New EMBO members' review: actin cytoskeleton regulation through modulation of PI(4,5)P(2) rafts

Three SAC1-like genes show overlapping patterns of expression in Arabidopsis but are remarkably silent during embryo development

In Saccharomyces cerevisiae, the SAC1 gene encodes a polyphosphoinositide phosphatase (PPIPase) that modulates the levels of phosphoinositides, which are key regulators of a number of signal transduction processes. SAC1p has been implicated in multiple cellular functions: actin cytoskeleton organization, secretory functions, inositol metabolism, ATP transport, and multiple-drug sensitivity. Here, we describe the characterization of three genes in Arabidopsis thaliana, AtSAC1a, AtSAC1b, and AtSAC1c, encoding proteins similar to those of yeast SAC1p. We demonstrated that the three AtSAC1 proteins are functional homologs of the yeast SAC1p because they can rescue the cold-sensitive and inositol auxotroph yeast sac1-null mutant strain. The fact that Arabidopsis and yeast SAC1 genes derived from a common ancestor suggests that this plant multigenic family is involved in the phosphoinositide pathway and in a range of cellular functions similar to those in yeast. Using GFP fusion experiments, we demonstrate that the three AtSAC1 proteins are targeted to the endoplasmic reticulum. Their expression patterns are overlapping, with at least two members expressed in each organ. Remarkably, AtSAC1 genes are not expressed during seed development, and therefore additional phosphatases are required to control phosphoinositide levels in seeds.

Characterization of two novel proteins, NgRH1 and NgRH2, structurally and biochemically homologous to the Nogo-66 receptor

Nogo-66 receptor (NgR) has recently been identified as the neuronal receptor of the myelin-associated proteins Nogo-A, oligodendrocyte protein (OMgp) and myelin-associated glycoprotein (MAG), and mediates inhibition of axonal regeneration both in vitro and in vivo. Through database searches, we have identified two novel proteins (NgRH1 and NgRH2) that turned out to be homologous in their primary structures, biochemical properties and expression patterns to NgR. Like NgR, the homologues contain eight leucine-rich repeats (LRR) flanked by a leucine-rich repeat C-terminus (LRRCT) and a leucine-rich repeat N-terminus (LRRNT), and also have a C-terminal GPI signal sequence. Northern blot analysis showed predominant expression of NgRH1 and NgRH2 mRNA in the brain. In situ hybridization and immunohistochemistry on rat brain slices revealed neuronal expression of the genes. NgRH1 and NgRH2 were detected on the cell surface of recombinant cell lines as N-glycosylated GPI anchored proteins and, consistent with other GPI anchored proteins, were localized within the lipid rafts of cellular membranes. In addition, an N-terminal proteolytic fragment of NgR comprising the majority of the ectodomain was found to be constitutively secreted from cells. Our data indicate that NgR, NgRH1 and NgRH2 constitute a novel receptor protein family, which may play related roles within the CNS.

Imaging metabolism of phosphatidylinositol 4,5-bisphosphate in T-cell GM1-enriched domains containing Ras proteins

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and Ras proteins are involved in signalling pathways originating at the plasma membrane. The localisation and metabolism of PI(4,5)P(2) was studied in Jurkat T cells using fluorescence microscopic imaging with EGFP-tagged and antibody probes. Software was developed to objectively quantitate colocalisation and was used to show that plasma membrane PI(4,5)P(2) was enriched in lipid raft-containing patches of GM1 ganglioside, formed by crosslinking cholera toxin B-subunit (CT-B). The PI(4,5)P(2) metabolites phosphatidylinositol 3,4,5-trisphosphate and diacylglycerol appeared in plasma membrane CT-B-GM1 patches upon induction of signalling. Transferrin receptor and the CD45 tyrosine phosphatase did not colocalise with CT-B-GM1 patches, whereas the tyrosine kinase Lck, the scaffolding protein LAT, and endogenous Ras proteins did partially colocalise with CT-B-GM1 patches as did transfected EGFP-K-Ras(4B) and EGFP-H-Ras. The results demonstrate that T-cell PI(4,5)P(2) metabolism is occurring in GM1-enriched domains and that Ras proteins are present in these domains in vivo.

