Simultaneous binding of PtdIns(4,5)P2 and clathrin by AP180 in the nucleation of clathrin lattices on membranes

Phosphatidylinositol 4,5-bisphosphate stimulates vesicle formation from liposomes by brain cytosol

As a step toward the elucidation of mechanisms in vesicle budding, a cell-free assay that measures cytosol-induced vesicle generation from liposomes was established. This assay then was used to explore the role of phosphoinositides in vesicle formation. Liposomes incubated with brain cytosol in the presence of ATP and GTP massively generated small vesicles, as assessed both quantitatively and qualitatively by a dynamic light-scattering assay. Both ATP and GTP were required. Vesicle formation was inhibited greatly by the immunodepletion of dynamin 1 from the cytosol, indicating a major contribution of this GTPase in this reaction and suggesting that it mimics endocytic vesicle fission. Increasing the concentration of l-alpha-phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] but not of l-alpha-phosphatidylinositol 4-monophosphate or l-alpha-phosphatidylinositol in the lipid membranes enhanced vesicle formation. Lipid analysis revealed rapid degradation of PtdIns(4,5)P2 to l-alpha-phosphatidylinositol during the incubation with the reaction reaching a maximum within 5 sec, whereas vesicle formation proceeded with a longer time course. PtdIns(4,5)P2 degradation was independent of vesicle formation and occurred also in the presence of guanosine 5'-O-(thiotriphosphate), where few vesicle formations occurred. These results suggest that PtdIns(4,5)P2 plays a critical role in the early step of vesicle formation, possibly in the recruitment of coats and fission factors to membranes.

The ENTH domain

The epsin NH2-terminal homology (ENTH) domain is a membrane interacting module composed by a superhelix of alpha-helices. It is present at the NH2-terminus of proteins that often contain consensus sequences for binding to clathrin coat components and their accessory factors, and therefore function as endocytic adaptors. ENTH domain containing proteins have additional roles in signaling and actin regulation and may have yet other actions in the nucleus. The ENTH domain is structurally similar to the VHS domain. These domains define two families of adaptor proteins which function in membrane traffic and whose interaction with membranes is regulated, in part, by phosphoinositides.

Biological basket weaving: formation and function of clathrin-coated vesicles

There has recently been considerable progress in understanding the regulation of clathrin-coated vesicle (CCV) formation and function. These advances are due to the determination of the structure of a number of CCV coat components at molecular resolution and the identification of novel regulatory proteins that control CCV formation in the cell. In addition, pathways of (a) phosphorylation, (b) receptor signaling, and (c) lipid modification that influence CCV formation, as well as the interaction between the cytoskeleton and CCV transport pathways are becoming better defined. It is evident that although clathrin coat assembly drives CCV formation, this fundamental reaction is modified by different regulatory proteins, depending on where CCVs are forming in the cell. This regulatory difference likely reflects the distinct biological roles of CCVs at the plasma membrane and trans-Golgi network, as well as the distinct properties of these membranes themselves. Tissue-specific functions of CCVs require even more-specialized regulation and defects in these pathways can now be correlated with human diseases.

AP180 binds to the C-terminal SH2 domain of phospholipase C-gamma1 and inhibits its enzymatic activity

The role of phospholipase Cgamma1 (PLCgamma1) in signal transduction was investigated by characterizing its SH domain-binding proteins that may represent components of a novel signaling pathway. A 180-kDa protein that binds to the SH2 domain of PLCgamma1 was purified from rat brain. The amino acid sequence of peptide derived from the purified protein is now identified as AP180, a clathrin assembly protein that has been implicated in clathrin-mediated synaptic vesicle recycling in synapses. In this report, we demonstrate the stable association of PLCgamma1 with AP180 in a clathrin-coated vesicle complex, which not only binds to the carboxyl-terminal SH2 domain of PLCgamma1, but also inhibits its enzymatic activity in a dose-dependent manner.

