Structure of molecular aggregates of 1-(3-sn-phosphatidyl)-L-myo-inositol 3,4-bis(phosphate) in water

PI(4,5)P2 Clustering and Its Impact on Biological Functions

Located at the inner leaflet of the plasma membrane (PM), phosphatidyl-inositol 4,5-bisphosphate [PI(4,5)P₂] composes only 1-2 mol% of total PM lipids. With its synthesis and turnover both spatially and temporally regulated, PI(4,5)P₂ recruits and interacts with hundreds of cellular proteins to support a broad spectrum of cellular functions. Several factors contribute to the versatile and dynamic distribution of PI(4,5)P₂ in membranes. Physiological multivalent cations such as Ca²⁺ and Mg²⁺ can bridge between PI(4,5)P₂ headgroups, forming nanoscopic PI(4,5)P₂-cation clusters. The distinct lipid environment surrounding PI(4,5)P₂ affects the degree of PI(4,5)P₂ clustering. In addition, diverse cellular proteins interacting with PI(4,5)P₂ can further regulate PI(4,5)P₂ lateral distribution and accessibility. This review summarizes the current understanding of PI(4,5)P₂ behavior in both cells and model membranes, with emphasis on both multivalent cation- and protein-induced PI(4,5)P₂ clustering. Understanding the nature of spatially separated pools of PI(4,5)P₂ is fundamental to cell biology.

PIP2 Reshapes Membranes through Asymmetric Desorption

Phosphatidylinositol-4,5-bisphosphate (PIP2) is an important signaling lipid in eukaryotic cell plasma membranes, playing an essential role in diverse cellular processes. The headgroup of PIP2 is highly negatively charged, and this lipid displays a high critical micellar concentration compared to housekeeping phospholipid analogs. Given the crucial role of PIP2, it is imperative to study its localization, interaction with proteins, and membrane-shaping properties. Biomimetic membranes have served extensively to elucidate structural and functional aspects of cell membranes including protein-lipid and lipid-lipid interactions, as well as membrane mechanics. Incorporation of PIP2 into biomimetic membranes, however, has at times resulted in discrepant findings described in the literature. With the goal to elucidate the mechanical consequences of PIP2 incorporation, we studied the desorption of PIP2 from biomimetic giant unilamellar vesicles by means of a fluorescent marker. A decrease in fluorescence intensity with the age of the vesicles suggested that PIP2 lipids were being desorbed from the outer leaflet of the membrane. To evaluate whether this desorption was asymmetric, the vesicles were systematically diluted. This resulted in an increase in the number of internally tubulated vesicles within minutes after dilution, suggesting that the desorption was asymmetric and also generated membrane curvature. By means of a saturated chain homolog of PIP2, we showed that the fast desorption of PIP2 is facilitated by presence of an arachidonic lipid tail and is possibly due to its oxidation. Through measurements of the pulling force of membrane tethers, we quantified the effect of this asymmetric desorption on the spontaneous membrane curvature. Furthermore, we found that the spontaneous curvature could be modulated by externally increasing the concentration of PIP2 micelles. Given that the local concentration of PIP2 in biological membranes is variable, spontaneous curvature generated by PIP2 may affect the formation of highly curved structures that can serve as initiators for signaling events.

Role of Phosphorylation in Moesin Interactions with PIP2-Containing Biomimetic Membranes

Moesin, a protein of the ezrin, radixin, and moesin family, which links the plasma membrane to the cytoskeleton, is involved in multiple physiological and pathological processes, including viral budding and infection. Its interaction with the plasma membrane occurs via a key phosphoinositide, the phosphatidyl(4,5)inositol-bisphosphate (PIP2), and phosphorylation of residue T558, which has been shown to contribute, in cellulo, to a conformationally open protein. We study the impact of a double phosphomimetic mutation of moesin (T235D, T558D), which mimics the phosphorylation state of the protein, on protein/PIP2/microtubule interactions. Analytical ultracentrifugation in the micromolar range showed moesin in the monomer and dimer forms, with wild-type (WT) moesin containing a slightly larger fraction (∼30%) of dimers than DD moesin (10-20%). Only DD moesin was responsive to PIP2 in its micellar form. Quantitative cosedimentation assays using large unilamellar vesicles and quartz crystal microbalance on supported lipid bilayers containing PIP2 reveal a specific cooperative interaction for DD moesin with an ability to bind two PIP2 molecules simultaneously, whereas WT moesin was able to bind only one. In addition, DD moesin could subsequently interact with microtubules, whereas WT moesin was unable to do so. Altogether, our results point to an important role of these two phosphorylation sites in the opening of moesin: since DD moesin is intrinsically in a more open conformation than WT moesin, this intermolecular interaction is reinforced by its binding to PIP2. We also highlight important differences between moesin and ezrin, which appear to be finely regulated and to exhibit distinct molecular behaviors.