Lipid rafts in the maintenance of synapses, dendritic spines, and surface AMPA receptor stability

Cholesterol/sphingolipid microdomains (lipid rafts) in the membrane are involved in protein trafficking, formation of signaling complexes, and regulation of actin cytoskeleton. Here, we show that lipid rafts exist abundantly in dendrites of cultured hippocampal neurons, in which they are associated with several postsynaptic proteins including surface AMPA receptors. Depletion of cholesterol/sphingolipid leads to instability of surface AMPA receptors and gradual loss of synapses (both inhibitory and excitatory) and dendritic spines. The remaining synapses and spines in raft-depleted neurons become greatly enlarged. The importance of lipid rafts for normal synapse density and morphology could explain why cholesterol promotes synapse maturation in retinal ganglion cells (Mauch et al., 2001) and offers a potential link between disordered cholesterol metabolism and the synapse loss seen in neurodegenerative disease.

Myofibroblast differentiation: plasma membrane microdomains and cell phenotype

Myofibroblast differentiation characterizes a prominent cellular phenotype identified in experimental models of progressive kidney disease and human kidney biopsies. Mesangial cells, tubulointerstitial fibroblasts and, perhaps, tubular epithelial cells undergo myofibroblast differentiation, a process characterized by alpha-actin expression, synthesis of interstitial collagens and a growth response. Inhibition of myofibroblast differentiation could prevent kidney disease progression but may be difficult to accomplish, since inhibition of multiple signaling pathways would be required. Cell biology advances have enabled a better understanding of how information from many microenvironmental stimuli are integrated by spatial compartmentalization of extracellular receptors and cytosolic signaling molecules within specialized plasma membrane domains, such as focal adhesions and lipid rafts. We review this information and hypothesize that myofibroblast differentiation of renal cells can only proceed if the spatial arrangement of intracellular molecules, in large part determined by extracellular matrix-regulated cytoskeletal organization, permits activation of appropriate signaling pathways by soluble molecules interacting with receptors in specialized plasma membrane microdomains. If proven, this hypothesis suggests targeting key molecules within adhesion complexes and rafts (in some cases with drugs that are already clinically available) may provide more effective therapy for kidney disease progression.

Engagement of CD99 triggers the exocytic transport of ganglioside GM1 and the reorganization of actin cytoskeleton

We studied the role of lipid rafts and actin cytoskeleton in CD99-mediated signaling to elucidate the mechanism of protein transport upon CD99 engagement. CD99 engagement in Jurkat cells elicited the exocytic transport of GM1 as well as several surface molecules closely related with CD99 functions. In addition, CD99 molecules were rapidly incorporated into lipid rafts and appeared to rearrange the actin cytoskeleton upon CD99 stimulation. Association of CD99 with actin cytoskeleton was inhibited by methyl-beta-cyclodextrin, while CD99-mediated GM1 clustering was inhibited by cytochalasin D. Therefore, we suggest that CD99 may play a role in the vesicular transport of transmembrane proteins and lipid rafts from the intracellular location to the cell surface, possibly by effecting actin cytoskeleton reorganization.

