Dynamin and the actin cytoskeleton cooperatively regulate plasma membrane invagination by BAR and F-BAR proteins

Dynamin-2 function and dysfunction along the secretory pathway

Dynamin-2 is a ubiquitously expressed mechano-GTPase involved in different stages of the secretory pathway. Its most well-known function relates to the scission of nascent vesicles from the plasma membrane during endocytosis; however, it also participates in the formation of new vesicles from the Golgi network, vesicle trafficking, fusion processes and in the regulation of microtubule, and actin cytoskeleton dynamics. Over the last 8 years, more than 20 mutations in the dynamin-2 gene have been associated to two hereditary neuromuscular disorders: Charcot-Marie-Tooth neuropathy and centronuclear myopathy. Most of these mutations are grouped in the pleckstrin homology domain; however, there are no common mutations associated with both disorders, suggesting that they differently impact on dynamin-2 function in diverse tissues. In this review, we discuss the impact of these disease-related mutations on dynamin-2 function during vesicle trafficking and endocytotic processes.

srGAP1 regulates lamellipodial dynamics and cell migratory behavior by modulating Rac1 activity

srGAP1 limits Rac1 activity at lamellipodia in a negative feedback manner, allowing concomitant activation of Rac1 and RhoA at lamellipodia. Rho signaling causes membrane ruffling through actomyosin contractility and removes the protrusive structures. Such coordination of Rac and Rho determines migratory behavior through lamellipodial dynamics.

The distinct levels of Rac activity differentially regulate the pattern of intrinsic cell migration. However, it remains unknown how Rac activity is modulated and how the level of Rac activity controls cell migratory behavior. Here we show that Slit-Robo GAP 1 (srGAP1) is a modulator of Rac activity in locomotive cells. srGAP1 possesses a GAP activity specific to Rac1 and is recruited to lamellipodia in a Rac1-dependent manner. srGAP1 limits Rac1 activity and allows concomitant activation of Rac1 and RhoA, which are mutually inhibitory. When both GTPases are activated, the protrusive structures caused by Rac1-dependent actin reorganization are spatially restricted and periodically destabilized, causing ruffling by RhoA-induced actomyosin contractility. Depletion of srGAP1 overactivates Rac1 and inactivates RhoA, resulting in continuous spatiotemporal spreading of lamellipodia and a modal shift of intrinsic cell motility from random to directionally persistent. Thus srGAP1 is a key determinant of lamellipodial dynamics and cell migratory behavior.

The Bin/amphiphysin/Rvs (BAR) domain protein endophilin B2 interacts with plectin and controls perinuclear cytoskeletal architecture

Proteins of the Bin/amphiphysin/Rvs (BAR) domain superfamily are essential in controlling the shape and dynamics of intracellular membranes. Here, we present evidence for the unconventional function of a member of the endophilin family of BAR and Src homology 3 domain-containing proteins, namely endophilin B2, in the perinuclear organization of intermediate filaments. Using mass spectrometry analysis based on capturing endophilin B2 partners in in situ pre-established complexes in cells, we unravel the interaction of endophilin B2 with plectin 1, a variant of the cytoskeleton linker protein plectin as well as with vimentin. Endophilin B2 directly binds the N-terminal region of plectin 1 via Src homology 3-mediated interaction and vimentin indirectly via plectin-mediated interaction. The relevance of these interactions is strengthened by the selective and drastic reorganization of vimentin around nuclei upon overexpression of endophilin B2 and by the extensive colocalization of both proteins in a meshwork of perinuclear filamentous structures. By generating mutants of the endophilin B2 BAR domain, we show that this phenotype requires the BAR-mediated membrane binding activity of endophilin B2. Plectin 1 or endophilin B2 knockdown using RNA interference disturbed the perinuclear organization of vimentin. Altogether, these data suggest that the endophilin B2-plectin 1 complex functions as a membrane-anchoring device organizing and stabilizing the perinuclear network of vimentin filaments. Finally, we present evidence for the involvement of endophilin B2 and plectin 1 in nuclear positioning in individual cells. This points to the potential importance of the endophilin B2-plectin complex in the biological functions depending on nuclear migration and positioning.

PICK1 interacts with PACSIN to regulate AMPA receptor internalization and cerebellar long-term depression

The dynamic trafficking of AMPA receptors (AMPARs) into and out of synapses is crucial for synaptic transmission, plasticity, learning, and memory. The protein interacting with C-kinase 1 (PICK1) directly interacts with GluA2/3 subunits of the AMPARs. Although the role of PICK1 in regulating AMPAR trafficking and multiple forms of synaptic plasticity is known, the exact molecular mechanisms underlying this process remain unclear. Here, we report a unique interaction between PICK1 and all three members of the protein kinase C and casein kinase II substrate in neurons (PACSIN) family and show that they form a complex with AMPARs. Our results reveal that knockdown of the neuronal-specific protein, PACSIN1, leads to a significant reduction in AMPAR internalization following the activation of NMDA receptors in hippocampal neurons. The interaction between PICK1 and PACSIN1 is regulated by PACSIN1 phosphorylation within the variable region and is required for AMPAR endocytosis. Similarly, the binding of PICK1 to the ubiquitously expressed PACSIN2 is also regulated by the homologous phosphorylation sites within the PACSIN2-variable region. Genetic deletion of PACSIN2, which is highly expressed in Purkinje cells, eliminates cerebellar long-term depression. This deficit can be fully rescued by overexpressing wild-type PACSIN2, but not by a PACSIN2 phosphomimetic mutant, which does not bind PICK1 efficiently. Taken together, our data demonstrate that the interaction of PICK1 and PACSIN is required for the activity-dependent internalization of AMPARs and for the expression of long-term depression in the cerebellum.

Dynamin-2 regulates fusion pore expansion and quantal release through a mechanism that involves actin dynamics in neuroendocrine chromaffin cells

Over the past years, dynamin has been implicated in tuning the amount and nature of transmitter released during exocytosis. However, the mechanism involved remains poorly understood. Here, using bovine adrenal chromaffin cells, we investigated whether this mechanism rely on dynamin’s ability to remodel actin cytoskeleton. According to this idea, inhibition of dynamin GTPase activity suppressed the calcium-dependent de novo cortical actin and altered the cortical actin network. Similarly, expression of a small interfering RNA directed against dynamin-2, an isoform highly expressed in chromaffin cells, changed the cortical actin network pattern. Disruption of dynamin-2 function, as well as the pharmacological inhibition of actin polymerization with cytochalasine-D, slowed down fusion pore expansion and increased the quantal size of individual exocytotic events. The effects of cytochalasine-D and dynamin-2 disruption were not additive indicating that dynamin-2 and F-actin regulate the late steps of exocytosis by a common mechanism. Together our data support a model in which dynamin-2 directs actin polymerization at the exocytosis site where both, in concert, adjust the hormone quantal release to efficiently respond to physiological demands.