Structural genomics and signaling domains

Many novel signal transduction domains are being identified in the wake of genome sequencing projects and improved sensitivity in homology-detection techniques. The functions of these domains are being discovered by hypothesis-driven experiments and structural genomics approaches. This article reviews the recent highlights of research on modular signaling domains, and the relative contributions and limitations of the various approaches being used.

The Phox homology (PX) domain, a new player in phosphoinositide signalling

Phosphoinositides are key regulators of diverse cellular processes. The pleckstrin homology (PH) domain mediates the action of PtdIns(3,4)P(2), PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3), while the FYVE domain relays the pulse of PtdIns3P. The recent establishment that the Phox homology (PX) domain interacts with PtdIns3P and other phosphoinositides suggests another mechanism by which phosphoinositides can regulate/integrate multiple cellular events via a spectrum of PX domain-containing proteins. Together with the recent discovery that the epsin N-terminal homologue (ENTH) domain interacts with PtdIns(4,5)P(2), it is becoming clear that phosphoinositides regulate diverse cellular events through interactions with several distinct structural motifs present in many different proteins.

Phosphatidylinositol-4-phosphate 5-kinase-1beta is essential for epidermal growth factor receptor-mediated endocytosis

Phosphatidylinositol-4,5-bisphosphate (PIP(2)) is known to play an important role in signal transduction and membrane trafficking. We show that one enzyme responsible for PIP(2) production, phosphatidylinositol-4-phosphate 5-kinase type 1beta (PIPKbeta), is essential for epidermal growth factor receptor (EGFR)-mediated endocytosis. Expression of murine PIPKbeta in NR6 cells expressing EGFR strikingly increased receptor internalization. Moreover, the kinase was shown to form an immunoprecipitable complex with EGFR. Expression of either a truncated kinase or a kinase dead mutant inhibited EGFR endocytosis and also blocked the membrane recruitment of PIPKbeta and both clathrin light chain and dynamin. Our results delineate a novel mechanism by which PIPKbeta regulates receptor-mediated endocytosis and receptor tyrosine kinase membrane traffic.

Clathrin- and AP-2-binding sites in HIP1 uncover a general assembly role for endocytic accessory proteins

Clathrin-mediated endocytosis is a major pathway for the internalization of macromolecules into the cytoplasm of eukaryotic cells. The principle coat components, clathrin and the AP-2 adaptor complex, assemble a polyhedral lattice at plasma membrane bud sites with the aid of several endocytic accessory proteins. Here, we show that huntingtin-interacting protein 1 (HIP1), a binding partner of huntingtin, copurifies with brain clathrin-coated vesicles and associates directly with both AP-2 and clathrin. The discrete interaction sequences within HIP1 that facilitate binding are analogous to motifs present in other accessory proteins, including AP180, amphiphysin, and epsin. Bound to a phosphoinositide-containing membrane surface via an epsin N-terminal homology (ENTH) domain, HIP1 associates with AP-2 to provide coincident clathrin-binding sites that together efficiently recruit clathrin to the bilayer. Our data implicate HIP1 in endocytosis, and the similar modular architecture and function of HIP1, epsin, and AP180 suggest a common role in lipid-regulated clathrin lattice biogenesis.

All phox homology (PX) domains from Saccharomyces cerevisiae specifically recognize phosphatidylinositol 3-phosphate

Phox homology (PX) domains are named for a 130-amino acid region of homology shared with part of two components of the phagocyte NADPH oxidase (phox) complex. They are found in proteins involved in vesicular trafficking, protein sorting, and lipid modification. It was recently reported that certain PX domains specifically recognize phosphatidylinositol 3-phosphate (PtdIns-3-P) and drive recruitment of their host proteins to the cytoplasmic leaflet of endosomal and/or vacuolar membranes where this phosphoinositide is enriched. We have analyzed phosphoinositide binding by all 15 PX domains encoded by the Saccharomyces cerevisiae genome. All yeast PX domains specifically recognize PtdIns-3-P in protein-lipid overlay experiments, with just one exception (a significant sequence outlier). In surface plasmon resonance studies, four of the yeast PX domains bind PtdIns-3-P with high (micromolar range) affinity. Although the remaining PX domains specifically recognize PtdIns-3-P, they bind this lipid with only low affinity. Interestingly, many proteins with "low affinity" PX domains are known to form large multimeric complexes, which may increase the overall avidity for membranes. Our results establish that PtdIns-3-P, and not other phosphoinositides, is the target of all PX domains in S. cerevisiae and suggest a role for PX domains in assembly of multiprotein complexes at specific membrane surfaces.

