Structure of human annexin a6 at the air-water interface and in a membrane-bound state

How to gather useful and valuable information from protein binding measurements using Langmuir lipid monolayers

This review presents data on the influence of various experimental parameters on the binding of proteins onto Langmuir lipid monolayers. The users of the Langmuir methodology are often unaware of the importance of choosing appropriate experimental conditions to validate the data acquired with this method. The protein Retinitis pigmentosa 2 (RP2) has been used throughout this review to illustrate the influence of these experimental parameters on the data gathered with Langmuir monolayers. The methods detailed in this review include the determination of protein binding parameters from the measurement of adsorption isotherms, infrared spectra of the protein in solution and in monolayers, ellipsometric isotherms and fluorescence micrographs.

Annexins: Ca2+ Effectors Determining Membrane Trafficking in the Late Endocytic Compartment

Despite the discovery of annexins 40 years ago, we are just beginning to understand some of the functions of these still enigmatic proteins. Defined and characterized by their ability to bind anionic membrane lipids in a Ca²⁺-dependent manner, each annexin has to be considered a multifunctional protein, with a multitude of cellular locations and diverse activities. Underlying causes for this considerable functional diversity include their capability to associate with multiple cytosolic and membrane proteins. In recent years, the increasingly recognized establishment of membrane contact sites between subcellular compartments opens a new scenario for annexins as instrumental players to link Ca²⁺ signalling with the integration of membrane trafficking in many facets of cell physiology. In this chapter, we review and discuss current knowledge on the contribution of annexins in the biogenesis and functioning of the late endocytic compartment, affecting endo- and exocytic pathways in a variety of physiological consequences ranging from membrane repair, lysosomal exocytosis, to cell migration.

Annexin-phospholipid interactions. Functional implications

Annexins constitute an evolutionary conserved multigene protein superfamily characterized by their ability to interact with biological membranes in a calcium dependent manner. They are expressed by all living organisms with the exception of certain unicellular organisms. The vertebrate annexin core is composed of four (eight in annexin A6) homologous domains of around 70 amino acids, with the overall shape of a slightly bent ring surrounding a central hydrophilic pore. Calcium- and phospholipid-binding sites are located on the convex side while the N-terminus links domains I and IV on the concave side. The N-terminus region shows great variability in length and amino acid sequence and it greatly influences protein stability and specific functions of annexins. These proteins interact mainly with acidic phospholipids, such as phosphatidylserine, but differences are found regarding their affinity for lipids and calcium requirements for the interaction. Annexins are involved in a wide range of intra- and extracellular biological processes in vitro, most of them directly related with the conserved ability to bind to phospholipid bilayers: membrane trafficking, membrane-cytoskeleton anchorage, ion channel activity and regulation, as well as antiinflammatory and anticoagulant activities. However, the in vivo physiological functions of annexins are just beginning to be established.

Human islet amyloid polypeptide at the air-aqueous interface: a Langmuir monolayer approach

Human islet amyloid polypeptide (hIAPP) is the source of the major component of the amyloid deposits found in the islets of Langerhans of around 95 per cent type 2 diabetic patients. The formation of aggregates and mature fibrils is thought to be responsible for the dysfunction and death of the insulin-producing pancreatic β-cells. Investigation on the conformation, orientation and self-assembly of the hIAPP at time zero could be beneficial for our understanding of its stability and aggregation process. To obtain these insights, the hIAPP at time zero was studied at the air-aqueous interface using the Langmuir monolayer technique. The properties of the hIAPP Langmuir monolayer at the air-aqueous interface on a NaCl subphase with pH 2.0, 5.6 and 9.0 were examined by surface pressure- and potential-area isotherms, UV-Vis absorption, fluorescence spectroscopy and Brewster angle microscopy. The conformational and orientational changes of the hIAPP Langmuir monolayer under different surface pressures were characterized by p-polarized infrared-reflection absorption spectroscopy, and the results did not show any prominent changes of conformation or orientation. The predominant secondary structure of the hIAPP at the air-aqueous interface was α-helix conformation, with a parallel orientation to the interface during compression. These results showed that the hIAPP Langmuir monolayer at the air-aqueous interface was stable, and no aggregate or domain of the hIAPP at the air-aqueous interface was observed during the time of experiments.