Cut, via CrebA, transcriptionally regulates the COPII secretory pathway to direct dendrite development in Drosophila

Dendrite development is crucial in the formation of functional neural networks. Recent studies have provided insights into the involvement of secretory transport in dendritogenesis, raising the question of how the secretory pathway is controlled to direct dendritic elaboration. Here, we identify a functional link between transcriptional regulatory programs and the COPII secretory machinery in driving dendrite morphogenesis in Drosophila dendritic arborization (da) sensory neurons. MARCM analyses and gain-of-function studies reveal cell-autonomous requirements for the COPII coat protein Sec31 in mediating da neuron dendritic homeostasis. We demonstrate that the homeodomain protein Cut transcriptionally regulates Sec31 in addition to other components of COPII secretory transport, to promote dendrite elaboration, accompanied by increased satellite secretory endoplasmic reticulum (ER) and Golgi outposts primarily localized to dendritic branch points. We further establish a novel functional role for the transcription factor CrebA in regulating dendrite development and show that Cut initiates a gene expression cascade through CrebA that coordinately affects the COPII machinery to mediate dendritic morphology.

Dual role of BAR domain-containing proteins in regulating vesicle release catalyzed by the GTPase, dynamin-2

Dynamin-2 (Dyn2) is ubiquitously expressed and catalyzes membrane fission during clathrin-mediated endocytosis in nonneuronal cells. We have previously shown that Dyn2 inefficiently generates membrane curvature and only mediates fission of highly curved membranes. This led to the hypothesis that other endocytic accessory proteins (EAPs) generate curvature needed to sculpt a sufficiently narrow neck to trigger Dyn2 assembly and fission. Candidates for this activity are EAPs that bind to the dynamin proline/arginine-rich domain (PRD) through their SH3 (src homology-3) domains and also encode curvature-generating BAR (Bin/Amphiphysin/Rvs) domains. We show that at low concentrations, amphiphysin and endophilin, but not SNX9 or the curvature-generating epsin N-terminal homology (ENTH) domain, are able to generate tubules from planar membrane templates and to synergize with Dyn2ΔPRD to catalyze vesicle release. Unexpectedly, SH3-PRD interactions were inhibitory and reciprocally regulate scaffold assembly. Of the three proteins studied, only full-length amphiphysin functions synergistically with full-length Dyn2 to catalyze vesicle release. The differential activity of these proteins correlates with the relative potency of their positive, curvature-generating activity, and the negative regulatory effects mediated by SH3 domain interactions. Our findings reveal opportunities for the spatio-temporal coordination of membrane curvature generation, dynamin assembly, and fission during clathrin-mediated endocytosis.

Mechanics of dynamin-mediated membrane fission

In eukaryotic cells, membrane compartments are split into two by membrane fission. This ensures discontinuity of membrane containers and thus proper compartmentalization. The first proteic machinery implicated in catalyzing membrane fission was dynamin. Dynamin forms helical collars at the neck of endocytic buds. This structural feature suggested that the helix of dynamin could constrict in order to promote fission of the enclosed membrane. However, verifying this hypothesis revealed itself to be a challenge, which inspired many in vitro and in vivo studies. The primary goal of this review is to discuss recent structural and physical data from biophysical studies that have refined our understanding of the dynamin mechanism. In addition to the constriction hypothesis, other models have been proposed to explain how dynamin induces membrane fission. We present experimental data supporting these various models and assess which model is the most probable.

Formation of membrane ridges and scallops by the F-BAR protein Nervous Wreck

Eukaryotic cells are defined by extensive intracellular compartmentalization, which requires dynamic membrane remodeling. FER/Cip4 homology-Bin/amphiphysin/Rvs (F-BAR) domain family proteins form crescent-shaped dimers, which can bend membranes into buds and tubules of defined geometry and lipid composition. However, these proteins exhibit an unexplained wide diversity of membrane-deforming activities in vitro and functions in vivo. We find that the F-BAR domain of the neuronal protein Nervous Wreck (Nwk) has a novel higher-order structure and membrane-deforming activity that distinguishes it from previously described F-BAR proteins. The Nwk F-BAR domain assembles into zigzags, creating ridges and periodic scallops on membranes in vitro. This activity depends on structural determinants at the tips of the F-BAR dimer and on electrostatic interactions of the membrane with the F-BAR concave surface. In cells, Nwk-induced scallops can be extended by cytoskeletal forces to produce protrusions at the plasma membrane. Our results define a new F-BAR membrane-deforming activity and illustrate a molecular mechanism by which positively curved F-BAR domains can produce a variety of membrane curvatures. These findings expand the repertoire of F-BAR domain mediated membrane deformation and suggest that unique modes of higher-order assembly can define how these proteins sculpt the membrane.

CIP4 controls CCL19-driven cell steering and chemotaxis in chronic lymphocytic leukemia

Solid tumor dissemination relies on the reprogramming of molecular pathways controlling chemotaxis. Whether the motility of nonsolid tumors such as leukemia depends on the deregulated expression of molecules decoding chemotactic signals remains an open question. We identify here the membrane remodeling F-BAR adapter protein Cdc42-interacting protein 4 (CIP4) as a key regulator of chemotaxis in chronic lymphocytic leukemia (CLL). CIP4 is expressed at abnormally high levels in CLL cells, where it is required for CCL19-induced chemotaxis. Upon CCL19 stimulation of CLL cells, CIP4 associates with GTP-bound Cdc42 and is recruited to the rear of the lamellipodium and along microspikes radiating through the lamellipodium. Consistent with its cellular distribution, CIP4 removal impairs both the assembly of the polarized lamellipodium and directional migration along a diffusible CCL19 gradient. Furthermore, CIP4 depletion results in decreased activation of WASP, but increased activation of PAK1 and p38 mitogen-activated protein kinase (MAPK). Notably, p38 MAPK inhibition results in impaired lamellipodium assembly and loss of directional migration. This suggests that CIP4 modulates both the WASP and p38 MAPK pathways to promote lamellipodium assembly and chemotaxis. Overall, our study reveals a critical role of CIP4 in mediating chemotaxis of CLL cells by controlling the dynamics of microspike-containing protrusions and cell steering.

CIP4 coordinates with phospholipids and actin-associated proteins to localize to the protruding edge and produce actin ribs and veils

Cdc42-interacting protein 4 (CIP4), a member of the F-BAR family of proteins, plays important roles in a variety of cellular events by regulating both membrane and actin dynamics. In many cell types, CIP4 functions in vesicle formation, endocytosis and membrane tubulation. However, recent data indicate that CIP4 is also involved in protrusion in some cell types, including cancer cells (lamellipodia and invadopodia) and neurons (ribbed lamellipodia and veils). In neurons, CIP4 localizes specifically to extending protrusions and functions to limit neurite outgrowth early in development. The mechanism by which CIP4 localizes to the protruding edge membrane and induces lamellipodial/veil protrusion and actin rib formation is not known. Here, we show that CIP4 localization to the protruding edge of neurons is dependent on both the phospholipid content of the plasma membrane and the underlying organization of actin filaments. Inhibiting phosphatidylinositol (3,4,5)-trisphosphate (PIP3) production decreases CIP4 at the membrane. CIP4 localization to the protruding edge is also dependent on Rac1/WAVE1, rather than Cdc42/N-WASP. Capping actin filaments with low concentrations of cytochalasin D or by overexpressing capping protein dramatically decreases CIP4 at the protruding edge, whereas inactivating Arp2/3 drives CIP4 to the protruding edge. We also demonstrate that CIP4 dynamically colocalizes with Ena/VASP and DAAM1, two proteins known to induce unbranched actin filament arrays and play important roles in neuronal development. Together, this is the first study to show that the localization of an F-BAR protein depends on both actin filament architecture and phospholipids at the protruding edge of developing neurons.