Lipid polyunsaturation determines the extent of membrane structural changes induced by Amphotericin B in Pichia pastoris yeast

The activity of the potent but highly toxic antifungal drug Amphotericin B (AmB), used intravenously to treat systemic fungal and parasitic infections, is widely accepted to result from its specific interaction with the fungal sterol ergosterol. While the effect of sterols on AmB activity has been intensely investigated, the role of membrane phospholipid composition has largely been ignored, and structural studies of native membranes have been hampered by their complex and disordered nature. We show for the first time that the structure of fungal membranes derived from Pichia pastoris yeast depends on the degree of lipid polyunsaturation, which has an impact on the structural consequences of AmB activity. AmB inserts in yeast membranes even in the absence of ergosterol, and forms an extra-membraneous layer whose thickness is resolved to be 4-5 nm. In ergosterol-containing membranes, AmB insertion is accompanied by ergosterol extraction into this layer. The AmB-sponge mediated depletion of ergosterol from P. pastoris membranes gives rise to a significant membrane thinning effect that depends on the degree of lipid polyunsaturation. The resulting hydrophobic mismatch is likely to interfere with a much broader range of membrane protein functions than those directly involving ergosterol, and suggests that polyunsaturated lipids could boost the efficiency of AmB. Furthermore, a low degree of lipid polyunsaturation leads to least AmB insertion and may protect host cells against the toxic effects of AmB. These results provide a new framework based on lipid composition and membrane structure through which we can understand its antifungal action and develop better treatments.

Counterion-mediated cluster formation by polyphosphoinositides

Polyphosphoinositides (PPI) and in particular PI(4,5)P2, are among the most highly charged molecules in cell membranes, are important in many cellular signaling pathways, and are frequently targeted by peripheral polybasic proteins for anchoring through electrostatic interactions. Such interactions between PIP2 and proteins containing polybasic stretches depend on the physical state and the lateral distribution of PIP2 within the inner leaflet of the cell's lipid bilayer. The physical and chemical properties of PIP2 such as pH-dependent changes in headgroup ionization and area per molecule as determined by experiments together with molecular simulations that predict headgroup conformations at various ionization states have revealed the electrostatic properties and phase behavior of PIP2-containing membranes. This review focuses on recent experimental and computational developments in defining the physical chemistry of PIP2 and its interactions with counterions. Ca(2+)-induced changes in PIP2 charge, conformation, and lateral structure within the membrane are documented by numerous experimental and computational studies. A simplified electrostatic model successfully predicts the Ca(2+)-driven formation of PIP2 clusters but cannot account for the different effects of Ca(2+) and Mg(2+) on PIP2-containing membranes. A more recent computational study is able to see the difference between Ca(2+) and Mg(2+) binding to PIP2 in the absence of a membrane and without cluster formation. Spectroscopic studies suggest that divalent cation- and multivalent polyamine-induced changes in the PIP2 lateral distribution in model membrane are also different, and not simply related to the net charge of the counterion. Among these differences is the capacity of Ca(2+) but not other polycations to induce nm scale clusters of PIP2 in fluid membranes. Recent super resolution optical studies show that PIP2 forms nanoclusters in the inner leaflet of a plasma membrane with a similar size distribution as those induced by Ca(2+) in model membranes. The mechanisms by which PIP2 forms nanoclusters and other structures inside a cell remain to be determined, but the unique electrostatic properties of PIP2 and its interactions with multivalent counterions might have particular physiological relevance.

High affinity binding to profilin by a covalently constrained, soluble mimic of phosphatidylinositol-4,5-bisphosphate micelles

Phosphoinositide (PI) lipids are essential regulators of a wide variety of cellular functions. We present here the preparation of a multivalent analogue of a phosphatidylinositol-4,5-bisphosphate (PIP(2)) micelle containing only the polar headgroup portion of this lipid. We show that this dendrimer binds to the cytoskeletal protein profilin with an affinity indistinguishable from that of PIP(2), despite the fact that profilin discriminates between PIP(2) and its monomeric hydrolysis product inositol-1,4,5-triphosphate (IP(3)) under physiological conditions. These data demonstrate that the diacylglycerol (DAG) moiety of PIP(2) is not required for high-affinity binding and suggest that profilin uses multivalency as a key means to distinguish between the intact lipid and IP(3). The class of soluble membrane analogues described here is likely to have broad applicability in the study of protein.PI interactions.