Annexins as organizers of cholesterol- and sphingomyelin-enriched membrane microdomains in Niemann-Pick type C disease

Growing evidence suggests that membrane microdomains enriched in cholesterol and sphingomyelin are sites for numerous cellular processes, including signaling, vesicular transport, interaction with pathogens, and viral infection, etc. Recently some members of the annexin family of conserved calcium and membrane-binding proteins have been recognized as cholesterol-interacting molecules and suggested to play a role in the formation, stabilization, and dynamics of membrane microdomains to affect membrane lateral organization and to attract other proteins and signaling molecules onto their territory. Furthermore, annexins were implicated in the interactions between cytosolic and membrane molecules, in the turnover and storage of cholesterol and in various signaling pathways. In this review, we focus on the mechanisms of interaction of annexins with lipid microdomains and the role of annexins in membrane microdomains dynamics including possible participation of the domain-associated forms of annexins in the etiology of human lysosomal storage disease called Niemann-Pick type C disease, related to the abnormal storage of cholesterol in the lysosome-like intracellular compartment. The involvement of annexins and cholesterol/sphingomyelin-enriched membrane microdomains in other pathologies including cardiac dysfunctions, neurodegenerative diseases, obesity, diabetes mellitus, and cancer is likely, but is not supported by substantial experimental observations, and therefore awaits further clarification.

Annexin A6 is an organizer of membrane microdomains to regulate receptor localization and signalling

Annexin A6 (AnxA6) belongs to the conserved annexin protein family--a group of Ca(2+) -dependent membrane binding proteins. It is the largest of all annexin proteins and upon activation, binds to negatively charged phospholipids in the plasma membrane and endosomes. In addition, AnxA6 associates with cholesterol-rich membrane microdomains termed lipid rafts. Membrane cholesterol triggers Ca(2+) -independent translocation of AnxA6 to membranes and AnxA6 levels determine the number of caveolae, a form of specialized rafts at the cell surface. AnxA6 also has an F-actin binding domain and interacts with cytoskeleton components. Taken together, this suggests that AnxA6 has a scaffold function to link membrane microdomains with the organization of the cytoskeleton. Such a link facilitates AnxA6 to participate in plasma membrane repair and it would also impact on receptor signalling at the cell surface, growth factor, and lipoprotein receptor trafficking, Ca(2+) -channel activity and T cell activation. Hence, the regulation of cell surface receptors by AnxA6 may be facilitated by its unique structure that allows recruitment of interaction partners and simultaneously bridging specialized membrane domains with cortical actin surrounding activated receptors.

Matrix vesicles isolated from mineralization-competent Saos-2 cells are selectively enriched with annexins and S100 proteins

Matrix vesicles (MVs) are cell-derived membranous entities crucial for mineral formation in the extracellular matrix. One of the dominant groups of constitutive proteins present in MVs, recognised as regulators of mineralization in norm and pathology, are annexins. In this report, besides the annexins already described (AnxA2 and AnxA6), we identified AnxA1 and AnxA7, but not AnxA4, to become selectively enriched in MVs of Saos-2 cells upon stimulation for mineralization. Among them, AnxA6 was found to be almost EGTA-non extractable from matrix vesicles. Moreover, our report provides the first evidence of annexin-binding S100 proteins to be present in MVs of mineralizing cells. We observed that S100A10 and S100A6, but not S100A11, were selectively translocated to the MVs of Saos-2 cells upon mineralization. This observation provides the rationale for more detailed studies on the role of annexin-S100 interactions in MV-mediated mineralization.

Annexin A6-Linking Ca(2+) signaling with cholesterol transport

Annexin A6 (AnxA6) belongs to a conserved family of Ca(2+)-dependent membrane-binding proteins. Like other annexins, the function of AnxA6 is linked to its ability to bind phospholipids in cellular membranes in a dynamic and reversible fashion, in particular during the regulation of endocytic and exocytic pathways. High amounts of AnxA6 sequester cholesterol in late endosomes, thereby lowering the levels of cholesterol in the Golgi and the plasma membrane. These AnxA6-dependent redistributions of cellular cholesterol pools give rise to reduced cytoplasmic phospholipase A2 (cPLA(2)) activity, retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. In addition to regulating cholesterol and caveolin distribution, AnxA6 acts as a scaffold/targeting protein for several signaling proteins, the best characterized being the Ca(2+)-dependent membrane targeting of p120GAP to downregulate Ras activity. AnxA6 also stimulates the Ca(2+)-inducible involvement of PKC in the regulation of HRas and possibly EGFR signal transduction pathways. The ability of AnxA6 to recruit regulators of the EGFR/Ras pathway is likely potentiated by AnxA6-induced actin remodeling. Accordingly, AnxA6 may function as an organizer of membrane domains (i) to modulate intracellular cholesterol homeostasis, (ii) to create a scaffold for the formation of multifactorial signaling complexes, and (iii) to regulate transient membrane-actin interactions during endocytic and exocytic transport. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Mitochondrial creatine kinase binding to phospholipid monolayers induces cardiolipin segregation