Antagonistic regulation of F-BAR protein assemblies controls actin polymerization during podosome formation

FBP17, an F-BAR domain protein, has emerged as a crucial factor linking the plasma membrane to WASP-mediated actin polymerization. Although it is well established that FBP17 has a powerful self-polymerizing ability that promotes actin nucleation on membranes in vitro, knowledge of inhibitory factors that counteract this activity in vivo is limited. Here, we demonstrate that the assembly of FBP17 on the plasma membranes is antagonized by PSTPIP2, another F-BAR protein implicated in auto-inflammatory disorder. Knockdown of PSTPIP2 in macrophage promotes the assembly of FBP17 as well as subsequent actin nucleation at podosomes, resulting in an enhancement of matrix degradation. This phenotype is rescued by expression of PSTPIP2 in a manner dependent on its F-BAR domain. Time-lapse total internal reflection fluorescence (TIRF) microscopy observations reveal that the self-assembly of FBP17 at the podosomal membrane initiates actin polymerization, whereas the clustering of PSTPIP2 has an opposite effect. Biochemical analysis and live-cell imaging show that PSTPIP2 inhibits actin polymerization by competing with FBP17 for assembly at artificial as well as the plasma membrane. Interestingly, the assembly of FBP17 is dependent on WASP, and its dissociation by WASP inhibition strongly induces a self-organization of PSTPIP2 at podosomes. Thus, our data uncover a previously unappreciated antagonism between different F-BAR domain assemblies that determines the threshold of actin polymerization for the formation of functional podosomes and may explain how the absence of PSTPIP2 causes auto-inflammatory disorder.

Deleterious effects of soluble amyloid-β oligomers on multiple steps of synaptic vesicle trafficking

Growing evidence supports a role for soluble amyloid-β oligomer intermediates in the synaptic dysfunction associated with Alzheimer's disease (AD), but the molecular mechanisms underlying this effect remain unclear. We found that acute treatment of cultured rat hippocampal neurons with nanomolar concentrations of Aβ oligomers reduced the recycling pool and increased the resting pool of synaptic vesicles. Endocytosis of synaptic vesicles and the regeneration of fusion-competent vesicles were also severely impaired. Furthermore, the release probability of the readily-releasable pool (RRP) was increased, and recovery of the RRP was delayed. All these effects were prevented by antibody against Aβ. Moreover reduction of the pool size was prevented by inhibiting calpain or CDK5, while the defects in endocytosis were averted by overexpressing phosphatidylinositol-4-phosphate-5-kinase type I-γ, indicating that these two downstream pathways are involved in Aβ oligomers-induced presynaptic dysfunction.

Targeting and functional mechanisms of the cytokinesis-related F-BAR protein Hof1 during the cell cycle

Hof1 targets to the division site by interacting with septins and myosin II sequentially during the cell cycle. It plays a role in cytokinesis by coupling actomyosin ring constriction to primary septum formation through interactions with Myo1 and Chs2.

F-BAR proteins are membrane‑associated proteins believed to link the plasma membrane to the actin cytoskeleton in cellular processes such as cytokinesis and endocytosis. In the budding yeast Saccharomyces cerevisiae, the F‑BAR protein Hof1 localizes to the division site in a complex pattern during the cell cycle and plays an important role in cytokinesis. However, the mechanisms underlying its localization and function are poorly understood. Here we show that Hof1 contains three distinct targeting domains that contribute to cytokinesis differentially. The N‑terminal half of Hof1 localizes to the bud neck and the sites of polarized growth during the cell cycle. The neck localization is mediated mainly by an interaction between the second coiled‑coil region in the N‑terminus and the septin Cdc10, whereas the localization to the sites of polarized growth is mediated entirely by the F‑BAR domain. In contrast, the C‑terminal half of Hof1 interacts with Myo1, the sole myosin‑II heavy chain in budding yeast, and localizes to the bud neck in a Myo1‑dependent manner from the onset to the completion of cytokinesis. We also show that the SH3 domain in the C‑terminus plays an important role in maintaining the symmetry of Myo1 ring constriction during cytokinesis and that Hof1 interacts with Chs2, a chitin synthase that is required for primary septum formation. Together these data define a mechanism that accounts for the localization of Hof1 during the cell cycle and suggest that Hof1 may function in cytokinesis by coupling actomyosin ring constriction to primary septum formation through interactions with Myo1 and Chs2.

Hof1 and Rvs167 have redundant roles in actomyosin ring function during cytokinesis in budding yeast

The Hof1 protein (Homologue of Fifteen) regulates formation of the primary septum during cytokinesis in the budding yeast Saccharomyces cerevisiae, whereas the orthologous Cdc15 protein in fission yeast regulates the actomyosin ring by using its F-BAR domain to recruit actin nucleators to the cleavage site. Here we show that budding yeast Hof1 also contributes to actin ring assembly in parallel with the Rvs167 protein. Simultaneous deletion of the HOF1 and RVS167 genes is lethal, and cells fail to assemble the actomyosin ring as they progress through mitosis. Although Hof1 and Rvs167 are not orthologues, they both share an analogous structure, with an F-BAR or BAR domain at the amino terminus, capable of inducing membrane curvature, and SH3 domains at the carboxyl terminus that bind to specific proline-rich targets. The SH3 domain of Rvs167 becomes essential for assembly of the actomyosin ring in cells lacking Hof1, suggesting that it helps to recruit a regulator of the actin cytoskeleton. This new function of Rvs167 appears to be independent of its known role as a regulator of the Arp2/3 actin nucleator, as actin ring assembly is not abolished by the simultaneous inactivation of Hof1 and Arp2/3. Instead we find that recruitment to the bud-neck of the Iqg1 actin regulator is defective in cells lacking Hof1 and Rvs167, though future studies will be needed to determine if this reflects a direct interaction between these factors. The redundant role of Hof1 in actin ring assembly suggests that the mechanism of actin ring assembly has been conserved to a greater extent across evolution than anticipated previously.

FCHSD1 and FCHSD2 are expressed in hair cell stereocilia and cuticular plate and regulate actin polymerization in vitro

Mammalian FCHSD1 and FCHSD2 are homologous proteins containing an amino-terminal F-BAR domain and two SH3 domains near their carboxyl-termini. We report here that FCHSD1 and FCHSD2 are expressed in mouse cochlear sensory hair cells. FCHSD1 mainly localizes to the cuticular plate, whereas FCHSD2 mainly localizes along the stereocilia in a punctuate pattern. Nervous Wreck (Nwk), the Drosophila ortholog of FCHSD1 and FCHSD2, has been shown to bind Wsp and play an important role in F-actin assembly. We show that, like its Drosophila counterpart, FCHSD2 interacts with WASP and N-WASP, the mammalian orthologs of Drosophila Wsp, and stimulates F-actin assembly in vitro. In contrast, FCHSD1 doesn’t bind WASP or N-WASP, and can’t stimulate F-actin assembly when tested in vitro. We found, however, that FCHSD1 binds via its F-BAR domain to the SH3 domain of Sorting Nexin 9 (SNX9), a well characterized BAR protein that has been shown to promote WASP-Arp2/3-dependent F-actin polymerization. FCHSD1 greatly enhances SNX9’s WASP-Arp2/3-dependent F-actin polymerization activity. In hair cells, SNX9 was detected in the cuticular plate, where it colocalizes with FCHSD1. Our results suggest that FCHSD1 and FCHSD2 could modulate F-actin assembly or maintenance in hair cell stereocilia and cuticular plate.