The synaptojanin-like protein Inp53/Sjl3 functions with clathrin in a yeast TGN-to-endosome pathway distinct from the GGA protein-dependent pathway

Yeast TGN resident proteins that frequently cycle between the TGN and endosomes are much more slowly transported to the prevacuolar/late endosomal compartment (PVC) than other proteins. However, TGN protein transport to the PVC is accelerated in mutants lacking function of Inp53p. Inp53p contains a SacI polyphosphoinositide phosphatase domain, a 5-phosphatase domain, and a proline-rich domain. Here we show that all three domains are required to mediate "slow delivery" of TGN proteins into the PVC. Although deletion of the proline-rich domain did not affect general membrane association, it caused localization to become less specific. The proline-rich domain was shown to bind to two proteins, including clathrin heavy chain, Chc1p. Unlike chc1 mutants, inp53 mutants do not mislocalize TGN proteins to the cell surface, consistent with the idea that Chc1p and Inp53p act at a common vesicular trafficking step but that Chc1p is used at other steps also. Like mutations in the AP-1 adaptor complex, mutations in INP53 exhibit synthetic growth and transport defects when combined with mutations in the GGA proteins. Taken together with other recent studies, our results suggest that Inp53p and AP-1/clathrin act together in a TGN-to-early endosome pathway distinct from the direct TGN-to-PVC pathway mediated by GGA/clathrin.

Distinct cytoskeletal tracks direct individual vesicle populations to the apical membrane of epithelial cells

A key aspect in the structure of epithelial and neuronal cells is the maintenance of a polarized organization based on highly specific sorting machinery at the exit site of the trans Golgi network (TGN). Epithelial cells sort protein and lipid components into different sets of carriers for the apical or basolateral plasma membrane. The two intestinal proteins lactase-phlorizin hydrolase (LPH) and sucrase-isomaltase (SI) are delivered to the apical plasma membrane of epithelial cells with high fidelity but differ in their affinity to detergent-insoluble, glycolipid-enriched complexes (DIGs). Using a two-color labeling technique, we have recently characterized two post-Golgi vesicle populations that direct LPH and SI separately to the apical cell surface. Here, we investigated the structure and identification of protein components in these vesicle populations and assessed the role of cytoskeletal post-Golgi transport routes for apical cargo. Apart from the central role of microtubules in vesicle transport, we demonstrate that the transport of SI-carrying apical vesicles (SAVs) occurs along actin tracks in the cellular periphery, whereas LPH-carrying apical vesicles (LAVs) are transferred in an actin-independent fashion to the apical membrane. Our data further indicate that myosin 1A is the actin-associated motor protein that drives SAVs along actin filaments to the apical cell surface.

Phosphoinositide recognition domains

Domains or modules known to bind phosphoinositides have increased dramatically in number over the past few years, and are found in proteins involved in intracellular trafficking, cellular signaling, and cytoskeletal remodeling. Analysis of lipid binding by these domains and its structural basis has provided significant insight into the mechanism of membrane recruitment by the different cellular phosphoinositides. Domains that target only the rare (3-phosphorylated) phosphoinositides must bind with very high affinity, and with exquisite specificity. This is achieved solely by headgroup interactions in the case of certain pleckstrin homology (PH) domains [which bind PtdIns(3,4,5)P3 and/or PtdIns(3,4)P2], but requires an additional membrane-insertion and/or oligomerization component in the case of the PtdIns(3)P-targeting phox homology (PX) and FYVE domains. Domains that target PtdIns(4,5)P2, which is more abundant by some 25-fold, do not require the same stringent affinity and specificity characteristics, and tend to be more diverse in structure. The mode of phosphoinositide binding by different domains also appears to reflect their distinct functions. For example, pleckstrin homology domains that serve as simple targeting domains recognize only phosphoinositide headgroups. By contrast, certain other domains, notably the epsin ENTH domain, appear to promote bilayer curvature by inserting into the membrane upon binding.