Giant unilamellar vesicles containing phosphatidylinositol(4,5)bisphosphate: characterization and functionality

Biomimetic systems such as giant unilamellar vesicles (GUVs) are increasingly used for studying protein/lipid interactions due to their size (similar to that of cells) and to their ease of observation by light microscopy techniques. Biophysicists have begun to complexify GUVs to investigate lipid/protein interactions. In particular, composite GUVs have been designed that incorporate lipids that play important physiological roles in cellulo, such as phosphoinositides and among those the most abundant one, phosphatidylinositol(4,5)bisphosphate (PIP2). Fluorescent lipids are often used as tracers to observe GUV membranes by microscopy but they can not bring quantitative information about the insertion of unlabeled lipids. In this study, we carried out zeta-potential measurements to prove the effective incorporation of PIP2 as well as that of phosphatidylserine in the membrane of GUVs prepared by electroformation and to follow the stability of PIP2-containing GUVs. Using confocal microscopy, we found that long-chain (C16) fluorescent PIP2 analogs used as tracers (0.1% of total lipids) show a uniform distribution in the membrane whereas PIP2 antibodies show PIP2 clustering. However, the clustering effect, which is emphasized when tertiary antibodies are used in addition to secondary ones to enhance the size of the detection complex, is artifactual. We showed that divalent ions (Ca2+ and Mg2+) can induce aggregation of PIP2 in the membrane depending on their concentration. Finally, the interaction of ezrin with PIP2-containing GUVs was investigated. Using either labeled ezrin and unlabeled GUVs or both labeled ezrin and GUVs, we showed that clusters of PIP2 and proteins are formed.

Quantitative analysis of the binding of ezrin to large unilamellar vesicles containing phosphatidylinositol 4,5 bisphosphate

The plasma membrane-cytoskeleton interface is a dynamic structure participating in a variety of cellular events. Among the proteins involved in the direct linkage between the cytoskeleton and the plasma membrane is the ezrin/radixin/moesin (ERM) family. The FERM (4.1 ezrin/radixin/moesin) domain in their N-terminus contains a phosphatidylinositol 4,5 bisphosphate (PIP(2)) (membrane) binding site whereas their C-terminus binds actin. In this work, our aim was to quantify the interaction of ezrin with large unilamellar vesicles (LUVs) containing PIP(2). For this purpose, we produced human recombinant ezrin bearing a cysteine residue at its C-terminus for subsequent labeling with Alexa488 maleimide. The functionality of labeled ezrin was checked by comparison with that of wild-type ezrin. The affinity constant between ezrin and LUVs was determined by cosedimentation assays and fluorescence correlation spectroscopy. The affinity was found to be approximately 5 microM for PIP(2)-LUVs and 20- to 70-fold lower for phosphatidylserine-LUVs. These results demonstrate, as well, that the interaction between ezrin and PIP(2)-LUVs is not cooperative. Finally, we found that ezrin FERM domain (area of approximately 30 nm(2)) binding to a single PIP(2) can block access to neighboring PIP(2) molecules and thus contributes to lower the accessible PIP(2) concentration. In addition, no evidence exists for a clustering of PIP(2) induced by ezrin addition.

Phosphatidylinositol 4,5-Bisphosphate Induced Flunitrazepam Sensitive-GABAA Receptor Increase in Synaptosomes from Chick Forebrain

The flunitrazepam sensitive-GABA(A) receptor density was increased by cytochalasins C and D at 37 degrees C suggesting that microfilament depolymerization induces exposure to the radioligand of a GABA(A) receptor in synaptosomes (Pharm Biochem Behav 72 (2002) 497). Similarly, phosphatidylinositol-4,5-bisphosphate (1-5 microM), but not a mixture of phospholipids, induced an increase of GABA(A) receptors in synaptosomes. Furthermore, N-ethyl maleimide, an inactivator of the sensitive fusion protein, which interacts with GABA(A) receptor, abolished the receptor increase induced by phosphatidylinositol-4,5-bisphosphate. Together, the results suggest that phosphatidylinositol-4,5-bisphosphate, acts via microfilament depolymerization increasing the binding of the radioligand to receptors possibly by modulation of their interaction with proteins involved in trafficking and docking mechanisms.

Mapping the Phosphoinositide-Binding Site on Chick Cofilin Explains How PIP2 Regulates the Cofilin-Actin Interaction

Cofilin plays a key role in the choreography of actin dynamics via its ability to sever actin filaments and increase the rate of monomer dissociation from pointed ends. The exact manner by which phosphoinositides bind to cofilin and inhibit its interaction with actin has proven difficult to ascertain. We determined the structure of chick cofilin and used NMR chemical shift mapping and structure-directed mutagenesis to unambiguously locate its recognition site for phosphoinositides (PIs). This structurally unique recognition site requires both the acyl chain and head group of the PI for a productive interaction, and it is not inhibited by phosphorylation of cofilin. We propose that the interaction of cofilin with membrane-bound PIs abrogates its binding to both actin and actin-interacting protein 1, and facilitates spatiotemporal regulation of cofilin activity.