The F-BAR protein Cip4/Toca-1 antagonizes the formin Diaphanous in membrane stabilization and compartmentalization

During Drosophila embryogenesis, the first epithelium with defined cortical compartments is established during cellularization. Actin polymerization is required for the separation of lateral and basal domains as well as suppression of tubular extensions in the basal domain. The actin nucleator mediating this function is unknown. We found that the formin Diaphanous (Dia) is required for establishing and maintaining distinct lateral and basal domains during cellularization. In dia mutant embryos lateral marker proteins, such as Discs-large and Armadillo/β-Catenin spread into the basal compartment. Furthermore, high-resolution and live-imaging analysis of dia mutant embryos revealed an increased number of membrane extensions and endocytic activity at the basal domain, indicating a suppressing function of dia on membrane invaginations. Dia function might be based on an antagonistic interaction with the F-BAR protein Cip4/Toca-1, a known activator of the WASP/WAVE-Arp2/3 pathway. Dia and Cip4 physically and functionally interact and overexpression of Cip4 phenocopies dia loss-of-function. In vitro, Cip4 inhibits mainly actin nucleation by Dia. Thus, our data support a model in which linear actin filaments induced by Dia stabilize cortical compartmentalization by antagonizing membrane turnover induced by WASP/WAVE-Arp2/3.

Biochemical and functional significance of F-BAR domain proteins interaction with WASP/N-WASP

The Bin-Amphiphysin-Rvs (BAR) domain family of proteins includes groups which promote positive (classical BAR, N-BAR, and F-BAR) and negative (I-BAR) membrane deformation. Of these groups, the F-BAR subfamily is the most diverse in its biochemical properties. F-BAR domain proteins dimerize to form a tight scaffold about the membrane. The F-BAR domain provides a banana-shaped, alpha-helical structure that senses membrane curvature. Different types of F-BAR domain proteins contain tyrosine kinase or GTPase activities; some interact with phosphatases and RhoGTPases. Most possess an SH3 domain that facilitates the recruitment and activation of WASP/N-WASP. Thus, F-BAR domain proteins affect remodeling of both membrane and the actin cytoskeleton. The purpose of this review is to highlight the role of F-BAR proteins in coupling WASP/N-WASP to cytoskeletal remodeling. A role for F-BAR/WASP interaction in human diseases affecting nervous, blood, and neoplastic tissues is discussed.

Membrane sculpting by F-BAR domains studied by molecular dynamics simulations

Interplay between cellular membranes and their peripheral proteins drives many processes in eukaryotic cells. Proteins of the Bin/Amphiphysin/Rvs (BAR) domain family, in particular, play a role in cellular morphogenesis, for example curving planar membranes into tubular membranes. However, it is still unclear how F-BAR domain proteins act on membranes. Electron microscopy revealed that, in vitro, F-BAR proteins form regular lattices on cylindrically deformed membrane surfaces. Using all-atom and coarse-grained (CG) molecular dynamics simulations, we show that such lattices, indeed, induce tubes of observed radii. A 250 ns all-atom simulation reveals that F-BAR domain curves membranes via the so-called scaffolding mechanism. Plasticity of the F-BAR domain permits conformational change in response to membrane interaction, via partial unwinding of the domains 3-helix bundle structure. A CG simulation covering more than 350 µs provides a dynamic picture of membrane tubulation by lattices of F-BAR domains. A series of CG simulations identified the optimal lattice type for membrane sculpting, which matches closely the lattices seen through cryo-electron microscopy.

To generate organelles, eukaryotic cells sculpt their membranes into compartments, often employing proteins as chaperones, for example, F-BAR domains. The latter induce formation of tubular and vesicular membranes. Functional and structural studies suggest that F-BAR domains sculpt membranes through electrostatic interactions, driving the membrane to match the concave surface of the protein's banana-like shape. Cryo-electron microscopy (cryo-EM) studies provide an average static picture of how F-BAR domains form lattices on the surface of membranes to induce tube formation. Complementing the cryo-EM images, molecular dynamics simulations reported here offer a detailed, dynamic picture of membrane tubulation by a lattice of F-BAR domains and identified lattice types optimally attuned to producing high membrane curvature. The simulations reproduced also a process lasting 350 µs in which lattices of F-BAR domains form a complete tube out of an initially flat membrane. The molecular dynamics study offers, thereby, both a large-scale picture of membrane sculpting by F-BAR domain lattices as well as atomic-level dynamic information about the involvement of the individual F-BAR domain and its interactions with partner F-BAR domains and membrane in the sculpting process.

Wiskott-Aldrich syndrome protein-mediated actin dynamics control type-I interferon production in plasmacytoid dendritic cells

Wiskott-Aldrich Syndrome protein (WASp)–mediated actin polymerization controls intracellular trafficking and compartmentalization of TLR9 ligands in plasmacytoid dendritic cells.

Mutations in Wiskott-Aldrich syndrome (WAS) protein (WASp), a regulator of actin dynamics in hematopoietic cells, cause WAS, an X-linked primary immunodeficiency characterized by recurrent infections and a marked predisposition to develop autoimmune disorders. The mechanisms that link actin alterations to the autoimmune phenotype are still poorly understood. We show that chronic activation of plasmacytoid dendritic cells (pDCs) and elevated type-I interferon (IFN) levels play a role in WAS autoimmunity. WAS patients display increased expression of type-I IFN genes and their inducible targets, alteration in pDCs numbers, and hyperresponsiveness to TLR9. Importantly, ablating IFN-I signaling in WASp null mice rescued chronic activation of conventional DCs, splenomegaly, and colitis. Using WASp-deficient mice, we demonstrated that WASp null pDCs are intrinsically more responsive to multimeric agonist of TLR9 and constitutively secrete type-I IFN but become progressively tolerant to further stimulation. By acute silencing of WASp and actin inhibitors, we show that WASp-mediated actin polymerization controls intracellular trafficking and compartmentalization of TLR9 ligands in pDCs restraining exaggerated activation of the TLR9–IFN-α pathway. Together, these data highlight the role of actin dynamics in pDC innate functions and imply the pDC–IFN-α axis as a player in the onset of autoimmune phenomena in WAS disease.

Calcium oscillations-coupled conversion of actin travelling waves to standing oscillations

Dynamic spatial patterns of signaling factors or macromolecular assemblies in the form of oscillations or traveling waves have emerged as important themes in cell physiology. Feedback mechanisms underlying these processes and their modulation by signaling events and reciprocal cross-talks remain poorly understood. Here we show that antigen stimulation of mast cells triggers cyclic changes in the concentration of actin regulatory proteins and actin in the cell cortex that can be manifested in either spatial pattern. Recruitment of FBP17 and active Cdc42 at the plasma membrane, leading to actin polymerization, are involved in both events, whereas calcium oscillations, which correlate with global fluctuations of plasma membrane PI(4,5)P(2), are tightly linked to standing oscillations and counteract wave propagation. These findings demonstrate the occurrence of a calcium-independent oscillator that controls the collective dynamics of factors linking the actin cytoskeleton to the plasma membrane. Coupling between this oscillator and the one underlying global plasma membrane PI(4,5)P2 and calcium oscillations spatially regulates actin dynamics, revealing an unexpected pattern-rendering mechanism underlying plastic changes occurring in the cortical region of the cell.