Membrane lipid organization is critical for human neutrophil polarization

In response to chemoattractants neutrophils extend an actin-rich pseudopod, which imparts morphological polarity and is required for migration. Even when stimulated by an isotropic bath of chemoattractant, neutrophils exhibit persistent polarization and continued lamellipod formation at the front, suggesting that the cells establish an internal polarity. In this report, we show that perturbing lipid organization by depleting plasma membrane cholesterol levels reversibly inhibits cell polarization and migration. Among other receptor-mediated responses, beta(2) integrin up-regulation was unaffected, and initial calcium mobilization was only partially reduced by cholesterol depletion, indicating that this treatment did not abrogate initial receptor-mediated signal transduction. Interestingly, cholesterol depletion did not prevent initial activation of the GTPase Rac or an initial burst of actin polymerization, but rather it inhibited prolonged activation of Rac and sustained actin polymerization. Collectively, these findings support a model in which the plasma membrane is organized into domains that aid in amplifying the chemoattractant gradient and maintaining cell polarization.

Sterol-rich plasma membrane domains in the fission yeast Schizosaccharomyces pombe

Sterol-rich membrane domains exist in unicellular and multicellular eukaryotes. They are thought to provide a structural framework for interactions among a subset of proteins by selectively incorporating some proteins while excluding others. Although most studies have focused on the biophysical and biochemical properties of sterol-rich membrane domains and incorporated proteins, relatively little is known about their intracellular distribution. Using a cytological approach we show here that in the fission yeast Schizosaccharomyces pombe, sterols are enriched in the plasma membrane at the growing cell tips and at the site of cytokinesis. The distribution of sterols is regulated in a cell-cycle-dependent manner and requires a functional secretory pathway. By manipulating the integrity of sterol-rich membrane domains using sterol sequestering agents and genetic means, we show that these domains are important for multiple processes regulating cytokinesis. In these cells, defects in proper maintenance of the actomyosin ring and/or its attachment to the overlying plasma membrane were observed. Furthermore, the stability of a plasma membrane protein that colocalises with sterol-rich membrane domains was compromised. Taken together, our studies establish S. pombe as a genetically tractable model organism in which to study the role(s) of sterol-rich membrane domains in cell polarity and cytokinesis.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

Phosphorylation of FcgammaRIIA is required for the receptor-induced actin rearrangement and capping: the role of membrane rafts

Activation of Fcgamma receptor II (FcgammaRII) induces rearrangement of the actin-based cytoskeleton that serves as a driving force for FcgammaRII-mediated phagocytosis and FcgammaRII capping. To get insight into the signaling events that lead to the actin reorganization we investigated the role of raft-associated Src family tyrosine kinases in capping of FcgammaRII in U937 cells. After crosslinking, FcgammaRII was found to be recruited to detergent-resistant membrane domains (DRMs), rafts, where it coexisted with Lyn kinase and underwent tyrosine phosphorylation. Lyn was displaced from DRMs under the influence of DL-alpha-hydroxymyristic acid and 2-bromopalmitic acid, agents blocking N-terminal myristoylation and palmitoylation of proteins, respectively, and after disruption of DRM integrity by depletion of plasma membrane cholesterol with beta-cyclodextrin. Under these conditions, phosphorylation of the crosslinked FcgammaRII was diminished and assembly of FcgammaRII caps was blocked. The similar reduction of FcgammaRII cap formation correlated with inhibition of receptor phosphorylation was achieved with the use of PP1 and herbimycin A, specific inhibitors of Src family tyrosine kinases. Phosphorylation of FcgammaRIIA expressed in BHK cells, lacking endogenous FcgammaRs, was abolished by substitution of tyrosine 298 by phenylalanine in the ITAM of the receptor. The mutant receptor did not undergo translocation towards cap-like structures and failed to promote the receptor-mediated spreading of the cells, as compared to BHK cells transfected with the wild-type FcgammaRIIA. On the basis of these data, we suggest that tyrosine phosphorylation of activated FcgammaRIIA by raft-residing tyrosine kinases of the Src family triggers signaling pathways that control the rearrangement of the actin cytoskeleton required for FcgammaRII-mediated motility.