Huntingtin interacting protein 1 Is a clathrin coat binding protein required for differentiation of late spermatogenic progenitors

Huntingtin-interacting protein 1 (HIP1) interacts with huntingtin, the protein whose gene is mutated in Huntington's disease. In addition, a fusion between HIP1 and platelet-derived growth factor beta receptor causes chronic myelomonocytic leukemia. The HIP1 proteins, including HIP1 and HIP1-related (HIP1r), have an N-terminal polyphosphoinositide-interacting epsin N-terminal homology, domain, which is found in proteins involved in clathrin-mediated endocytosis. HIP1 and HIP1r also share a central leucine zipper and an actin binding TALIN homology domain. Here we show that HIP1, like HIP1r, colocalizes with clathrin coat components. We also show that HIP1 physically associates with clathrin and AP-2, the major components of the clathrin coat. To further understand the putative biological role(s) of HIP1, we have generated a targeted deletion of murine HIP1. HIP1(-/-) mice developed into adulthood, did not develop overt neurologic symptoms in the first year of life, and had normal peripheral blood counts. However, HIP1-deficient mice exhibited testicular degeneration with increased apoptosis of postmeiotic spermatids. Postmeiotic spermatids are the only cells of the seminiferous tubules that express HIP1. These findings indicate that HIP1 is required for differentiation, proliferation, and/or survival of spermatogenic progenitors. The association of HIP1 with clathrin coats and the requirement of HIP1 for progenitor survival suggest a role for HIP1 in the regulation of endocytosis.

AtPIP5K1, an Arabidopsis thaliana phosphatidylinositol phosphate kinase, synthesizes PtdIns(3,4)P(2) and PtdIns(4,5)P(2) in vitro and is inhibited by phosphorylation

PtdIns phosphate kinases (PIPkins), which generate PtdInsP(2) isomers, have been classified into three subfamilies that differ in their substrate specificities. We demonstrate here that the previously identified AtPIP5K1 gene from Arabidopsis thaliana encodes a PIPkin with dual substrate specificity in vitro, capable of phosphorylating PtdIns3P and PtdIns4P to PtdIns(3,4)P(2) and PtdIns(4,5)P(2) respectively. We also show that recombinant AtPIP5K1 is phosphorylated by protein kinase A and a soluble protein kinase from A. thaliana. Phosphorylation of AtPIP5K1 by protein kinase A is accompanied by a 40% inhibition of its catalytic activity. Full activity is recovered by treating phosphorylated AtPIP5K1 with alkaline phosphatase.

The dephosphins: dephosphorylation by calcineurin triggers synaptic vesicle endocytosis

When nerve terminals in the brain are stimulated, a group of phosphoproteins called the dephosphins are coordinately dephosphorylated by calcineurin, the Ca(2+)-dependent protein phosphatase. Amazingly, the seven presently known dephosphins are not structurally related, yet each has been independently shown to be essential for synaptic vesicle endocytosis (SVE). Nowhere else in biology is there a similar example of the coordinated dephosphorylation of such a large group of proteins each sharing roles in the same biological response. This suggests that dephosphorylation and phosphorylation of the dephosphins is essential for SVE. Recent studies in synaptosomes have confirmed this view, with calcineurin-mediated dephosphorylation of the dephosphins essential for triggering SVE. The phosphorylation cycle of the dephosphins might regulate SVE by targeting the proteins to sites of action and by stimulating the assembly of several large essential endocytic protein complexes.