Dynamin: expanding its scope to the cytoskeleton

The large GTPase dynamin is well known for its actions on budded cellular membranes to generate vesicles, most often, clathrin-coated endocytic vesicles. The scope of cellular processes in which dynamin-mediated vesicle formation occurs, has expanded to include secretory vesicle formation at the Golgi, from other endosomes and nonclathrin structures, such as caveolae, as well as membrane remodeling during exocytosis and vesicle fusion. An intriguing new facet of dynamin's sphere of influence is the cytoskeleton. Cytoskeletal filament networks maintain cell shape, provide cell movement, execute cell division and orchestrate vesicle trafficking. Recent evidence supports the hypothesis that dynamin influences actin filaments and microtubules via mechanisms that are independent of its membrane-remodeling activities. This chapter discusses this emerging evidence and considers possible mechanisms of action.

Ciliated sensory hair cell formation and function require the F-BAR protein syndapin I and the WH2 domain-based actin nucleator Cobl

During development, general body plan information must be translated into distinct morphologies of individual cells. Shaping cells is thought to involve cortical cytoskeletal components and Bin-Amphiphysin-Rvs167 (BAR) superfamily proteins. We therefore conducted comprehensive side-by-side loss-of-function studies of zebrafish orthologs of the F-BAR protein syndapin I and the actin nucleator Cobl. Zebrafish syndapin I associates with Cobl. The loss-of-function phenotypes of these proteins were remarkably similar and suggested a common function. Both cobl- and syndapin I-morphant fish showed severe swimming and balance-keeping defects, reflecting an impaired organization and function of the lateral line organ. Their lateral line organs lacked several neuromasts and showed an impaired functionality of the sensory hair cells within the neuromasts. Scanning electron microscopy revealed that sensory hair cells of both cobl- and syndapin I-morphant animals showed defects in the formation of both microtubule-dependent kinocilia and F-actin-rich stereocilia. Consistent with the kinocilia defects in sensory hair cells, body length was shortened and the development of body laterality, a process depending on motile cilia, was also impaired. Interestingly, Cobl and syndapin I both localized to the base of forming cilia. Rescue experiments demonstrated that proper formation of ciliated sensory hair cell rosettes relied on Cobl's syndapin I-binding Cobl homology domain, the actin-nucleating C-terminus of Cobl and the membrane curvature-inducing F-BAR domain of syndapin I. Our data thus suggest that the formation of distinct types of ciliary structures relies on membrane topology-modulating mechanisms that are based on F-BAR domain functions and on complex formation of syndapin I with the actin nucleator Cobl.

Monomeric synucleins generate membrane curvature

Synucleins are a family of presynaptic membrane binding proteins. α-Synuclein, the principal member of this family, is mutated in familial Parkinson disease. To gain insight into the molecular functions of synucleins, we performed an unbiased proteomic screen and identified synaptic protein changes in αβγ-synuclein knock-out brains. We observed increases in the levels of select membrane curvature sensing/generating proteins. One of the most prominent changes was for the N-BAR protein endophilin A1. Here we demonstrate that the levels of synucleins and endophilin A1 are reciprocally regulated and that they are functionally related. We show that all synucleins can robustly generate membrane curvature similar to endophilins. However, only monomeric but not tetrameric α-synuclein can bend membranes. Further, A30P α-synuclein, a Parkinson disease mutant that disrupts protein folding, is also deficient in this activity. This suggests that synucleins generate membrane curvature through the asymmetric insertion of their N-terminal amphipathic helix. Based on our findings, we propose to include synucleins in the class of amphipathic helix-containing proteins that sense and generate membrane curvature. These results advance our understanding of the physiological function of synucleins.

[Regulation of intercellular adhesion during epithelial morphogenesis]

The epithelium is one of the most abundant tissues in metazoans. It is required to generate stable chemical and mechanical barriers between physiological compartments (fluid matrix/external environment). This function is based on multiple intercellular junctions, which insulate and stabilize cell-cell contacts in the tissue. Despite this apparent robustness, epithelia can be extensively remodeled during wound healing, embryogenesis and tumor progression. The capacity to be remodeled while keeping tissue cohesion requires a perfect balance between stability and plasticity of intercellular junctions. The balance is partially regulated by intercellular adhesion, which is mostly based on adherens junctions and the transmembrane protein E-cadherin. The aim of this review is to report the molecular basis of the balance between plasticity and robustness in the epithelium. We will first present the minimal physical framework used to describe epithelial cell shape. We will then describe the main processes involved in intercellular adhesion regulation and their functions during epithelial morphogenesis. Eventually, we will analyze the relationship and the coupling between adhesive forces and cortical tension.

The functions of anionic phospholipids during clathrin-mediated endocytosis site initiation and vesicle formation

Anionic phospholipids PI(4,5)P2 and phosphatidylserine (PS) are enriched in the cytosolic leaflet of the plasma membrane where endocytic sites form. In this study, we investigated the roles of PI(4,5)P2 and PS in clathrin-mediated endocytosis (CME) site initiation and vesicle formation in Saccharomyces cerevisiae. Live-cell imaging of endocytic protein dynamics in an mss4(ts) mutant, which has severely reduced PI(4,5)P2 levels, revealed that PI(4,5)P2 is required for endocytic membrane invagination but is less important for endocytic site initiation. We also demonstrated that, in various deletion mutants of genes encoding components of the Rcy1-Ypt31/32 GTPase pathway, endocytic proteins dynamically assemble not only on the plasma membrane but also on intracellular membrane compartments, which are likely derived from early endosomes. In rcy1Δ cells, fluorescent biosensors indicated that PI(4,5)P2 only localized to the plasma membrane while PS localized to both the plasma membrane and intracellular membranes. Furthermore, we found that polarized endocytic patch establishment is defective in the PS-deficient cho1Δ mutant. We propose that PS is important for directing endocytic proteins to the plasma membrane and that PI(4,5)P2 is required to facilitate endocytic membrane invagination.

Plasma membrane reshaping during endocytosis is revealed by time-resolved electron tomography

Endocytosis, like many dynamic cellular processes, requires precise temporal and spatial orchestration of complex protein machinery to mediate membrane budding. To understand how this machinery works, we directly correlated fluorescence microscopy of key protein pairs with electron tomography. We systematically located 211 endocytic intermediates, assigned each to a specific time window in endocytosis, and reconstructed their ultrastructure in 3D. The resulting virtual ultrastructural movie defines the protein-mediated membrane shape changes during endocytosis in budding yeast. It reveals that clathrin is recruited to flat membranes and does not initiate curvature. Instead, membrane invagination begins upon actin network assembly followed by amphiphysin binding to parallel membrane segments, which promotes elongation of the invagination into a tubule. Scission occurs on average 9 s after initial bending when invaginations are ∼100 nm deep, releasing nonspherical vesicles with 6,400 nm2 mean surface area. Direct correlation of protein dynamics with ultrastructure provides a quantitative 4D resource.