Plasticity of cortical projections after stroke

Ischemic stroke produces cell death and disability, and a process of repair and partial recovery. Plasticity within cortical connections after stroke leads to partial recovery of function after the initial injury. Physiologically, cortical connections after stroke become hyperexcitable and more susceptible to the induction of LTP Stroke produces changes in the distribution and laterality of sensory, motor, and language representations within the brain that correlate with functional recovery. Anatomically, ischemic lesions induce axonal sprouting within local, intracortical projections and long distance, interhemispheric projections. This postischemic axonal sprouting establishes substantially new patterns of cortical connections with de-afferented or partially damaged brain areas. Axonal sprouting after ischemic lesions is induced by a transient pattern of synchronous, low-frequency neuronal activity in a network of cortical areas connected to the infarct. This pattern of neuronal activity that induces axonal sprouting in the adult after ischemic lesions resembles that seen in the developing brain during axonal elongation and synaptogenesis. Thus, stroke induces a process of remapping and reconnection within the adult brain through changes in neuronal activity that may involve a reactivation of developmental programs in areas connected to the infarct.

Remodeling the cytoskeleton for growth and form: an overview with some new views

The cytoskeleton coordinates all aspects of growth in plant cells, including exocytosis of membrane and wall components during cell expansion. This review seeks to integrate current information about cytoskeletal components in plants and the role they play in generating cell form. Advances in genome analysis have fundamentally changed the nature of research strategies and generated an explosion of new information on the cytoskeleton-associated proteins, their regulation, and their role in signaling to the cytoskeleton. Some of these proteins appear novel to plants, but many have close homologues in other eukaryotic systems. It is becoming clear that the mechanisms behind cell growth are essentially similar across the growth continuum, which ranges from tip growth to diffuse expansion. Remodeling of the actin cytoskeleton at sites of exocytosis is an especially critical feature of polarized and may also contribute to axial growth. We evaluate the most recent work on the signaling mechanisms that continually remodel the actin cytoskeleton via the activation of actin-binding proteins (ABPs) and consider the role the microtubule cytoskeleton plays in this process.

Phospholipid-based signaling in plants

Phospholipids are emerging as novel second messengers in plant cells. They are rapidly formed in response to a variety of stimuli via the activation of lipid kinases or phospholipases. These lipid signals can activate enzymes or recruit proteins to membranes via distinct lipid-binding domains, where the local increase in concentration promotes interactions and downstream signaling. Here, the latest developments in phospholipid-based signaling are discussed, including the lipid kinases and phospholipases that are activated, the signals they produce, the domains that bind them, the downstream targets that contain them and the processes they control.

Lateral sequestration of phosphatidylinositol 4,5-bisphosphate by the basic effector domain of myristoylated alanine-rich C kinase substrate is due to nonspecific electrostatic interactions

A peptide corresponding to the basic (+13), unstructured effector domain of myristoylated alanine-rich C kinase substrate (MARCKS) binds strongly to membranes containing phosphatidylinositol 4,5-bisphosphate (PIP(2)). Although aromatic residues contribute to the binding, three experiments suggest the binding is driven mainly by nonspecific local electrostatic interactions. First, peptides with 13 basic residues, Lys-13 and Arg-13, bind to PIP(2)-containing vesicles with the same high affinity as the effector domain peptide. Second, removing basic residues from the effector domain peptide reduces the binding energy by an amount that correlates with the number of charges removed. Third, peptides corresponding to a basic region in GAP43 and MARCKS effector domain-like regions in other proteins (e.g. MacMARCKS, adducin, Drosophila A kinase anchor protein 200, and N-methyl-d-aspartate receptor) also bind with an energy that correlates with the number of basic residues. Kinetic measurements suggest the effector domain binds to several PIP(2). Theoretical calculations show the effector domain produces a local positive potential, even when bound to a bilayer with 33% monovalent acidic lipids, and should thus sequester PIP(2) laterally. This electrostatic sequestration was observed experimentally using a phospholipase C assay. Our results are consistent with the hypothesis that MARCKS could reversibly sequester much of the PIP(2) in the plasma membrane.