Phosphoinositides, key molecules for regulation of actin cytoskeletal organization and membrane traffic from the plasma membrane

Phosphoinositide plays a critical role not only in generating second messengers, such as inositol 1,4,5-trisphosphate and diacylglycerol, but also in modulating a variety of cellular functions including cytoskeletal organization and membrane trafficking. Many inositol lipid kinases and phosphatases appear to regulate the concentration of a variety of phosphoinositides in a specific area, thereby inducing spatial and temporal changes in their availability. For example, local concentration changes in phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) in response to extracellular stimuli cause the reorganization of actin filaments and a change in cell shape. PI(4,5)P(2) uncaps the barbed end of actin filaments and increases actin nucleation by modulating a variety of actin regulatory proteins, leading to de novo actin polymerization. PI(4,5)P(2) also plays a key role in membrane trafficking processes. In endocytosis, PI(4,5)P(2) targets clathrin-associated proteins to endocytic vesicles, leading to clathrin-coated pit formation. On the contrary, PI(4,5)P(2) must be dephosphorylated when they shed clathrin coats to fuse endosome. Thus, through regulating actin cytoskeleton organization and membrane trafficking, phosphoinositides play crucial roles in a variety of cell functions such as growth, polarity, movement, and pattern formation.

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.

HIP1 functions in clathrin-mediated endocytosis through binding to clathrin and adaptor protein 2

Polyglutamine expansion in huntingtin is the underlying mutation leading to neurodegeneration in Huntington disease. This mutation influences the interaction of huntingtin with different proteins, including huntingtin-interacting protein 1 (HIP1), in which affinity to bind to mutant huntingtin is profoundly reduced. Here we demonstrate that HIP1 colocalizes with markers of clathrin-mediated endocytosis in neuronal cells and is highly enriched on clathrin-coated vesicles (CCVs) purified from brain homogenates. HIP1 binds to the clathrin adaptor protein 2 (AP2) and the terminal domain of the clathrin heavy chain, predominantly through a small fragment encompassing amino acids 276-335. This region, which contains consensus clathrin- and AP2-binding sites, functions in conjunction with the coiled-coil domain to target HIP1 to CCVs. Expression of various HIP1 fragments leads to a potent block of clathrin-mediated endocytosis. Our findings demonstrate that HIP1 is a novel component of the endocytic machinery.

Phosphoinositides as key regulators of synaptic function

Phosphoinositides have recently emerged as key regulators of a variety of synaptic processes, including neurosecretory vesicle targeting, exo-endocytosis, and ion channel modulation. These pleiotropic activities derive from their ability to serve either as membrane targeting sites for cytosolic factors, as allosteric ligands, or as nucleation points for coat proteins and cytoskeletal elements. This versatility depends upon the existence of highly diversified enzymatic machinery for their synthesis and degradation, which governs, both temporally and spatially, their appearance in the microenvironment of the synapse.

PIP kinase Igamma is the major PI(4,5)P(2) synthesizing enzyme at the synapse

Disruption of the presynaptically enriched polyphosphoinositide phosphatase synaptojanin 1 leads to an increase of clathrin-coated intermediates and of polymerized actin at endocytic zones of nerve terminals. These changes correlate with elevated levels of PI(4,5)P(2) in neurons. We report that phosphatidylinositol phosphate kinase type Igamma (PIPKIgamma), a major brain PI(4)P 5-kinase, is concentrated at synapses. Synaptojanin 1 and PIPKIgamma antagonize each other in the recruitment of clathrin coats to lipid membranes. Like synaptojanin 1 and other proteins involved in endocytosis, PIPKIgamma undergoes stimulation-dependent dephosphorylation. These results implicate PIPKIgamma in the synthesis of a PI(4,5)P(2) pool that acts as a positive regulator of clathrin coat recruitment and actin function at the synapse.