Membrane bending is critical for the stability of voltage sensor segments in the membrane

The interaction between membrane proteins and the surrounding membrane is becoming increasingly appreciated for its role in regulating protein function, protein localization, and membrane morphology. In particular, recent studies have suggested that membrane deformation is needed to stably accommodate proteins harboring charged amino acids in their transmembrane (TM) region, as it is energetically prohibitive to bury charge in the hydrophobic core of the bilayer. Unfortunately, current computational methods are poorly equipped for describing such deformations, as atomistic simulations are often too short to observe large-scale membrane reorganization and most continuum approaches assume a flat membrane. Previously, we developed a method that overcomes these shortcomings by using elasticity theory to characterize equilibrium membrane distortions in the presence of a TM protein, while using traditional continuum electrostatic and nonpolar energy models to determine the energy of the protein in the membrane. Here, we linked the elastostatics, electrostatics, and nonpolar numeric solvers to permit the calculation of energies for nontrivial membrane deformations. We then coupled this procedure to a robust search algorithm that identifies optimal membrane shapes for a TM protein of arbitrary chemical composition. This advance now permits us to explore a host of biological phenomena that were beyond the scope of our original method. We show that the energy required to embed charged residues in the membrane can be highly nonadditive, and our model provides a simple mechanical explanation for this nonadditivity. Our results also predict that isolated voltage sensor segments do not insert into rigid membranes, but membrane bending dramatically stabilizes these proteins in the bilayer despite their high charge content. Additionally, we use the model to explore hydrophobic mismatch with regard to nonpolar peptides and mechanosensitive channels. Our method is in quantitative agreement with molecular dynamics simulations at a tiny fraction of the computational cost.

A novel contiguous gene deletion of AVPR2 and ARHGAP4 genes in male dizygotic twins with nephrogenic diabetes insipidus and intellectual disability

The clinical features of loss of ARHGAP4 function remain unclear despite several reports of different patterns of deletions inactivating different functional regions of the protein. The protein encoded by ARHGAP4 is thought to function as a Rho GTPase activating protein. Characterization of the genetic defect causing X-linked nephrogenic diabetes insipidus (NDI) and intellectual disability in two dizygotic twin brothers revealed a novel contiguous deletion of 17,905 bp encompassing the entire AVPR2 gene and extending into intron 7 of the ARHGAP4 gene. Examination of their mother showed that she was a carrier of this deletion. An attempt was made to distinguish the putative clinical signs of an ARHGAP4 deletion from the well-defined phenotype of X-linked NDI caused by an AVPR2 gene deletion. By reviewing all characterized deletions encompassing ARHGAP4, we reconsider the potential role of ARHGAP4 in cognition.

Synaptic vesicle endocytosis

Neurons can sustain high rates of synaptic transmission without exhausting their supply of synaptic vesicles. This property relies on a highly efficient local endocytic recycling of synaptic vesicle membranes, which can be reused for hundreds, possibly thousands, of exo-endocytic cycles. Morphological, physiological, molecular, and genetic studies over the last four decades have provided insight into the membrane traffic reactions that govern this recycling and its regulation. These studies have shown that synaptic vesicle endocytosis capitalizes on fundamental and general endocytic mechanisms but also involves neuron-specific adaptations of such mechanisms. Thus, investigations of these processes have advanced not only the field of synaptic transmission but also, more generally, the field of endocytosis. This article summarizes current information on synaptic vesicle endocytosis with an emphasis on the underlying molecular mechanisms and with a special focus on clathrin-mediated endocytosis, the predominant pathway of synaptic vesicle protein internalization.

Membrane-mediated interactions between rigid inclusions: an effective field theory

An approach based on effective field theory (EFT) is discussed and applied to the problem of surface-mediated interactions between rigid inclusions of circular footprint on a membrane. Instead of explicitly constraining the surface fluctuations in accord with the boundary conditions around the inclusions, the EFT formalism rewrites the theory; the Hamiltonian of a freely fluctuating surface is augmented by pointwise localized terms that capture the same constraints. This allows one to compute the interaction free energy as an asymptotic expansion in inverse separations in a systematic, efficient, and transparent way. Both entropic (fluctuation-induced, Casimir-like) and curvature-elastic (ground-state) forces are considered. Our findings include higher-order corrections to known asymptotic results, on both the pair and the multibody levels. We also show that the few previous attempts in the literature at predicting subleading orders missed some terms due to an uncontrolled point-particle approximation.

Roles for actin assembly in endocytosis

Endocytosis includes a number of processes by which cells internalize segments of their plasma membrane, enclosing a wide variety of material from outside the cell. Endocytosis can contribute to uptake of nutrients, regulation of signaling molecules, control of osmotic pressure, and function of synapses. The actin cytoskeleton plays an essential role in several of these processes. Actin assembly can create protrusions that encompass extracellular materials. Actin can also support the processes of invagination of a membrane segment into the cytoplasm, elongation of the invagination, scission of the new vesicle from the plasma membrane, and movement of the vesicle away from the membrane. We briefly discuss various types of endocytosis, including phagocytosis, macropinocytosis, and clathrin-independent endocytosis. We focus mainly on new findings on the relative importance of actin in clathrin-mediated endocytosis (CME) in yeast versus mammalian cells.

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) specifically induces membrane penetration and deformation by Bin/amphiphysin/Rvs (BAR) domains

Cellular proteins containing Bin/amphiphysin/Rvs (BAR) domains play a key role in clathrin-mediated endocytosis. Despite extensive structural and functional studies of BAR domains, it is still unknown how exactly these domains interact with the plasma membrane containing phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) and whether they function by a universal mechanism or by different mechanisms. Here we report that PtdIns(4,5)P(2) specifically induces partial membrane penetration of the N-terminal amphiphilic α-helix (H(0)) of two representative N-BAR domains from Drosophila amphiphysin (dAmp-BAR) and rat endophilin A1 (EndoA1-BAR). Our quantitative fluorescence imaging analysis shows that PtdIns(4,5)P(2)-dependent membrane penetration of H(0) is important for self-association of membrane-bound dAmp-BAR and EndoA1-BAR and their membrane deformation activity. EndoA1-BAR behaves differently from dAmp-BAR because the former has an additional amphiphilic α-helix that penetrates the membrane in a PtdIns(4,5)P(2)-independent manner. Depletion of PtdIns(4,5)P(2) from the plasma membrane of HEK293 cells abrogated the membrane deforming activity of EndoA1-BAR and dAmp-BAR. Collectively, these studies suggest that the local PtdIns(4,5)P(2) concentration in the plasma membrane may regulate the membrane interaction and deformation by N-BAR domain-containing proteins during clathrin-mediated endocytosis.

A novel patch assembly domain in Num1 mediates dynein anchoring at the cortex during spindle positioning

During mitosis in budding yeast, cortically anchored dynein generates pulling forces on astral microtubules to position the mitotic spindle across the mother-bud neck. The attachment molecule Num1 is required for dynein anchoring at the cell membrane, but how Num1 assembles into stationary cortical patches and interacts with dynein is unknown. We show that an N-terminal Bin/Amphiphysin/Rvs (BAR)-like domain in Num1 mediates the assembly of morphologically distinct patches and its interaction with dynein for spindle translocation into the bud. We name this domain patch assembly domain (PA; residues 1-303), as it was both necessary and sufficient for the formation of functional dynein-anchoring patches when it was attached to a pleckstrin homology domain or a CAAX motif. Distinct point mutations targeting the predicted BAR-like PA domain differentially disrupted patch assembly, dynein anchoring, and mitochondrial attachment functions of Num1. We also show that the PA domain is an elongated dimer and discuss the mechanism by which it drives patch assembly.