Gelsolin binds to polyphosphoinositide-free lipid vesicles and simultaneously to actin microfilaments

Gelsolin is a calcium-, pH- and lipid-dependent actin filament severing/capping protein whose main function is to regulate the assembly state of the actin cytoskeleton. Gelsolin is associated with membranes in cells, and it is generally assumed that this interaction is mediated by PPIs (polyphosphoinositides), since an interaction with these lipids has been characterized in vitro. We demonstrate that non-PPI lipids also bind gelsolin, especially at low pH. The data suggest further that gelsolin becomes partially buried in the lipid bilayer under mildly acidic conditions, in a manner that is not dependent of the presence of PPIs. Our data also suggest that lipid binding involves a number of sites that are spread throughout the gelsolin molecule. Linker regions between gelsolin domains have been implicated by other work, notably the linker between G1 and G2 (gelsolin domains 1 and 2 respectively), and we postulate that the linker region between the N-terminal and C-terminal halves of gelsolin (between G3 and G4) is also involved in the interaction with lipids. This region is compatible with other studies in which additional binding sites have been located within G4-6. The lipid-gelsolin interactions reported in the present paper are not calcium-dependent, and are likely to involve significant conformational changes to the gelsolin molecule, as the chymotryptic digest pattern is altered by the presence of lipids under our conditions. We also report that vesicle-bound gelsolin is capable of binding to actin filaments, presumably through barbed end capping. Gelsolin bound to vesicles can nucleate actin assembly, but is less active in severing microfilaments.

Cytoskeletal regulation: rich in lipids

Phosphorylated derivatives of the phospholipid phosphatidylinositol, or phosphoinositides, are implicated in many aspects of cell function. Binding of phosphoinositides that are localized within cell membranes to soluble protein ligands allows spatially selective regulation at the cytoplasm-membrane interface. Recently, studies that relate phosphoinositide production to membrane domains are converging with those that show effects of these lipids on the assembly of cellular actin, and are therefore linking membrane and cytoskeletal structures in new ways.

Evidence for two interaction regions for phosphatidylinositol(4,5)-bisphosphate on mammalian profilin I

The binding of phosphatidylinositol(4,5)-bisphosphate (PI(4,5)P(2)) to profilin at a region distinct from the actin interaction surface is demonstrated by experiments with covalently cross-linked profilin:beta-actin. The result is in agreement with observations made with several mutant profilins and provides strong evidence for two regions on mammalian profilin mediating electrostatic interaction with phosphatidylinositol lipids; one close to the binding site for poly(L-proline), and one partially overlapping with the actin-binding surface. Congruent with this, two plant profilins, which have a reduced number of positive amino acids in one of these regions, displayed a dramatically lower binding to PI(4,5)P(2) compared to human profilin I.

Phosphatidylinositol-dependent actin filament binding by the SWI/SNF-like BAF chromatin remodeling complex

Recently, several chromatin remodeling complexes in yeast, Drosophila, and mammals have been shown to contain actin and actin-related proteins (arps). However, the function of actin in these complexes is unclear. Here, we show that the mammalian SWI/SNF-like BAF complex binds to phosphatidylinositol 4,5-bisphosphate (PIP2) micelles and PIP2-containing mixed lipid vesicles, and that PIP2 binding allows the complex to associate with actin pointed ends and branch points. Actin binds to at least two distinct domains in the C terminus of the Brg1 protein, and interaction with only one of these domains is sensitive to PIP2. Based on these findings, we propose a model for PIP2 activation of actin binding by relief of intramolecular capping of actin by Brg1.

The distribution of polyphosphoinositides in lipid films

Fluorescence microscopy of Langmuir films is used to determine the effect of polyphosphoinositides (PPIs) on the structure of phosphatidylcholine-containing monolayers. Dramatic alterations in the texture of these films occur as the fraction of PPI in the film is altered. These changes depend on the ionic strength of the underlying subphase and can be accounted for by considering the electrostatic interactions among PPIs. Surface adsorption of a fluorescent peptide derivative based on the PPI binding site of the protein gelsolin co-localizes with PPI-rich domains. Localization of polyphosphoinositides in domains within the inner leaflet of the plasma membrane is proposed to be a key element in some aspects of intracellular signaling, and these data have implications for explaining the cause of restructuring of such domains.

Fluorescent phosphoinositide derivatives reveal specific binding of gelsolin and other actin regulatory proteins to mixed lipid bilayers

