocally in only a few cases S100 proteins are thought to function by binding to, and thereby regulating, cellular target proteins in a Ca2+ dependent manner. To describe for one member of the family, S100A1, structural requirements underlying target protein binding, we generated specifically mutated S100A1 derivatives and characterized their interaction with the alpha subunit of the actin capping protein CapZ shown here to represent a direct binding partner for S100A1. Chemical cross-linking, ligand blotting and fluorescence emission spectroscopy reveal that removal of, or mutations within, the sequence encompassing residues 88-90 in the unique C-terminal region of S100A1 interfere with binding to CapZ alpha and to TRTK-12, a synthetic CapZ alpha peptide. The S100A1 sequence identified contains a cluster of three hydrophobic residues (Phe-88, Phe-89 and Trp-90) at least one of which--as revealed by qualitative phenyl Sepharose binding and hydrophobic fluorescent probe spectroscopy--is exposed on the protein surface of Ca2+ bound S100A1. As homologous hydrophobic residues in the closely related S100B protein were shown by NMR spectroscopy of Ca(2+)-free S100B dimers to provide intersubunit contacts [Kilby P.M., van Eldik L.J., Roberts G.C.K. The solution structure of the bovine S100B dimer in the calcium-free state. Structure 1996; 4: 1041-1052; Drohat A.C., Amburgey J.C., Abildgaard F., Starich M.R., Baldisseri D., Weber D.J. Solution structure of rat apo-S100B (beta beta) as determined by NMR spectroscopy. Biochemistry 1996; 35: 11,577-11,588], we characterized the physical state of the various S100A1 derivatives. Analytical gel filtration and chemical cross-linking show that dimer formation is not compromised in S100A1 mutants lacking residues 88-90 or containing specific amino acid substitutions in this sequence. Thus a cluster of hydrophobic residues in the C-terminal region of S100A1 is essential for target protein binding but dispensable for dimerization, a situation possibly met in other S100 proteins as well.

Differential expression of annexins I, II and IV in human tissues: an immunohistochemical study

Annexins constitute a family of Ca2+- and phospholipid-binding proteins. Although their functions are still not clearly defined, several members of the annexin family have been implicated in membrane-related events along exocytotic and endocytotic pathways. To elucidate a possible correlation of those functional proposals with the tissue distribution of annexins, we analysed immunohistochemically the expression of annexins I, II and IV in a broad variety of human tissues. Annexins I and II were chosen for this study since their functionally relevant N-terminal domains are structurally closely related, whilst annexin IV is structurally less related to the former two proteins. The study revealed distinct expression patterns of annexins I, II and IV throughout the body. Annexin I was found in leucocytes of peripheral blood, tissue macrophages and T-lymphocytes and in certain epithelial cells (respiratory and urinary system, superficial cells of non-keratinised squamous epithelium), annexin II in endothelial cells, myoepithelial cells and certain epithelial cells (mainly respiratory and urinary system), whereas annexin IV was almost exclusively found in epithelial cells. Epithelia of the upper respiratory system, Bowman's capsule, urothelial cells, mesothelial cells, peripheral nerves, the choroid plexus, ependymal cells and pia mater and arachnoid of meninges generally strongly expressed all three annexins investigated. The characteristic expression in different tissues and the intracellular distribution indicates that the three annexins investigated are involved in aspects of differentiation and/or physiological functions specific to these tissues.

The annexin II-p11 complex is involved in regulated exocytosis in bovine pulmonary artery endothelial cells

Annexin II is a member of a multigene family of Ca2+-regulated, membrane-binding proteins implicated through biochemical and perforated cell experiments in Ca2+-triggered secretion. Within most cells annexin II resides in a tight heterotetrameric complex with a cellular protein ligand, p11, and complex formation is mediated via the N-terminal 14 residues of annexin II including the N-terminal acetyl group. To analyze at the single cell level whether the annexin II-p11 complex is involved in regulated secretion, we used membrane capacitance measurements to follow exocytotic fusion events in bovine aortic endothelial cells manipulated with respect to their annexin II-p11 complex formation. Upon guanosine 5'-O-(thiotriphosphate) (GTPgammaS) stimulation, the endothelial cells show a significant increase in membrane capacitance which is generally preceded by a transient rise in intracellular Ca2+ and thus indicative of the occurrence of Ca2+-regulated secretion. The GTPgammaS-induced capacitance increase is markedly reduced in cells loaded with a synthetic peptide, Ac1-14, which corresponds in sequence to the N-terminal 14 residues of annexin II in their correctly acetylated form and which is capable of disrupting preformed annexin II-p11 complexes. The effect of the peptide is highly specific as the nonacetylated variant, N1-14, which is incapable of disrupting annexin II-p11, does not interfere with the GTPgammaS-induced increase in membrane capacitance. These data show that intact annexin II-p11 complexes are indispensable for regulated exocytosis to occur in an efficient manner in endothelial cells.