Agonist-induced PIP(2) hydrolysis inhibits cortical actin dynamics: regulation at a global but not at a micrometer scale

Phosphatidylinositol 4, 5-bisphosphate (PIP(2)) at the inner leaflet of the plasma membrane has been proposed to locally regulate the actin cytoskeleton. Indeed, recent studies that use GFP-tagged pleckstrin homology domains (GFP-PH) as fluorescent PIP(2) sensors suggest that this lipid is enriched in membrane microdomains. Here we report that this concept needs revision. Using three distinct fluorescent GFP-tagged pleckstrin homology domains, we show that highly mobile GFP-PH patches colocalize perfectly with various lipophilic membrane dyes and, hence, represent increased lipid content rather than PIP(2)-enriched microdomains. We show that bright patches are caused by submicroscopical folds and ruffles in the membrane that can be directly visualized at approximately 15 nm axial resolution with a novel numerically enhanced imaging method. F-actin motility is inhibited significantly by agonist-induced PIP(2) breakdown, and it resumes as soon as PIP(2) levels are back to normal. Thus, our data support a role for PIP(2) in the regulation of cortical actin, but they challenge a model in which spatial differences in PIP(2) regulation of the cytoskeleton exist at a micrometer scale.

Role of phosphatidylinositol(4,5)bisphosphate organization in membrane transport by the Unc104 kinesin motor

Unc104 (KIF1A) kinesin transports membrane vesicles along microtubules in lower and higher eukaryotes. Using an in vitro motility assay, we show that Unc104 uses a lipid binding pleckstrin homology (PH) domain to dock onto membrane cargo. Through its PH domain, Unc104 can transport phosphatidylinositol(4,5)bisphosphate (PtdIns(4,5)P2)-containing liposomes with similar properties to native vesicles. Interestingly, liposome movement by monomeric Unc104 motors shows a very steep dependence on PtdIns(4,5)P2 concentration (Hill coefficient of approximately 20), even though liposome binding is noncooperative. This switch-like transition for movement can be shifted to lower PtdIns(4,5)P2 concentrations by the addition of cholesterol/sphingomyelin or GM1 ganglioside/cholera toxin, conditions that produce raft-like behavior of Unc104 bound to lipid bilayers. These studies suggest that clustering of Unc104 in PtdIns(4,5)P2-containing rafts provides a trigger for membrane transport.

Endocytosis via caveolae

Caveolae are flask-shaped invaginations present in the plasma membrane of many cell types. They have long been implicated in endocytosis, transcytosis, and cell signaling. Recent work has confirmed that caveolae are directly involved in the internalization of membrane components (glycosphingolipids and glycosylphosphatidylinositol-anchored proteins), extracellular ligands (folic acid, albumin, autocrine motility factor), bacterial toxins (cholera toxin, tetanus toxin), and several nonenveloped viruses (Simian virus 40, Polyoma virus). Unlike clathrin-mediated endocytosis, internalization through caveolae is a triggered event that involves complex signaling. The mechanism of internalization and the subsequent intracellular pathways that the internalized substances take are starting to emerge.

Human immunodeficiency virus type 1 uses lipid raft-colocalized CD4 and chemokine receptors for productive entry into CD4(+) T cells

