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The domain III of annexin 5 undergoes a Ca2+‐ and a pH‐dependent conformational transition of large amplitude. Modeling of the transition pathway by computer simulations suggested that the interactions between D226 and T229 in the IIID–IIIE loop on the one hand and the H‐bond interactions between W187 and T224 on the other hand, are important in this process [Sopkova et al. (2000) Biochemistry 39, 14065–14074]. In agreement with the modeling, we demonstrate in this work that the D226K mutation behaves as a molecular switch of the pH‐ and Ca2+‐mediated conformational transition. In contrast, the hydrogen bonds between W187 and T224 seem marginal.

The domain III of annexin 5 undergoes a Ca 2+- and a pH-dependent conformational transition of large amplitude. Modeling of the transition pathway by computer simulations suggested that the interactions between D226 and T229 in the IIID–IIIE loop on the one hand and the H-bond interactions between W187 and T224 on the other hand, are important in this process [Sopkova et al. (2000) Biochemistry 39, 14065–14074]. In agreement with the modeling, we demonstrate in this work that the D226K mutation behaves as a molecular switch of the pH- and Ca 2+-mediated conformational transition. In contrast, the hydrogen bonds between W187 and T224 seem marginal.

Annexins constitute a family of phospholipid- and Ca²⁺-binding proteins involved in a variety of membrane-related processes. The property of several annexins, including annexin A5, to self-organize at the surface of lipid membranes into 2D ordered arrays has been proposed to be functionally relevant in cellular contexts. To further address this question, we investigated the high-resolution structure of annexin A5 trimers in membrane-bound 2D crystals by cryo-electron microscopy (Cryo-EM). A new 2D crystal form was discovered, with p32₁ symmetry, which is significantly better ordered than the 2D crystals reported before. A 2D projection map was obtained at 6.5 Å resolution, revealing protein densities within each of the four domains characteristic of annexins. A quantitative comparison was performed between this structure and models generated from the structure of the soluble form of annexin A5 in pseudo-R3 3D crystals. This analysis indicated that both structures are essentially identical, except for small local changes attributed to membrane binding. As a consequence, and contrary to the common view, annexin A5 molecules maintain their bent shape and do not flatten upon membrane binding, which implies either that the four putative Ca²⁺ and membrane-binding loops present different types of interaction with the membrane surface, or that the membrane surface is locally perturbed. We propose that the trimerization of annexin A5 molecules is the relevant structural change occurring upon membrane binding. The evidence that 2D arrays of annexin A5 trimers are responsible for its in vitro property of blood coagulation inhibition supports this conclusion. Copyright 2000 Academic Press

Several annexins have been shown to bind proteins that belong to the S100 calcium-binding protein family. The two best-characterized complexes are annexin II with p11 and annexin I with S100C, the former of which has been implicated in membrane fusion processes. We have solved the crystal structures of the complexes of p11 with annexin II N-terminus and of S100C with annexin I N-terminus. Using these structural results, as well as electron microscopy observations of liposome junctions formed in the presence of such complexes (Lambert et al., 1997 J Mol Biol 272, 42–55), we propose a computer generated model for the entire annexin II/p11 complex.

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.

The H4‐H5loop of the α‐subunit of mouse brain Na,K‐ATPase was expressed and isolated from Escherichia colicells. Using fluorescence analogues of ATP, this loop was shown to retain its capability to bind ATP. Isolation of a soluble H4‐H5loop with the native ATP binding site is a crucial step for detailed studies of the molecular mechanism of ATP binding and utilisation.

Three closely related rhombohedral crystal structures of human annexin V have been analysed and compared: a low‐calcium, a high‐calcium and an ytterbium‐soaked crystal. The occupancy of the calcium sites increases at higher calcium concentrations, but the calcium is removed rather than replaced during soaking in the ytterbium solution. Instead, other sites are substituted at high calcium concentrations as well as in the presence of ytterbium. Furthermore, a new site is revealed in the ytterbium‐soaked crystal which may give a clue to the mechanism of conformational change that takes place in the third domain of annexin V in the presence of very high calcium concentrations and of phospholipids.

Annexin III, a putative inositol (1,2)-phosphohydrolase, was co-crystallized with inositol 2-phosphate, the inhibitor of the reaction, and its structure was solved to 1.95 A resolution. No enzyme active site was observed in the structure. Assays for enzymatic activity were also negative. Search for annexin III-inositol phosphate interactions using the BIAcoreTM system revealed an affinity for inositol cyclic (1,2)-phosphate, suggesting annexin III may sequester the molecule in the cell. The BIAcoreTM system used with different phospholipids showed that annexin III displays specificity for phosphatidylethanolamine, but not for phosphatidylinositols. Interestingly, a molecule of ethanolamine was found bound to the protein in the crystal structure. Coupled with the fact that this is a particularly abundant phospholipid in granules specific to neutrophils, cells where annexin III is highly expressed, our finding could be pointing to a physiological role of annexin III.

Four distinct areas of primary sequence conservation between known tumour necrosis factor and lymphotoxin polypeptides from various species can be recognized. When these amino acid sequences are highlighted in the three-dimensional structure, all are found in the same region, constituting the framework of the trimeric structure.

Three closely related rhombohedral crystal structures of human annexin V have been analysed and compared: a low-calcium, a high-calcium and an ytterbium-soaked crystal. The occupancy of the calcium sites increases at higher calcium concentrations, but the calcium is removed rather than replaced during soaking in the ytterbium solution. Instead, other sites are substituted at high calcium concentrations as well as in the presence of ytterbium. Furthermore, a new site is revealed in the ytterbium-soaked crystal which may give a clue to the mechanism of conformational change that takes place in the third domain of annexin V in the presence of very high calcium concentrations and of phospholipids.

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).

Annexin V was crystallized in the presence of a high concentration of calcium and the structure refined at 1·9 Å resolution. The crystals are triclinic (P1) with three molecules per asymmetric unit and pseudo-R3 symmetry, reflecting a tendency of annexin to form trimers. The overall structure of the protein is similar to that seen in other crystal forms. There are, however, significant changes in domain III, where a new calcium site is formed. The whole region surrounding this site is reorganized in our structure, rendering annexin V more symmetrical and more alike annexin I. The formation of the new calcium site causes the displacement of Trp187 from a buried to an exposed conformation, a change that has recently been demonstrated by fluorescence measurements. The affinity of the different potential calcium sites is modulated: there is no calcium bound in domains II and IV, while up to two secondary calcium ions sites (in domains I and III) can substitute, depending on the calcium concentration present. We suggest that annexin can act as a calcium buffer, binding or releasing calcium depending on its local concentration. Our results also show that annexin displays inherent mobility which, together with its capacity to modulate the calcium affinity of its sites, can be of importance for its function on the membrane surface.

Four distinct areas of primary sequence conservation between known tumour necrosis factor and lymphotoxin polypeptides from various species can be recognized. When these amino acid sequences are highlighted in the three‐dimensional structure, all are found in the same region, constituting the framework of the trimeric structure.