Fluorescent derivatives of phosphatidyl inositol (PtdIns)-(4,5)-P2 were synthesized and used to test the effects of the PtdIns-(4, 5)-P2-regulated proteins gelsolin, tau, cofilin, and profilin on labeled PtdIns-(4,5)-P2 that was either in micellar form or mixed with phosphatidylcholine (PtdCho) in bilayer vesicles. Gelsolin increased the fluorescence of 7-nitrobenz-2-oxa-1,3-diazole (NBD)- or pyrene-labeled PtdIns-(4,5)-P2 and NBD-PtdIns-(3,4,5)-P3. Cofilin and profilin produced no detectable change at equimolar ratios to PtdIns-(4,5)-P2, while tau decreased NBD-PtdIns-(4,5)-P2 fluorescence. Fluorescence enhancement by gelsolin of NBD-PtdIns-(4, 5)-P2 in mixed lipid vesicles depended on the mole fraction of PtdIns-(4,5)-P2 in the bilayer. Specific enhancement of 3% NBD-PtdIns-(4,5)-P2 : 97% PtdCho was much lower than that of 10% PtdIns-(4,5)-P2 : 90% PtdCho, but the enhancement of 3% NBD-PtdIns-(4,5)-P2 could be increased by addition of 7% unlabeled PtdIns-(4,5)-P2. The gelsolin-dependent increase in NBD-PtdIns-(4, 5)-P2 fluorescence was reversed by addition of Ca2+ or G-actin. Significant, but weaker, fluorescence enhancement was observed with the gelsolin N-terminal domain (residues 1-160) and a peptide comprised of gelsolin residues 150-169. Fluorescence energy transfer from gelsolin to pyrene-PtdIns-(4,5)-P2 was much stronger with intact gelsolin than the N-terminal region of gelsolin containing the PtdIns-(4,5)-P2 binding sites, suggesting that PtdIns-(4,5)-P2 may bind near a site formed by the juxtaposition of the N- and C-terminal domains of gelsolin.

Identification of a suppressor of the Dictyostelium profilin-minus phenotype as a CD36/LIMP-II homologue

Profilin is an ubiquitous G-actin binding protein in eukaryotic cells. Lack of both profilin isoforms in Dictyostelium discoideum resulted in impaired cytokinesis and an arrest in development. A restriction enzyme-mediated integration approach was applied to profilin-minus cells to identify suppressor mutants for the developmental phenotype. A mutant with wild-type-like development and restored cytokinesis was isolated. The gene affected was found to code for an integral membrane glycoprotein of a predicted size of 88 kD containing two transmembrane domains, one at the NH2 terminus and the other at the COOH terminus. It is homologous to mammalian CD36/LIMP-II and represents the first member of this family in D. discoideum, therefore the name DdLIMP is proposed. Targeted disruption of the lmpA gene in the profilin-minus background also rescued the mutant phenotype. Immunofluorescence revealed a localization in vesicles and ringlike structures on the cell surface. Partially purified DdLIMP bound specifically to PIP2 in sedimentation and gel filtration assays. A direct interaction between DdLIMP and profilin could not be detected, and it is unclear how far upstream in a regulatory cascade DdLIMP might be positioned. However, the PIP2 binding of DdLIMP points towards a function via the phosphatidylinositol pathway, a major regulator of profilin.

The structure of divalent cation-induced aggregates of PIP2 and their alteration by gelsolin and tau

Phosphatidylinositol bisphosphate (PIP2) serves as a precursor for diacylglycerol and inositol trisphosphate in signal transduction cascades and regulates the activities of several actin binding proteins that influence the organization of the actin cytoskeleton. Molecules of PIP2 form 6-nm diameter micelles in water, but aggregate into larger, multilamellar structures in physiological concentrations of divalent cations. Electron microscopic analysis of these aggregates reveals that they are clusters of striated filaments, suggesting that PIP2 aggregates form stacks of discoid micelles rather than multilamellar vesicles or inverted hexagonal arrays as previously inferred from indirect observations. The distance between striations within the filaments varies from 4.2 to 5.4 nm and the diameter of the filaments depends on the dehydrated ionic radius of the divalent cation, with average diameters of 19, 12, and 10 nm for filaments formed by Mg2+, Ca2+, and Ba2+, respectively. The structure of the divalent cation-induced aggregates can be altered by PIP2 binding proteins. Gelsolin and the microtubule associated protein tau both affect the formation of aggregates, indicating that tau acts as a PIP2 binding protein in a manner similar to gelsolin. In contrast, another PIP2 binding protein, profilin, does not modify the aggregates.

Characterization of a mutant profilin with reduced actin-binding capacity: effects in vitro and in vivo

We are investigating structure-function relationships in profilin and actin by site-specific mutagenesis using a yeast, Saccharomyces cerevisiae, expression system to produce wild-type and mutant proteins. This paper shows that deleting proline 96 and threonine 97, which are located close to the major actin binding site on profilin, did not significantly alter the interaction between profilin and phosphatidylinositol 4,5-bisphosphate, nor did it affect the profilin:poly(L-proline) interaction. The mutant protein, however, had a lower capacity to bind to actin in vitro than wild-type profilin, though it showed a slightly increased profilin-enhanced nucleotide exchange on the actin. When microinjected into Swiss 3T3 mouse fibroblasts or porcine aortic endothelial cells, the mutant profilin did not change the organization of the microfilament system like the wild-type profilin did. This provides further evidence that profilin controls microfilament organization in the cell by interacting directly with actin.