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).

The acidic C-terminal domain of rna1p is required for the binding of Ran.GTP and for RanGAP activity

The small GTP binding protein Ran is an essential component of the nuclear protein import machinery whose GTPase cycle is regulated by the nuclear guanosine nucleotide exchange factor RCC1 and by the cytosolic GTPase activating protein RanGAP. In the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae the RanGAP activity is encoded by the RNA1 genes which are essential for cell viability and nucleocytoplasmic transport in vivo. Although of limited sequence identity the two yeast proteins show a conserved structural organization characterized by an N-terminal domain of eight leucine-rich repeats, motifs implicated in protein-protein interactions, and a C-terminal domain rich in acidic amino acid residues. By analyzing the RanGAP activity of a series of recombinantly expressed rna1p mutant derivatives, we show that the highly acidic sequence in the C-terminal domain of both yeast proteins is indispensable for activating Ran-mediated GTP hydrolysis. Chemical cross-linking reveals that the same sequence in rna1p is required for rna1p.Ran complex formation indicating that the loss of GAP activity in the C-terminally truncated rna1p mutants results from an impaired interaction with Ran. The predominant species stabilized through the covalent cross-link is a rna1p.Ran heterodimer whose formation requires the GTP-bound conformation of Ran. As the acidic C-terminal domain of rna1p is required for establishing the interaction with Ran, the leucine-rich repeats domain in rna1p is potentially available for additional protein interactions perhaps required for directing a fraction of rna1p to the nuclear pore.

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

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

Annexin I targets S100C to early endosomes

Immunofluorescence and subcellular fractionation localize annexin I and the EF hand protein S100C to the same membranous structures which in part correspond to transferrin receptor-positive endosomes. The association of S100C with endosomal membranes is strictly dependent on annexin I binding since a D91stop-S100C mutant protein, in which the residues essential for annexin I binding have been removed, fails to colocalize with membraneous structures. This indicates that annexin I and S100C form a complex in vivo and that the endosomal localization of this complex is mediated through an interaction of annexin I with the endosomal membrane.

Localization of five annexins in J774 macrophages and on isolated phagosomes

Annexins are a family of structurally related proteins which bind phospholipids in a calcium-dependent manner. Although the precise functions of annexins are unknown, there is an accumulating set of data arguing for a role for some of them in vesicular transport and, specifically, in membrane-membrane or membrane-cytoskeletal interactions during these processes. Here we describe our qualitative and quantitative analysis of the localization of annexins I-V in J774 macrophages that had internalized latex beads, both with and without IgG opsonization. Our results show that whereas all these annexins are present on both the plasma membrane and on phagosomes, the localization on other organelles differs. Annexins I, II, III and V were detected on early endosomes, while only annexin V was seen on late endocytic organelles and mitochondria. Annexins I and II distributed along the plasma membrane non-uniformly and co-localized with F-actin at the sites of membrane protrusions. We also investigated by western blot analysis the association of annexins with purified phagosomes isolated at different time-points after latex bead internalization. While the amounts of annexins I, II, III and V associated with phagosomes were similar at all times after their formation, the level of annexin IV was significantly higher on older phagosomes. Whereas annexins I, II, IV and V could be removed from phagosome membranes with a Ca2+ chelator they remained membrane bound under low calcium conditions. In contrast, annexin III was removed under these conditions and needed a relatively high Ca2+ concentration to remain phagosome bound. Because of their purity and ease of preparation we suggest that phagosomes are a powerful system to study the potential role of annexins in membrane traffic.