It is well established that the octameric mitochondrial form of creatine kinase (mtCK) binds to the outer face of the inner mitochondrial membrane mainly via electrostatic interactions with cardiolipin (CL). However, little is known about the consequences of these interactions on membrane and protein levels. Brewster angle microscopy investigations provide, for the first time to our knowledge, images indicating that mtCK binding induced cluster formation on CL monolayers. The thickness of the clusters (10-12 nm) corresponds to the theoretical height of the mtCK-CL complex. Protein insertion into a condensed CL film, together with monolayer stabilization after protein addition, was observed by means of differential capacity measurements. Polarization modulation infrared reflection-absorption spectroscopy showed that the mean orientation of alpha-helices within the protein shifted upon CL binding from 30 degrees to 45 degrees with respect to the interface plane, demonstrating protein domain movements. A comparison of data obtained with CL and phosphatidylcholine/phosphatidylethanolamine/CL (2:1:1) monolayers indicates that mtCK is able to selectively recruit CL molecules within the mixed monolayer, consolidating and changing the morphology of the interfacial film. Therefore, CL-rich domains induced by mtCK binding could modulate mitochondrial inner membrane morphology into a raft-like organization and influence essential steps of mitochondria-mediated apoptosis.

Annexins: multifunctional components of growth and adaptation

Plant annexins are ubiquitous, soluble proteins capable of Ca(2+)-dependent and Ca(2+)-independent binding to endomembranes and the plasma membrane. Some members of this multigene family are capable of binding to F-actin, hydrolysing ATP and GTP, acting as peroxidases or cation channels. These multifunctional proteins are distributed throughout the plant and throughout the life cycle. Their expression and intracellular localization are under developmental and environmental control. The in vitro properties of annexins and their known, dynamic distribution patterns suggest that they could be central regulators or effectors of plant growth and stress signalling. Potentially, they could operate in signalling pathways involving cytosolic free calcium and reactive oxygen species.

Potential Role of Annexin AnnAt1 from Arabidopsis thaliana in pH-Mediated Cellular Response to Environmental Stimuli

Plant annexins, Ca(2+)- and membrane-binding proteins, are probably implicated in the cellular response to stress resulting from acidification of cytosol. To understand how annexins can contribute to cellular ion homeostasis, we investigated the pH-induced changes in the structure and function of recombinant annexin AnnAt1 from Arabidopsis thaliana. The decrease of pH from 7.0 to 5.8 reduced the time of the formation of ion channels by AnnAt1 in artificial lipid membranes from 3.5 h to 15-20 min and increased their unitary conductance from 32 to 63 pS. These changes were accompanied by an increase in AnnAt1 hydrophobicity as revealed by hydrophobicity predictions, by an increase in fluorescence of 2-(p-toluidino)naphthalene-6-sulfonic acid (TNS) bound to AnnAt1 and fluorescence resonance energy transfer from AnnAt1 tryptophan residues to TNS. Concomitant lipid partition of AnnAt1 at acidic pH resulted in its partial protection from proteolytic digestion. Secondary structures of AnnAt1 determined by circular dichroism and infrared spectroscopy were also affected by lowering the pH from 7.2 to 5.2. These changes were characterized by an increase in beta-sheet content at the expense of alpha-helical structures, and were accompanied by reversible formation of AnnAt1 oligomers as probed by ultracentrifugation in a sucrose gradient. A further decrease of pH from 5.2 to 4.5 or lower led to the formation of irreversible aggregates and loss of AnnAt1 ionic conductance. Our findings suggest that AnnAt1 can sense changes of the pH milieu over the pH range from 7 to 5 and respond by changes in ion channel conductance, hydrophobicity, secondary structure of the protein and formation of oligomers. Further acidification irreversibly inactivated AnnAt1. We suggest that the pH-sensitive ion channel activity of AnnAt1 may play a role in intracellular ion homeostasis.