A novel mechanism for localizing membrane proteins to yeast trans-Golgi network requires function of synaptojanin-like protein

Localization of resident membrane proteins to the yeast trans-Golgi network (TGN) involves both their retrieval from a prevacuolar/endosomal compartment (PVC) and a "slow delivery" mechanism that inhibits their TGN-to-PVC transport. A screen for genes required for the slow delivery mechanism uncovered INP53, a gene encoding a phosphoinositide phosphatase. A retrieval-defective model TGN protein, A(F-->A)-ALP, was transported to the vacuole in inp53 mutants approximately threefold faster than in wild type. Inp53p appears to function in a process distinct from PVC retrieval because combining inp53 with mutations that block retrieval resulted in a much stronger phenotype than either mutation alone. In vps27 strains defective for both anterograde and retrograde transport out of the PVC, a loss of Inp53p function markedly accelerated the rate of transport of TGN residents A-ALP and Kex2p into the PVC. Inp53p function is cargo specific because a loss of Inp53p function had no effect on the rate of Vps10p transport to the PVC in vps27 cells. The rate of early secretory pathway transport appeared to be unaffected in inp53 mutants. Cell fractionation experiments suggested that Inp53p associates with Golgi or endosomal membranes. Taken together, these results suggest that a phosphoinositide signaling event regulates TGN-to-PVC transport of select cargo proteins.

The crystal structure of the PX domain from p40(phox) bound to phosphatidylinositol 3-phosphate

More than 50 human proteins with a wide range of functions have a 120 residue phosphoinositide binding module known as the PX domain. The 1.7 A X-ray crystal structure of the PX domain from the p40(phox) subunit of NADPH oxidase bound to PtdIns(3)P shows that the PX domain embraces the 3-phosphate on one side of a water-filled, positively charged pocket and reveals how 3-phosphoinositide specificity is achieved. A chronic granulomatous disease (CGD)-associated mutation in the p47(phox) PX domain that abrogates PtdIns(3)P binding maps to a conserved Arg that does not directly interact with the phosphoinositide but instead appears to stabilize a critical lipid binding loop. The SH3 domain present in the full-length protein does not affect soluble PtdIns(3)P binding to the p40(phox) PX domain.

The actin-binding protein Hip1R associates with clathrin during early stages of endocytosis and promotes clathrin assembly in vitro

Huntingtin-interacting protein 1 related (Hip1R) is a novel component of clathrin-coated pits and vesicles and is a mammalian homologue of Sla2p, an actin-binding protein important for both actin organization and endocytosis in yeast. Here, we demonstrate that Hip1R binds via its putative central coiled-coil domain to clathrin, and provide evidence that Hip1R and clathrin are associated in vivo at sites of endocytosis. First, real-time analysis of Hip1R-YFP and DsRed-clathrin light chain (LC) in live cells revealed that these proteins show almost identical temporal and spatial regulation at the cell cortex. Second, at the ultrastructure level, immunogold labeling of 'unroofed' cells showed that Hip1R localizes to clathrin-coated pits. Third, overexpression of Hip1R affected the subcellular distribution of clathrin LC. Consistent with a functional role for Hip1R in endocytosis, we also demonstrated that it promotes clathrin cage assembly in vitro. Finally, we showed that Hip1R is a rod-shaped apparent dimer with globular heads at either end, and that it can assemble clathrin-coated vesicles and F-actin into higher order structures. In total, Hip1R's properties suggest an early endocytic function at the interface between clathrin, F-actin, and lipids.

Modulation of HIV-like particle assembly in vitro by inositol phosphates

HIV-1 Gag protein assembles into 100- to 120-nm diameter particles in mammalian cells. Recombinant HIV-1 Gag protein assembles in a fully defined system in vitro into particles that are only 25-30 nm in diameter and that differ significantly in other respects from authentic particles. However, particles with the size and other properties of authentic virions were obtained in vitro by addition of inositol phosphates or phosphatidylinsitol phosphates to the assembly system. Thus, the interactions between HIV-1 Gag protein molecules are altered by binding of inositol derivatives; this binding is apparently essential for normal HIV-1 particle assembly. This requirement is not seen in a deleted Gag protein lacking residues 16-99 within the matrix domain.