Modification of annexin II expression in PC12 cell lines does not affect Ca(2+)-dependent exocytosis

The Ca2+/phospholipid/cytoskeletal-binding protein annexin II has been proposed to play an important role in Ca(2+)-dependent exocytosis; however, the evidence for this role is inconclusive. More direct evidence obtained by manipulating annexin II levels in cells is still required. We have attempted to do this by generating stably transfected PC12 cell lines expressing proteins which elevate or lower functional annexin II levels and using these cell lines to investigate Ca(2+)-dependent exocytosis. Three cell lines were generated: one expressing an annexin II mutant which aggregates annexin II in at least a proportion of the cells, thereby removing functional protein from the cell; a mixed clonal cell line constitutively overexpressing human annexin II; and a clonal cell line capable of over-expressing annexin II in the presence of sodium butyrate. After digitonin permeabilization, Ca(2+)-dependent dopamine release from these cell lines was compared with that from control nontransfected cells, and, in addition, release was compared in induced to uninduced cells. There were no significant differences in Ca(2+)-dependent exocytosis between any of the transfected cell lines before or after induction and the control cells. In addition, nontransfected PC12 cells treated with nerve growth factor, which elevates annexin II levels severalfold, failed to increase Ca(2+)-dependent exocytosis after digitonin permeabilization, compared with control cells. We conclude that annexin II is not an important regulator of Ca(2+)-dependent exocytosis in PC12 cells.

Differential expression of the calpactin I subunits annexin II and p11 in cultured keratinocytes and during wound repair

Transforming growth factor beta1 (TGF-beta1) is an important modulator of skin morphogenesis and cutaneous wound repair. To gain insight into the mechanisms of TGF-beta1 action in the skin, we used the differential display RT-PCR technique to identify genes that are regulated by this factor in cultured human keratinocytes. We obtained several partial cDNA clones. One of them was identical to the 3'-end of p11, the small and regulatory subunit of the calpactin I complex [(annexin II)2(p11)2]. RNase protection and northern blot analysis revealed specific regulation of expression of both subunits of this heterotetrameric protein (p11 and annexin II) by TGF-beta1 as well as by other growth factors, although the time course and degree of induction or suppression were different for each gene. Furthermore, we analyzed p11 and annexin II expression in normal and wounded skin. Both p11 and annexin II mRNAs were found in the dermal and epidermal compartments of normal human skin. Immunohistochemical studies demonstrated the presence of p11 at equally high levels in all layers of normal epidermis and in the hyper-proliferative epithelium at the wound edge. By contrast, annexin II expression was high in the basal layer of normal epidermis but low in the suprabasal layers and in the hyper-proliferative epithelium at the wound edge, suggesting a differentiation-specific regulation of this calpactin I subunit. The differential expression and regulation of p11 and annexin II subunits in keratinocytes suggest the existence of different ratios of monomeric versus p11-complexed annexin II that might be associated with different cellular functions.

Identification and characterization of a novel type of annexin-membrane interaction: Ca2+ is not required for the association of annexin II with early endosomes

Annexin II, a member of a family of Ca2+ and membrane binding proteins, has been implicated in regulating membrane organization and membrane transport during endocytosis and Ca2+ regulated secretion. To characterize the mechanistic aspects of the annexin. II action we studied parameters which determine the endosomal association of annexin II. Immunoblot analysis of subcellular membrane fractions prepared from BHK cells in the presence of a Ca2+ chelating agent reveals that annexin II remains associated with endosomal membranes under such conditions. This annexin II behaviour is atypical for the Ca2+ regulated annexins and is corroborated by the finding that ectopically expressed annexin II mutants with inactivated Ca2+ binding sites continue to co-fractionate with endosomal membranes. The Ca(2+)-independent membrane association of annexin II is also not affected by introducing mutations interfering with the complex formation of annexin II with its intracellular protein ligand p11. However, a deletion of the unique N-terminal domain of annexin II, in particular the sequence spanning residues 15 to 24, abolishes the Ca(2+)-independent association of the protein with endosomes. These results describe a novel, Ca(2+)-independent type of annexin-membrane interaction and provide a first explanation for the observed preference of different annexins for different cellular membranes. In the case of annexin II this specificity could be mediated through specific membrane receptors interacting with a unique sequence in the annexin II molecule.

Annexin II modulates volume-activated chloride currents in vascular endothelial cells

The membrane-associated, microfilament-binding protein annexin II is abundantly expressed in endothelial cells from calf pulmonary artery (CPAE cells). We have analyzed its role in the regulation of volume-activated chloride currents (ICl, vol) by loading the cells via the patch pipette with a peptide comprising the N-terminal 14 residues of annexin II. This sequence harbors the binding site for the intracellular annexin II ligand, p11, and the peptide interferes with the annexin II-p11 complex formation. Loading of a CPAE cell with this peptide caused a gradual decrease in the amplitude of ICl, vol during repetitive stimulations with a 28% hypotonic extracellular solution. This run down of the current was virtually absent in untreated cells and in cells that were loaded with a mutated 14-amino acid peptide, which has a single amino acid replacement known to result in a more than 1000 times reduced affinity for binding to p11. We conclude that annexin II-p11 complex formation is either directly or indirectly involved in the activation of ICl, vol in endothelial cells.