Effects of mutagenesis of W343 in human annexin A6 isoform 1 on its interaction with GTP: nucleotide-induced oligomer formation and ion channel activity

Accumulated experimental evidence suggests that annexin A6 (AnxA6) is involved in ion transport in various tissues. Such a biological function is related either to the modulation of ion transport systems by AnxA6 or to the ion channel activity of the protein. While AnxA6 channel activity at low pH seems to be associated with a large conformational transition in the protein, the mechanism of GTP-induced ion channel formation remains obscure. This activity is not accompanied by changes in protein structure. The existence of a domain binding the phosphate groups of GTP in AnxA6 [Bandorowicz-Pikula, J., Kirilenko, A., van Deursen, R., Golczak, M., Kuhnel, M., Lancelin, J. M., Pikula, S., and Buchet, R. (2003) Biochemistry 42, 9137-9146] may provide some clues about the molecular mechanisms of GTP-induced ion channel formation. In addition, we observed that one of the AnxA6 tryptophan residues, W192 or W343, may be involved in GTP binding. Therefore, we created several site-directed mutants of AnxA6 in which selected amino acid residues within a consensus sequence of a putative nucleotide-binding domain of AnxA6 were replaced with other amino acid residues without affecting the overall structure of protein as examined by circular dichroism and infrared spectroscopies. Their properties were analyzed and compared to those of the native protein. In contrast to mutant W192S and wild-type annexin, mutant W343S neither bound GTP nor exhibited GTP-induced ion channel activity. In addition, we detected the likely formation of AnxA6 trimers in the presence of GTP. The ability of mutant W343S to form trimers was significantly impaired. Our findings suggest that W343 participates in the formation of AnxA6 trimers. We hypothesize that such trimers could lead to a functional unit of the GTP-induced ion channels formed by the annexin molecules.

Structure-function relationship in annexin A13, the founder member of the vertebrate family of annexins

Annexin A13 is considered the original progenitor of the 11 other members of vertebrate annexins, a superfamily of calcium/phospholipid-binding proteins. It is highly tissue-specific, being expressed only in intestinal and kidney epithelial cells. Alternative splicing generates two isoforms, both of which bind to rafts. In view of the lack of structural information supporting the physiological role of this annexin subfamily, we have cloned, expressed and purified human annexin A13b to investigate its structural and functional properties. The N-terminus of annexin A13b: (i) destabilizes the conserved protein core, as deduced from the low melting temperature in the absence (44 degrees C) or presence of calcium (55 degrees C), and (ii) impairs calcium-dependent binding to acidic phospholipids, requiring calcium concentrations >400 microM. Truncation of the N-terminus restores thermal stability and decreases the calcium requirement for phospholipid binding, confirming its essential role in the structure-function relationship of this annexin. Non-myristoylated annexin A13b only binds to acidic phospholipids at high calcium concentrations. We show for the first time that myristoylation of annexin A13b enables the direct binding to phosphatidylcholine, raft-like liposomes and acidic phospholipids in a calcium-independent manner. The conformational switch induced by calcium binding, from a 'closed' to an 'open' conformation with exposure of Trp227, can be mimicked by a decrease in pH, a process that may be relevant for membrane interactions. Our studies confirm that the common structural and functional characteristics that are dependent on the protein core of vertebrate annexins are likely to be common conserved features, whereas their variable N-termini confer distinct functional properties on annexins, as we report for myristoylation of annexin A13b.

GPI-alkaline phosphatase insertion into phosphatidylcholine monolayers: phase behavior and morphology changes

GPI-anchored proteins are localized on the outer layer of plasma membranes and clustered in microdomains generally called lipid rafts. To study the interactions between the lipidic GPI-anchor of the protein and phospholipids, we used phosphatidylcholine monolayers at the air-water interface as a biomimetic membrane system and GPI-alkaline phosphatase prepared from bovine intestinal mucosa (GPI-BIAP) as an GPI-anchored protein model. The monolayer technique allowed us to define GPI-BIAP interaction with DPPC and POPC, lipids differing only by the presence of one unsaturation in their acyl chains. Meanwhile the exclusion pressures were similar for the two phospholipids, the comparison of the Langmuir isotherms (i.e., pressure/area diagrams) indicates that GPI-BIAP interacted differently with DPPC and POPC monolayers. BAM images, acquired in order to visualize the interface organization induced by GPI-BIAP incorporation, confirm these differences.