In this report, we describe a crucial role of lipid raft-colocalized receptors in the entry of human immunodeficiency virus type 1 (HIV-1) into CD4(+) T cells. We show that biochemically isolated detergent-resistant fractions have characteristics of lipid rafts. Lipid raft integrity was required for productive HIV-1 entry as determined by (i) semiquantitative PCR analysis and (ii) single-cycle infectivity assay using HIV-1 expressing the luciferase reporter gene and pseudotyped with HIV-1 HXB2 envelope or vesicular stomatitis virus envelope glycoprotein (VSV-G). Depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) relocalized raft-resident markers to a nonraft environment but did not significantly change the surface expression of HIV-1 receptors. MbetaCD treatment inhibited productive infection of HIV-1 by 95% as determined by luciferase activity in cells infected with HXB2 envelope-pseudotyped virus. In contrast, infection with VSV-G-pseudotyped virus, which enters the cells through an endocytic pathway, was not suppressed. Biochemical fractionation and confocal imaging of HIV-1 receptor distribution in live cells demonstrated that CD4, CCR5, and CXCR4 colocalized with raft-resident markers, ganglioside GM1, and glycosylphosphatidylinositol-anchored CD48. While confocal microscopy analysis revealed that HIV-1 receptors localized most likely to the same lipid microdomains, sucrose gradient analysis of the receptor localization showed that, in contrast to CD4 and CCR5, CXCR4 was associated preferentially with the nonraft membrane fraction. The binding of HIV-1 envelope gp120 to lipid rafts in the presence, but not in the absence, of cholesterol strongly supports our hypothesis that raft-colocalized receptors are directly involved in virus entry. Dramatic changes in lipid raft and HIV-1 receptor redistribution were observed upon binding of HIV-1 NL4-3 to PM1 T cells. Colocalization of CCR5 with GM1 and gp120 upon engagement of CD4 and CXCR4 by HIV-1 further supports our observation that HIV-1 receptors localize to the same lipid rafts in PM1 T cells.

Phosphoinositides regulate membrane-dependent actin assembly by latex bead phagosomes

Actin assembly on membrane surfaces is an elusive process in which several phosphoinositides (PIPs) have been implicated. We have reconstituted actin assembly using a defined membrane surface, the latex bead phagosome (LBP), and shown that the PI(4,5)P(2)-binding proteins ezrin and/or moesin were essential for this process (). Here, we provide several lines of evidence that both preexisting and newly synthesized PI(4,5)P(2), and probably PI(4)P, are essential for phagosomal actin assembly; only these PIPs were routinely synthesized from ATP during in vitro actin assembly. Treatment of LBP with phospholipase C or with adenosine, an inhibitor of type II PI 4-kinase, as well as preincubation with anti-PI(4)P or anti-PI(4,5)P(2) antibodies all inhibited this process. Incorporation of extra PI(4)P or PI(4,5)P(2) into the LBP membrane led to a fivefold increase in the number of phagosomes that assemble actin. An ezrin mutant mutated in the PI(4,5)P(2)-binding sites was less efficient in binding to LBPs and in reconstituting actin assembly than wild-type ezrin. Our data show that PI 4- and PI 5-kinase, and under some conditions also PI 3-kinase, activities are present on LBPs and can be activated by ATP, even in the absence of GTP or cytosolic components. However, PI 3-kinase activity is not required for actin assembly, because the process was not affected by PI 3-kinase inhibitors. We suggest that the ezrin-dependent actin assembly on the LBP membrane may require active turnover of D4 and D5 PIPs on the organelle membrane.

Quantitative profiling of phospholipids by multiple precursor ion scanning on a hybrid quadrupole time-of-flight mass spectrometer

A hybrid quadrupole time-of-flight mass spectrometer featured with ion trapping capabilities was employed for quantitative profiling of total extracts of endogenous phospholipids. Simultaneous acquisition of precursor ion spectra of multiple fragment ions allowed detection of major classes of phospholipids in a single experiment. Relative changes in their concentration were monitored using a mixture of isotopically labeled endogenous lipids as a comprehensive internal standard. Precursor ion scanning spectra were acquired simultaneously for acyl anions of major fatty acids in negative ion mode and identified the fatty acid moieties and their relative position at the glycerol backbone in individual lipid species. Taken together, a combination of multiple precursor ion scans allowed quantitative monitoring of major perturbation in phospholipid composition and elucidating of molecular heterogeneity of individual lipid species.