Rapid actions of plasma membrane estrogen receptors regulate motility of mouse embryonic stem cells through a profilin-1/cofilin-1-directed kinase signaling pathway

Long-term estrogen actions are vital for driving cell growth, but more recent evidence suggests that estrogen mediates more rapid cellular effects. However, the function of estradiol-17β (E(2))-BSA in mouse embryonic stem cells has not been reported. Therefore, we examined the role of E(2)-BSA in mouse embryonic stem cell motility and its related signal pathways. E(2)-BSA (10(-8) m) significantly increased motility after 24 h incubation and increased filamentous (F)-actin expression; these effects were inhibited by the estrogen receptor antagonist ICI 182,780, indicating that E(2)-BSA bound membrane estrogen receptors and initiated a signal. E(2)-BSA increased c-Src and focal adhesion kinase (FAK) phosphorylation, which was attenuated by ICI 182,780. The E(2)-BSA-induced increase in epidermal growth factor receptor (EGFR) phosphorylation was inhibited by Src inhibitor PP2. As a downstream signal molecule, E(2)-BSA activated cdc42 and increased formation of a complex with the neural Wiskott-Aldrich syndrome protein (N-WASP)/cdc42/transducer of cdc42-dependent actin assembly-1 (TOCA-1), which was inhibited by FAK small interfering RNA (siRNA) and EGFR inhibitor AG 1478. In addition, E(2)-BSA increased profilin-1 expression and cofilin-1 phosphorylation, which was blocked by cdc42 siRNA. Subsequently, E(2)-BSA induced an increase in F-actin expression, and cell motility was inhibited by each signal pathway-related siRNA molecule or inhibitors but not by cofilin-1 siRNA. A combined treatment of cofilin-1 siRNA and E(2)-BSA increased F-actin expression and cell motility more than that of E(2)-BSA alone. These data demonstrate that E(2)-BSA stimulated motility by interacting with profilin-1/cofilin-1 and F-actin through FAK- and c-Src/EGFR transactivation-dependent N-WASP/cdc42/TOCA-1 complex.

Notch ligand endocytosis generates mechanical pulling force dependent on dynamin, epsins, and actin

Notch signaling induced by cell surface ligands is critical to development and maintenance of many eukaryotic organisms. Notch and its ligands are integral membrane proteins that facilitate direct cell-cell interactions to activate Notch proteolysis and release the intracellular domain that directs Notch-specific cellular responses. Genetic studies suggest that Notch ligands require endocytosis, ubiquitylation, and epsin endocytic adaptors to activate signaling, but the exact role of ligand endocytosis remains unresolved. Here we characterize a molecularly distinct mode of clathrin-mediated endocytosis requiring ligand ubiquitylation, epsins, and actin for ligand cells to activate signaling in Notch cells. Using a cell-bead optical tweezers system, we obtained evidence for cell-mediated mechanical force dependent on this distinct mode of ligand endocytosis. We propose that the mechanical pulling force produced by endocytosis of Notch-bound ligand drives conformational changes in Notch that permit activating proteolysis.

Sho1p connects the plasma membrane with proteins of the cytokinesis network through multiple isomeric interaction states

An understanding of cytokinesis at the molecular level requires a detailed description of the protein complexes that perform central activities during this process. The proteins Hof1p, Cyk3p, Inn1p and Myo1p each represent one of the four genetically defined and partially complementary pathways of cytokinesis in the yeast Saccharomyces cerevisiae. Here we show that the osmosensor Sho1p is required for correct cell-cell separation. Shortly before cytokinesis Sho1p sequentially assembles with Hof1p, Inn1p and Cyk3p, into a complex (the HICS complex) that might help to connect the membrane with the actin-myosin ring. The HICS complex is formed exclusively through interactions between three SH3 domains located in Cyk3p, Hof1p and Sho1p, and five acceptor sites found in Cyk3p, Hof1p and Inn1p. Owing to the overlapping binding specificities of its members the HICS complex is best described as ensembles of isomeric interaction states that precisely coordinate the different functions of the interactors during cytokinesis.

Actin- and dynamin-dependent maturation of bulk endocytosis restores neurotransmission following synaptic depletion

Bulk endocytosis contributes to the maintenance of neurotransmission at the amphibian neuromuscular junction by regenerating synaptic vesicles. How nerve terminals internalize adequate portions of the presynaptic membrane when bulk endocytosis is initiated before the end of a sustained stimulation is unknown. A maturation process, occurring at the end of the stimulation, is hypothesised to precisely restore the pools of synaptic vesicles. Using confocal time-lapse microscopy of FM1-43-labeled nerve terminals at the amphibian neuromuscular junction, we confirm that bulk endocytosis is initiated during a sustained tetanic stimulation and reveal that shortly after the end of the stimulation, nerve terminals undergo a maturation process. This includes a transient bulging of the plasma membrane, followed by the development of large intraterminal FM1-43-positive donut-like structures comprising large bulk membrane cisternae surrounded by recycling vesicles. The degree of bulging increased with stimulation frequency and the plasmalemma surface retrieved following the transient bulging correlated with the surface membrane internalized in bulk cisternae and recycling vesicles. Dyngo-4a, a potent dynamin inhibitor, did not block the initiation, but prevented the maturation of bulk endocytosis. In contrast, cytochalasin D, an inhibitor of actin polymerization, hindered both the initiation and maturation processes. Both inhibitors hampered the functional recovery of neurotransmission after synaptic depletion. Our data confirm that initiation of bulk endocytosis occurs during stimulation and demonstrates that a delayed maturation process controlled by actin and dynamin underpins the coupling between exocytosis and bulk endocytosis.

Fabrication of lipid tubules with embedded quantum dots by membrane tubulation protein

The first one-dimensional (1D) assembly of low-toxicity CuInS(2) /ZnS quantum dots (QDs) embedded in lipid nanotubules, formed from liposomes using the Amphiphysin-BAR (Bin-Amphiphysin-Rvs domain of human amphiphysin) protein to elongate the structure, is reported. The QD-containing lipid nanotubules display a high aspect ratio of ≈500:1 (≈40 nm diameter and 20 μm length) and are stable for more than 20 h. Furthermore, this methodology is extended to the assembly of various nanoparticle species within 1D lipid nanotubules, and includes materials such as CdSe and Au. Encapsulation within the hydrophobic core of the bilayer makes these materials highly biocompatible. The developed methodology and materials with these unique characteristics could be useful for various applications in nanobiotechnology and nanomedicine.