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.

Structural requirements for annexin I-S100C complex-formation

S100C is a member of the S100 family of EF-hand-type Ca(2+)-binding proteins which are thought to bind to and thereby regulate the activity of cellular target proteins in a Ca(2+)-dependent manner. An intracellular ligand for S100C is the Ca2+/phospholipid-binding protein annexin I and we show here that complex-formation is mediated through unique domains within S100C and annexin I. Using a proteolytically truncated annexin I derivative as well as a number of N-terminal annexin I peptides in liposome co-pelleting and ligand-blotting assays we map the S100C-binding site to the N-terminal 13 residues of annexin I. Similar analyses employing recombinantly expressed S100C mutants reveal that residues D91 to 194 in the unique C-terminal extension of this S100 protein are indispensable for annexin I binding. Interaction between S100C and an N-terminal annexin I peptide containing a tryptoplan at position 11 can also be monitored by fluorescence emission spectroscopy after tryptophan excitation. This analysis indicates that the local environment of the tryptophan in annexin I becomes less aqueous on S100C binding, suggesting a hydrophobic nature of the protein-protein interaction. Thus the structural basis of the annexin 1-S100C complex-formation probably resembles to a large extent that of the well-characterized annexin II-p11 interaction.

The association of annexin I with early endosomes is regulated by Ca2+ and requires an intact N-terminal domain

Annexin I is a member of a multigene family of Ca2+/phospholipid-binding proteins and a major substrate for the epidermal growth factor (EGF) receptor kinase, which has been implicated in membrane-related events along the endocytotic pathway, in particular in the sorting of internalized EGF receptors occurring in the multivesicular body. We analyzed in detail the intracellular distribution of this annexin by cell fractionation and immunoelectron microscopy. These studies used polyclonal as well as a set of species-specific monoclonal antibodies, whose epitopes were mapped to the lateral surface of the molecule next to a region thought to be involved in vesicle aggregation. Unexpectedly, the majority of annexin I was identified on early and not on multivesicular endosomes in a form that required micromolar levels of Ca2+ for the association. The specific cofractionation with early endosomes was also observed in transfected baby hamster kidney cells when the intracellular fate of ectopically expressed porcine annexin I was analyzed by using the species-specific monoclonal antibodies in Western blots of subcellular fractions. Interestingly, a truncation of the N-terminal 26, but not the N-terminal 13 residues of annexin I altered its intracellular distribution, shifting it from fractions containing early to those containing late and multivesicular endosomes. These findings underscore the regulatory importance of the N-terminal domain and provide evidence for an involvement of annexin I in early endocytotic processes.

Annexins in the human neuroblastoma SH-SY5Y: demonstration of relocation of annexins II and V to membranes in response to elevation of intracellular calcium by membrane depolarisation and by the calcium ionophore A23187

The human neuroblastoma SH-SY5Y was found to express annexins I, II, IV, V, and VI by western blot analysis. Calcium-dependent membrane-binding proteins were isolated from SH-SY5Y and analysed by 2-dimensional gel electrophoresis. Proteins with Mr and Pi values similar to those of annexins I, II, III, IV, V, and VI were observed. The identity of annexins II and V was confirmed by western blotting. The membrane association of annexins II and V was studied in cells that had been stimulated to release noradrenaline by K+ depolarisation or by treatment with the ionophore A23187. Annexins II and V were both found to associate with membranes in a manner that was resistant to elution with EGTA and required Triton X-100 for their solubilisation. Homogenisation of cells in calcium-containing buffers also resulted in the formation of EGTA-resistant membrane-associated annexins II and V. The results demonstrate calcium-dependent relocation of annexins II and V to membranes in intact cells and suggest that these annexins bind in a calcium-dependent manner to non-phospholipid components of SH-SY5Y membranes. Examination of cells by immunofluorescence microscopy demonstrated that annexin II was homogeneously associated with the plasma membrane before treatment with ionophore and relocated to discrete patches of staining after treatment. Annexin V was found by immunofluorescence to be present in the cytoplasm and in the nucleus, Stimulation of the cells produced no change in the cytoplasmic staining pattern but resulted in a partial relocation of nuclear annexin V to the periphery of the nucleus. The results argue for a general role for both annexins in calcium signalling at discrete intracellular locations. The results are not consistent with the specific involvement proposed previously for annexin II in membrane fusion at sites of vesicle exocytosis.