Clathrin-mediated endocytosis: membrane factors pull the trigger

Clathrin-mediated endocytosis is a vesicular transport event involved in the internalization and recycling of receptors participating in signal transduction events and nutrient import as well as in the reformation of synaptic vesicles. Recent studies in vitro and in living cells have provided a number of new insights into the initial steps of clathrin-coated vesicle formation and the membrane factors involved in this process. The unexpected complexity of these interactions at the cytosol-membrane interface suggests that clathrin-coated vesicle assembly is a highly cooperative process occurring under tight regulatory control. In this review, we focus on the role of membrane proteins and lipids in the nucleation of clathrin-coated pits and provide a hypothetical model for the early steps in clathrin-mediated endocytosis.

Interaction of two structurally distinct sequence types with the clathrin terminal domain beta-propeller

The amino-terminal domain of the clathrin heavy chain, which folds into a seven-bladed beta-propeller, binds directly to several endocytic proteins via short sequences based on the consensus residues LLDLD. In addition to a single LLDLD-based, type I clathrin-binding sequence, both amphiphysin and epsin contain a second, distinct sequence that is also capable of binding to clathrin directly. Here, we analyzed these sequences, which we term type II sequences, and show that the (257)LMDLA sequence in rat epsin 1 appears to be a weak clathrin-binding variant of the sequence (417)PWDLW originally found in human amphiphysin II. The structural features of the type II sequence required for association with clathrin are distinct from the LLDLD-based sequence. In the central segment of amphiphysin, the type I and type II sequences cooperate to effect optimal clathrin binding and the formation of sedimentable assemblies. Together, the data provide evidence for two interaction surfaces upon certain endocytic accessory proteins that could cooperate with other coat components to enhance clathrin bud formation at the cell surface.

Clathrin-binding proteins: got a motif? Join the network!

Clathrin plays a key function in membrane and protein trafficking through the endocytic and late secretory pathways. Its role as a molecular scaffold that drives formation of transport vesicles requires binding to a number of proteins with distinct functional and structural properties. Recent studies have revealed that most of these proteins interact with clathrin through surprisingly simple, linear arrangements of acidic and hydrophobic amino acid residues. This article discusses the different types of clathrin-binding proteins and motifs as well as the physiological significance of these proteins in clathrin-dependent events.

PI(4,5)P(2) regulation of surface membrane traffic

Phosphatidylinositol 4,5-biphosphate (PI[4,5]P(2)) has emerged as an important signaling molecule in the membrane for regulating vesicle exo- and endocytosis and the accompanying actin cytoskeletal rearrangements. Localization studies with GFP-tagged binding domains and antibodies provide new views of the non-uniform, dynamic distribution of PI(4,5)P(2) in membranes and its organization in raft-like domains. The targeting of phosphoinositide kinases by GTPases can coordinate the reactions of membrane fusion and fission with cytoskeletal assembly, providing a basis for membrane movement.

The role of phosphoinositides in membrane transport

Phosphoinositides serve as intrinsic membrane signals that regulate intracellular membrane trafficking. Recently, phosphoinositides have been found to direct the localization and activity of effector proteins containing consensus sequence motifs such as FYVE, PH and ENTH domains. In addition, recent results show that regulated synthesis and turnover of phosphoinositides by membrane-associated phosphoinoside kinases and phosphatases spatially restrict the location of effectors critical for cellular transport processes, such as clathrin-mediated endocytosis, autophagy, phagocytosis, macropinocytosis and biosynthetic trafficking.