Rigidity of wedge loop in PACSIN 3 protein is a key factor in dictating diameters of tubules

BAR (Bin/amphiphysin/Rvs) domain-containing proteins participate in cellular membrane remodeling. The F-BAR proteins normally generate low curvature tubules. However, in the PACSIN subfamily, the F-BAR domain from PACSIN 1 and 2 can induce both high and low curvature tubules. We found that unlike PACSIN 1 and 2, PACSIN 3 could only induce low curvature tubules. To elucidate the key factors that dictate the tubule curvature, crystal structures of all three PACSIN F-BAR domains were determined. A novel type of lateral interaction mediated by a wedge loop is observed between the F-BAR neighboring dimers. Comparisons of the structures of PACSIN 3 with PACSIN 1 and 2 indicate that the wedge loop of PACSIN 3 is more rigid, which influences the lateral interactions between assembled dimers. We further identified the residues that affect the rigidity of the loop by mutagenesis and determined the structures of two PACSIN 3 wedge loop mutants. Our results suggest that the rigidity-mediated conformations of the wedge loop correlate well with the various crystal packing modes and membrane tubulations. Thus, the rigidity of the wedge loop is a key factor in dictating tubule diameters.

Inhibition of SRGAP2 function by its human-specific paralogs induces neoteny during spine maturation

Structural genomic variations represent a major driving force of evolution, and a burst of large segmental gene duplications occurred in the human lineage during its separation from nonhuman primates. SRGAP2, a gene recently implicated in neocortical development, has undergone two human-specific duplications. Here, we find that both duplications (SRGAP2B and SRGAP2C) are partial and encode a truncated F-BAR domain. SRGAP2C is expressed in the developing and adult human brain and dimerizes with ancestral SRGAP2 to inhibit its function. In the mouse neocortex, SRGAP2 promotes spine maturation and limits spine density. Expression of SRGAP2C phenocopies SRGAP2 deficiency. It underlies sustained radial migration and leads to the emergence of human-specific features, including neoteny during spine maturation and increased density of longer spines. These results suggest that inhibition of SRGAP2 function by its human-specific paralogs has contributed to the evolution of the human neocortex and plays an important role during human brain development.

Membrane-deforming proteins play distinct roles in actin pedestal biogenesis by enterohemorrhagic Escherichia coli

Many bacterial pathogens reorganize the host actin cytoskeleton during the course of infection, including enterohemorrhagic Escherichia coli (EHEC), which utilizes the effector protein EspF(U) to assemble actin filaments within plasma membrane protrusions called pedestals. EspF(U) activates N-WASP, a host actin nucleation-promoting factor that is normally auto-inhibited and found in a complex with the actin-binding protein WIP. Under native conditions, this N-WASP/WIP complex is activated by the small GTPase Cdc42 in concert with several different SH3 (Src-homology-3) domain-containing proteins. In the current study, we tested whether SH3 domains from the F-BAR (FCH-Bin-Amphiphysin-Rvs) subfamily of membrane-deforming proteins are involved in actin pedestal formation. We found that three F-BAR proteins: CIP4, FBP17, and TOCA1 (transducer of Cdc42-dependent actin assembly), play different roles during actin pedestal biogenesis. Whereas CIP4 and FBP17 inhibited actin pedestal assembly, TOCA1 stimulated this process. TOCA1 was recruited to pedestals by its SH3 domain, which bound directly to proline-rich sequences within EspF(U). Moreover, EspF(U) and TOCA1 activated the N-WASP/WIP complex in an additive fashion in vitro, suggesting that TOCA1 can augment actin assembly within pedestals. These results reveal that EspF(U) acts as a scaffold to recruit multiple actin assembly factors whose functions are normally regulated by Cdc42.

Barfly: sculpting membranes at the Drosophila neuromuscular junction

The ability of a cell to change the shape of its membranes is intrinsic to many cellular functions. Proteins that can alter or recognize curved membrane structures and those that can act to recruit other proteins which stabilize the membrane curvature are likely to be essential in cell functions. The BAR (Bin, amphiphysin, RVS167 homology) domain is a protein domain that can either induce lipidic membranes to curve or can sense curved membranes. BAR domains are found in several proteins at neuronal synapses. We will review BAR domain structure and the role that BAR domain containing proteins play in regulating the morphology and function of the Drosophila neuromuscular junction. In flies the BAR domain containing proteins, endophilin and syndapin affect synaptic vesicle endocytosis, whereas CIP4, dRich, nervous wreck and syndapin affect synaptic morphology. We will review the growing evidence implicating mutations in BAR domain containing proteins being the cause of human pathologies.

Getting active: protein sorting in endocytic recycling

Endocytic recycling returns proteins to the plasma membrane in many physiological contexts. Studies of these events have helped to elucidate fundamental mechanisms that underlie recycling. Recycling was for some time considered to be the exception to a general mechanism of active cargo sorting in multiple intracellular pathways. In recent years, studies have begun to reconcile this seeming disparity and also suggest explanations for why early recycling studies did not detect active sorting. Further articulation of this emerging trend has far-reaching implications for a deeper understanding of many physiological and pathological events that require recycling.

The F-BAR domains from srGAP1, srGAP2 and srGAP3 regulate membrane deformation differently

Coordination of membrane deformation and cytoskeletal dynamics lies at the heart of many biological processes critical for cell polarity, motility and morphogenesis. We have recently shown that Slit-Robo GTPase-activating protein 2 (srGAP2) regulates neuronal morphogenesis through the ability of its F-BAR domain to regulate membrane deformation and induce filopodia formation. Here, we demonstrate that the F-BAR domains of two closely related family members, srGAP1 and srGAP3 [designated F-BAR(1) and F-BAR(3), respectively] display significantly different membrane deformation properties in non-neuronal COS7 cells and in cortical neurons. F-BAR(3) induces filopodia in both cell types, though less potently than F-BAR(2), whereas F-BAR(1) prevents filopodia formation in cortical neurons and reduces plasma membrane dynamics. These three F-BAR domains can heterodimerize, and they act synergistically towards filopodia induction in COS7 cells. As measured by fluorescence recovery after photobleaching, F-BAR(2) displays faster molecular dynamics than F-BAR(3) and F-BAR(1) at the plasma membrane, which correlates well with its increased potency to induce filopodia. We also show that the molecular dynamic properties of F-BAR(2) at the membrane are partially dependent on F-Actin. Interestingly, acute phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] depletion in cells does not interfere with plasma membrane localization of F-BAR(2), which is compatible with our result showing that F-BAR(2) binds to a broad range of negatively-charged phospholipids present at the plasma membrane, including phosphatidylserine (PtdSer). Overall, our results provide novel insights into the functional diversity of the membrane deformation properties of this subclass of F-BAR-domains required for cell morphogenesis.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

The BAR Domain Superfamily Proteins from Subcellular Structures to Human Diseases

Eukaryotic cells have complicated membrane systems. The outermost plasma membrane contains various substructures, such as invaginations and protrusions, which are involved in endocytosis and cell migration. Moreover, the intracellular membrane compartments, such as autophagosomes and endosomes, are essential for cellular viability. The Bin-Amphiphysin-Rvs167 (BAR) domain superfamily proteins are important players in membrane remodeling through their structurally determined membrane binding surfaces. A variety of BAR domain superfamily proteins exist, and each family member appears to be involved in the formation of certain subcellular structures or intracellular membrane compartments. Most of the BAR domain superfamily proteins contain SH3 domains, which bind to the membrane scission molecule, dynamin, as well as the actin regulatory WASP/WAVE proteins and several signal transduction molecules, providing possible links between the membrane and the cytoskeleton or other machineries. In this review, we summarize the current information about each BAR superfamily protein with an SH3 domain(s). The involvement of BAR domain superfamily proteins in various diseases is also discussed.