Tubulin, Gq, and phosphatidylinositol 4,5-bisphosphate interact to regulate phospholipase Cbeta1 signaling

The cytoskeletal protein, tubulin, has been shown to regulate adenylyl cyclase activity through its interaction with the specific G protein alpha subunits, Galphas or Galphai1. Tubulin activates these G proteins by transferring GTP and stabilizing the active nucleotide-bound Galpha conformation. To study the possibility of tubulin involvement in Galphaq-mediated phospholipase Cbeta1 (PLCbeta1) signaling, the m1 muscarinic receptor, Galphaq, and PLCbeta1 were expressed in Sf9 cells. A unique ability of tubulin to regulate PLCbeta1 was observed. Low concentrations of tubulin, with guanine nucleotide bound, activated PLCbeta1, whereas higher concentrations inhibited the enzyme. Interaction of tubulin with both Galphaq and PLCbeta1, accompanied by guanine nucleotide transfer from tubulin to Galphaq, is suggested as a mechanism for the enzyme activation. The PLCbeta1 substrate, phosphatidylinositol 4,5-bisphosphate, bound to tubulin and prevented microtubule assembly. This observation suggested a mechanism for the inhibition of PLCbeta1 by tubulin, since high tubulin concentrations might prevent the access of PLCbeta1 to its substrate. Activation of m1 muscarinic receptors by carbachol relaxed this inhibition, probably by increasing the affinity of Galphaq for tubulin. Involvement of tubulin in the articulation between PLCbeta1 signaling and microtubule assembly might prove important for the intracellular governing of a broad range of cellular events.

Clathrin assembly protein AP-2 induces aggregation of membrane vesicles: a possible role for AP-2 in endosome formation

We have examined the in vitro behavior of clathrin-coated vesicles that have been stripped of their surface coats such that the majority of the clathrin is removed but substantial amounts of clathrin assembly proteins (AP) remain membrane-associated. Aggregation of these stripped coated vesicles (s-CV) is observed when they are placed under conditions that approximate the pH and ionic strength of the cell interior (pH 7.2, approximately 100 mM salt). This s-CV aggregation reaction is rapid (t1/2 < or = 0.5 min), independent of temperature within a range of 4-37 degrees C, and unaffected by ATP, guanosine-5'-O- (3-thiophosphate), and in particular EGTA, distinguishing it from Ca(2+)-dependent membrane aggregation reactions. The process is driven by the action of membrane-associated AP molecules since partial proteolysis results in a full loss of activity and since aggregation is abolished by pretreatment of the s-CVs with a monoclonal antibody that reacts with the alpha subunit of AP-2. However, vesicle aggregation is not inhibited by PPPi, indicating that the previously characterized polyphosphate-sensitive AP-2 self-association is not responsible for the reaction. The vesicle aggregation reaction can be reconstituted: liposomes of phospholipid composition approximating that found on the cytoplasmic surfaces of the plasma membrane and of coated vesicles (70% L-alpha-phosphatidylethanolamine (type I-A), 15% L-alpha-phosphatidyl-L- serine, and 15% L-alpha-phosphatidylinositol) aggregated after addition of AP-2, but not of AP-1, AP-3 (AP180), or pure clathrin triskelions. Aggregation of liposomes is abolished by limited proteolysis of AP-2 with trypsin. In addition, a highly purified AP-2 alpha preparation devoid of beta causes liposome aggregation, whereas pure beta subunit does not, consistent with results obtained in the s-CV assay which also indicate the involvement of the alpha subunit. Using a fluorescence energy transfer assay we show that AP-2 does not cause fusion of liposomes under physiological solution conditions. However, since the fusion of membranes necessarily requires the close opposition of the two participating bilayers, the AP-2-dependent vesicle aggregation events that we have identified may represent an initial step in the formation and fusion of endosomes that occur subsequent to endocytosis and clathrin uncoating in vivo.

Modification of gelsolin with 4-fluoro-7-nitrobenz-2-oxa-1,3-diazole

Pig plasma gelsolin was modified with the fluorescent reagent 4-fluoro-7-nitrobenz-2-oxa-1,3-diazole (NBD-F) for lysyl residues. The relationship between the gelsolin activity and the degree of NBD labeling suggested that a single lysyl residue, which reacted five times slower than the other reactive lysyl residues, was essential for the activity. Taking advantage of the slow reactivity of the essential residue, active NBD-gelsolin was prepared. Limited cleavage of NBD-gelsolin by chymotrypsin indicated that the fluorescent reagents were randomly incorporated into all fragments observed. When NBD-gelsolin formed a gelsolin/actin (1:2) complex in the presence of micromolar Ca2+, the fluorescence spectra of NBD-gelsolin were red-shifted by 5 nm and the intensity decreased by 30%. However, on binding to phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), the fluorescence spectra were blue-shifted by 5 nm with a concomitant increase in intensity by 20%. The addition of PtdIns(4,5)P2 to the NBD-gelsolin actin (1:2) complex restored the fluorescence spectra to that obtained in the presence of PtdIns(4,5)P2 alone. These results indicated that NBD-gelsolin, selectively labeled on lysyl residues not essential for activity, can be a useful probe to monitor the binding of PtdIns(4,5)P2 and actin.