A lipid-regulated docking site on vinculin for protein kinase C

During cell spreading, binding of actin-organizing proteins to acidic phospholipids and phosphorylation are important for localization and activity of these proteins at nascent cell-matrix adhesion sites. Here, we report on a transient interaction between the lipid-dependent protein kinase Calpha and vinculin, an early component of these sites, during spreading of HeLa cells on collagen. In vitro binding of protein kinase Calpha to vinculin tail was found dependent on free calcium and acidic phospholipids but independent of a functional kinase domain. The interaction was enhanced by conditions that favor the oligomerization of vinculin. Phosphorylation by protein kinase Calpha reached 1.5 mol of phosphate/mol of vinculin tail and required the C-terminal hydrophobic hairpin, a putative phosphatidylinositol 4,5-bisphosphate-binding site. Mass spectroscopy of peptides derived from in vitro phosphorylated vinculin tail identified phosphorylation of serines 1033 and 1045. Inhibition of C-terminal phospholipid binding at the vinculin tail by mutagenesis or deletion reduced the rate of phosphorylation to < or =50%. We suggest a possible mechanism whereby phospholipid-regulated conformational changes in vinculin may lead to exposure of a docking site for protein kinase Calpha and subsequent phosphorylation of vinculin and/or vinculin interaction partners, thereby affecting the formation of cell adhesion complexes.

Calmodulin signaling via the IQ motif

The IQ motif is widely distributed in both myosins and non-myosins and is quite common in the database that includes more than 900 Pfam entries. An examination of IQ motif-containing proteins that are known to bind calmodulin (CaM) indicates a wide diversity of biological functions that parallel the Ca2+-dependent targets. These proteins include a variety of neuronal growth proteins, myosins, voltage-operated channels, phosphatases, Ras exchange proteins, sperm surface proteins, a Ras Gap-like protein, spindle-associated proteins and several proteins in plants. The IQ motif occurs in some proteins with Ca2+-dependent CaM interaction where it may promote Ca2+-independent retention of CaM. The action of the IQ motif may result in complex signaling as observed for myosins and the L-type Ca2+ channels and is highly localized as required for sites of neuronal polarized growth and plasticity, fertilization, mitosis and cytoskeletal organization. The IQ motif associated with the unconventional myosins also promotes Ca2+ regulation of the vectorial movement of cellular constituents to these sites. Additional regulatory roles for this versatile motif seem likely.

The formation of helical tubular vesicles by binary monolayers containing a nickel-chelating lipid and phosphoinositides in the presence of basic polypeptides

Binary lipid monolayers consisting of equimolar proportions of a phosphoinositide and a nickel-chelating lipid formed helical tubular vesicular structures, which appeared to be induced and/or stabilized by myelin basic protein (MBP). Another basic polypeptide, poly-L-lysine, had a similar effect but not to as great a degree as MBP; the proteins thus appeared to act as polycations. Although, the nickel-chelating lipid is a synthetic product, other endogenous divalent cations such as Zn(2+), as well as phosphoinositides, are integral and dynamic components of the myelin sheath in vivo. There, comparable helical tubular structures might represent a means for sequestration of these lipids into domains of high local concentration, perhaps in regions where the membrane is greatly curved.

Modular phosphoinositide-binding domains--their role in signalling and membrane trafficking

The membrane phospholipid phosphatidylinositol is the precursor of a family of lipid second-messengers, known as phosphoinositides, which differ in the phosphorylation status of their inositol group. A major advance in understanding phosphoinositide signalling has been the identification of a number of highly conserved modular protein domains whose function appears to be to bind various phosphoinositides. Such 'cut and paste' modules are found in a diverse array of multidomain proteins and recruit their host protein to specific regions in cells via interactions with phosphoinositides. Here, with particular reference to proteins involved in membrane traffic pathways, we discuss recent advances in our understanding of phosphoinositide-binding domains.