Endocytotic mechanisms in synapses

Nerve terminals are highly enriched in proteins needed for endocytosis. Although constitutive and ligand-stimulated endocytosis take place in nerve terminals, the primary type is compensatory endocytosis--the process by which a cell retrieves the additional membrane added to cell surface by a regulated secretory event. This process has been extensively characterized using electrophysiological techniques. Except for an unusual form of coupled exo- and endocytosis called kiss-and-run release, compensatory endocytosis appears to use basically the same clathrin-mediated mechanisms as the constitutive and ligand stimulated type. The remarkable speed and selectivity of compensatory endocytosis may be achieved by concentrating the machinery at specialized sites in the nerve terminal adjacent to exocytosis sites and by the use of neuronal isoforms of the proteins that mediate endocytosis.

Regulation of presynaptic phosphatidylinositol 4,5-biphosphate by neuronal activity

Phosphatidylinositol 4,5-biphosphate (PIP2) has been implicated in a variety of cellular processes, including synaptic vesicle recycling. However, little is known about the spatial distribution of this phospholipid in neurons and its dynamics. In this study, we have focused on these questions by transiently expressing the phospholipase C (PLC)-delta1 pleckstrin homology (PH) domain fused to green fluorescent protein (GFP) in cultured hippocampal neurons. This PH domain binds specifically and with high affinity to PIP2. Live confocal imaging revealed that in resting cells, PH-GFP is localized predominantly on the plasma membrane. Interestingly, no association of PH-GFP with synaptic vesicles in quiescent neurons was observed, indicating the absence of detectable PIP2 on mature synaptic vesicles. Electrical stimulation of hippocampal neurons resulted in a decrease of the PH-GFP signal at the plasma membrane, most probably due to a PLC-mediated hydrolysis of PIP2. This was accompanied in the majority of presynaptic terminals by a marked increase in the cytoplasmic PH-GFP signal, localized most probably on freshly endocytosed membranes. Further investigation revealed that the increase in PH-GFP signal was dependent on the activation of N-methyl-D-aspartate receptors and the consequent production of nitric oxide (NO). Thus, PIP2 in the presynaptic terminal appears to be regulated by postsynaptic activity via a retrograde action of NO.

The PX domains of p47phox and p40phox bind to lipid products of PI(3)K

PX domains are found in a variety of proteins that associate with cell membranes, but their molecular function has remained obscure. We show here that the PX domains in p47phox and p40phox subunits of the phagocyte NADPH oxidase bind to phosphatidylinositol-3,4-bisphosphate (PtdIns(3,4)P(2)) and phosphatidylinositol-3-phosphate (PtdIns(3)P), respectively. We also show that an Arg-to-Gln mutation in the PX domain of p47phox, which is found in patients with chronic granulomatous disease, eliminates phosphoinositide binding, as does the analogous mutation in the PX domain of p40phox. The PX domain of p40phox localizes specifically to PtdIns(3)P-enriched early endosomes, and this localization is disrupted by inhibition of phosphoinositide-3-OH kinase (PI(3)K) or by the Arg-to-Gln point mutation. These findings provide a molecular foundation to understand the role of PI(3)K in regulating neutrophil function and inflammation, and to identify PX domains as specific phosphoinositide-binding modules involved in signal transduction events in eukaryotic cells.

Clathrin-dependent and clathrin-independent endocytosis are differentially sensitive to insertion of poly (ethylene glycol)-derivatized cholesterol in the plasma membrane

We examined the effect of a cholesterol derivative, poly (ethylene glycol) cholesteryl ether on the structure/function of clathrin-coated pits and caveolae. Addition of the compound to cultured cells induced progressive smoothening of the surface. Markedly, when the incorporated amount exceeded 10% equivalent of the surface area, fluid pinocytosis, but not endocytosis of transferrin, became inhibited in K562 cells. In A431 cells, both clathrin-independent fluid phase uptake and the internalization of fluorescent cholera-toxin B through caveolae were inhibited with concomitant flattening of caveolae. In contrast, clathrin-mediated internalization of transferrin was not affected until the incorporated poly (ethylene glycol) cholesteryl ether exceeded 20% equivalent of the plasma membrane surface area, at which point opened clathrin-coated pits accumulated. The cells were ruptured upon further addition of poly (ethylene glycol) cholesteryl ether. We propose that the primary reason for the differential effect of poly (ethylene glycol) cholesteryl ether is that the bulk membrane phase and caveolae are both more elastic than the rigid clathrin-coated pits. We analyzed the results with the current mechanical model (Rauch and Farge, Biophys J 2000;78:3036-3047) and suggest here that the functional clathrin-lattice is much stiffer than typical phospholipid bilayers.