Reconstitution of the skeletal sarcoplasmic reticulum Ca2(+)-pump: influence of negatively charged phospholipids

The Ca2(+)-ATPase of skeletal sarcoplasmic reticulum was purified and reconstituted in the presence of phosphatidyl choline using the freeze-thaw sonication technique. The effect of incorporation of negatively charged phospholipids, phosphatidylserine and phosphatidylinositol phosphate, into the phosphatidylcholine proteoliposomes was investigated. Various ratios of phosphatidylserine or phosphatidylinositol phosphate to phosphatidylcholine were used, while the total amount of phospholipid in the reconstituted vesicles was kept constant. Enrichment of phosphatidylcholine proteoliposomes by phosphatidylserine or phosphatidylinositol phosphate was associated with activation of Ca2(+)-uptake and Ca2(+)-ATPase activities. The highest activation was obtained at a 50:50 molar ratio of phosphatidylserine:phosphatidylcholine and at a 10:90 molar ratio of phosphatidylinositol phosphate:phosphatidylcholine. The initial rates of Ca2(+)-uptake obtained at 1 microM Ca2+ were 2.6 +/- 0.1 mumol/min per mg of phosphatidylserine:phosphatidylcholine proteoliposomes and 1.5 +/- 0.1 mumol/min per mg of phosphatidylinositol phosphate:phosphatidylcholine proteoliposomes, compared to 0.9 +/- 0.05 mumol/min per mg of phosphatidylcholine proteoliposomes. These findings suggest that negatively charged phospholipids may be involved in the activation of the reconstituted skeletal muscle sarcoplasmic reticulum Ca2(+)-pump.

The actin-binding protein profilin binds to PIP2 and inhibits its hydrolysis by phospholipase C

Profilin is generally thought to regulate actin polymerization, but the observation that acidic phospholipids dissociate the complex of profilin and actin raised the possibility that profilin might also regulate lipid metabolism. Profilin isolated from platelets binds with high affinity to small clusters of phosphatidylinositol 4,5-bisphosphate (PIP2) molecules in micelles and also in bilayers with other phospholipids. The molar ratio of the complex of profilin with PIP2 is 1:7 in micelles of pure PIP2 and 1:5 in bilayers composed largely of other phospholipids. Profilin competes efficiently with platelet cytosolic phosphoinositide-specific phospholipase C for interaction with the PIP2 substrate and thereby inhibits PIP2 hydrolysis by this enzyme. The cellular concentrations and binding characteristics of these molecules are consistent with profilin being a negative regulator of the phosphoinositide signaling pathway in addition to its established function as an inhibitor of actin polymerization.

Neutron and X-ray diffraction structural analysis of phosphatidylinositol bilayers

Phosphatidylinositol (PI) bilayers, squeezed together by applied osmotic pressures, were studied by both neutron diffraction and X-ray diffraction. The lamellar repeat period for PI bilayers decreased from 9.5 nm at an applied pressure of 1.1.10(6) dyn/cm2 (1.1 atm) to 5.4 nm at an applied pressure of 1.6.10(7) dyn/cm2 (16 atm). Further increases in applied pressure, up to 2.7.10(9) dyn/cm2 (2700 atm) reduced the repeat period by only about 0.3 nm, to 5.1 nm. Thus, a plot of applied pressure versus repeat period shows a sharp upward break for repeat periods less than about 5.4 nm. For repeat periods of less than 5.4 nm, analysis of neutron-scattering density profiles and electron-density profiles indicates that the structure of the PI bilayers changes as the bilayers are dehydrated, even though there are only small changes in the repeat period. These structural changes are most likely due to removal of water from the headgroup regions of the bilayer. D2O/H2O exchange experiments show that, at an applied pressure of 2.8.10(7) dyn/cm2, water is located between adjacent PI headgroups in the plane of the bilayer. We conclude that, although electrostatics provide the dominant long-range repulsive interaction, hydration repulsion and steric hindrance between PI headgroups from apposing bilayers provide the major barriers for the close approach of adjacent PI bilayers for repeat periods less than 5.4 nm. This structural analysis also indicates that the phosphoinositol group extends from the plane of the bilayer into the fluid space between adjacent bilayers. This extended orientation for the headgroup is consistent with electrophoretic measurements on PI vesicles.

The effect of polyphosphoinositides and phosphatidic acid on the phosphatidylinositol transfer protein from bovine brain: a kinetic study

The phosphatidylinositol transfer protein from bovine brain (PI-TP) has lipid transfer characteristics which make it well suited to maintain phosphatidylinositol (PI) levels in intracellular membranes (Van Paridon, P.A., Gadella, Jr., T.W.J., Somerharju, P.J. and Wirtz, K.W.A. (1987) Biochim. Biophys. Acta 903, 68-77). Using a continuous fluorimetric transfer assay we have investigated in what way phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) affect the transfer activity of this protein in model systems. The effects were analysed by application of a kinetic model which yielded the association constant (K) and dissociation rate constant (k-) for the PI-TP/vesicle complex. Incorporation of PA, PIP and PIP2 into the phosphatidylcholine-containing vesicles increased the association constant solely by diminishing the dissociation rate constant. This effect could be completely accounted for by changes in the membrane surface charge density. In contrast to the inhibitory effect of PA, the inhibition caused by PIP2 was completely abolished by the addition of neomycin, in agreement with the observed preferential binding of this polyamine antibiotic to PIP2. A rise in pH from 5.5 to 8 drastically reduced the association constant for vesicles containing 16 mol% PA (e.g., from 38 to 2 mM-1), without affecting the Vmax. This effect could be mainly attributed to an increase in the negative charge on PI-TP (isoelectric point 5.5), resulting in an enhanced repulsion. Increasing the negative membrane surface charge at pH 7.4 had the opposite effect. This is interpreted to indicate that the membrane interaction site on PI-TP must be positively charged, overcoming the repulsive forces between PI-TP and the vesicle. Addition of PIP2 micelles as a third component in the transfer assay strongly inhibited PI-TP transfer activity. The extent of inhibition suggests a very high affinity of PI-TP for this lipid.

Specificity of the interaction between phosphatidylinositol 4,5-bisphosphate and the profilin:actin complex

Profilactin, the profilin:actin complex, which is present in large amounts in extracts of many types of eukaryotic cells, appears to serve as the precursor of microfilaments. It was reported recently that profilactin interacts specifically with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) (Lassing and Lindberg: Nature 314:472-474, 1985.) The present paper describes in detail the behaviour of profilactin and profilin in the presence of different types of phospholipids and neutral lipids under different conditions. PtdIns(4,5)P2 is the only phospholipid found so far which in the presence of 80 mM KCl and at Ca2+ concentrations below 10(-5) M effectively dissociates profilactin with the resulting polymerization of the actin. Phosphatidylinositol 4-monophosphate exhibits some activity but phosphatidylinositol is inactive. Both calf spleen profilin and profilin from human platelets form stable complexes with PtdIns(4,5)P2 micelles. PtdIns(4,5)P2 is active also when incorporated together with other phospholipids in mixed vesicles.

Modulation of gelsolin function by phosphatidylinositol 4,5-bisphosphate

The actin-binding protein gelsolin requires micromolar concentrations of calcium ions to sever actin filaments, to potentiate its binding to the end of the filament and to promote the polymerization of monomeric actin into filaments. Because transient increases in both intracellular [Ca2+] and actin polymerization accompany the cellular response to certain stimuli, it has been suggested that gelsolin regulates the reversible assembly of actin filaments that accompanies such cellular activations. But other evidence suggests that these activities do not need increased cytoplasmic [Ca2+] and that once actin-gelsolin complexes form in the presence of Ca2+ in vitro, removal of free Ca2+ causes dissociation of only one of two bound actin monomers from gelsolin and the resultant binary complexes cannot sever actin filaments. The finding that cellular gelsolin-actin complexes can be dissociated suggests that a Ca2+-independent regulation of gelsolin also occurs. Here we show that, like the dissociation of profilin-actin complexes, phosphatidylinositol 4,5-bisphosphate, which undergoes rapid turnover during cell stimulation, strongly inhibits the actin filament-severing properties of gelsolin, inhibits less strongly the nucleating ability of this protein and restores the potential for filament-severing activity to gelsolin-actin complexes.

Polyphosphoinositides undergo charge neutralization in the physiological pH range: a 31P-NMR study

The charge state of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was determined as a function of pH by way of 31P-NMR spectroscopy. The pK values for the first protonation of the phosphomonoester residues in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate were found to be 6.2 and 6.6, respectively, for the 4-phosphate moiety, and 7.7 for the 5-phosphate moiety.

Specific interaction between phosphatidylinositol 4,5-bisphosphate and profilactin

There is evidence that the polymerization of actin takes place at the plasma membrane, and that profilactin (profilin/actin complex), the unpolymerized form of actin found in extracts of many non-muscle cells, serves as the immediate precursor. Both isolated profilin and profilactin interact with detergent when analysed by charge shift electrophoresis, indicating that they have amphipathic properties and may be able to interact directly with the plasma membrane. We demonstrate here that isolated profilin, as well as the profilactin complex, interacts with anionic phospholipids. Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) was found to be the most active phospholipid, causing a rapid and efficient dissociation of profilactin with a concomitant polymerization of the actin in appropriate conditions. These and other observations suggest the possibility of a relationship between the induction of actin filament formation and the increased activity in the phosphatidylinositol cycle seen as a result of ligand-receptor interactions in various systems.