Phox domain interaction with PtdIns(3)P targets the Vam7 t-SNARE to vacuole membranes

Specific recognition of phosphoinositides is crucial for protein sorting and membrane trafficking. Protein transport to the yeast vacuole depends on the Vam7 t-SNARE and its phox homology (PX) domain. Here, we show that the PX domain of Vam7 targets to vacuoles in vivo in a manner dependent on phosphatidylinositol 3-phosphate generation. A novel phosphatidylinositol-3-phosphate-binding motif and an exposed loop that interacts with the lipid bilayer are identified by nuclear magnetic resonance spectroscopy. Conservation of key structural and binding site residues across the diverse PX family indicates a shared fold and phosphoinositide recognition function.

Membrane traffic: how do GGAs fit in with the adaptors?

The AP-1 adaptor complex has been cast as the major player in clathrin coat formation for vesicular transport from the trans-Golgi to the endocytic pathway. But new results on 'GGA' proteins have raised doubts about this paradigm and suggest both a new sorting mechanism and an unexpected complexity in the roles of clathrin.

Peptides as receptor ligand drugs and their relationship to G-coupled signal transduction

Peptides act as effector agents that regulate and/or mediate physiological processes, serving as hormones, neurotransmitters and signal transducing factors. The low molecular weight peptides affect receptor-mediated events, which influence cardiovascular, gastrointestinal and neurocranial systems. While some peptides have been marketed as drugs, many have served as leads or templates for the development of non-peptide drugs that mimic peptide actions. This review presents the advantages and disadvantages of using peptides as drugs that bind as ligands to cell-surface receptors and considers their applications in such events. The value of both the peptides and their mimics is based on their participation in the biomodulation of physiological processes, which frequently employ scaffolding proteins acting in a cascading sequence of protein-to-protein interactions. The peptides bind to G-coupled surface receptors to initiate a signal that is transduced to the interior of the cell through multiple layers of phosphorylating enzymes and binding proteins. Peptides have been further employed to identify the molecular targets of signal transduction, the uncoupling of which might provide a means for various disease therapies. The exploitation of such peptide-mediated signal pathways, which are of primary importance to tumour cells, may provide an attractive strategy for anticancer therapy in the future.

Phosphoinositides in membrane traffic at the synapse

Inositol phospholipids represent a minor fraction of membrane phospholipids; yet they play important regulatory functions in signaling pathways and membrane traffic. The phosphorylated inositol ring can act either as a precursor for soluble intracellular messengers or as a binding site for cytosolic or membrane proteins. Hence, phosphorylation-dephosphorylation of phosphoinositides represents a mechanism for regulation of recruitment to the membrane of coat proteins, cytoskeletal scaffolds or signaling complexes and for the regulation of membrane proteins. Recent work suggests that phosphoinositide metabolism has an important role in membrane traffic at the synapse. PtdIns(4,5)P(2) generation is implicated in the secretion of at least a subset of neurotransmitters. Furthermore, PtdIns(4,5)P(2) plays a role in the nucleation of clathrin coats and of an actin-based cytoskeletal scaffold at endocytic zones of synapses, and PtdIns(4,5)P(2) dephosphorylation accompanies the release of newly formed vesicles from these interactions. Thus, the reversible phosphorylation of inositol phospholipids may be one of the mechanisms governing the timing and vectorial progression of synaptic vesicle membranes during their exocytic-endocytic cycle.