Members of the Bin/amphiphysin/Rvs (BAR) domain protein superfamily are involved in membrane remodeling in various cellular pathways ranging from endocytic vesicle and T-tubule formation to cell migration and neuromorphogenesis. Membrane curvature induction and stabilization are encoded within the BAR or Fer-CIP4 homology-BAR (F-BAR) domains, α-helical coiled coils that dimerize into membrane-binding modules. BAR/F-BAR domain proteins often contain an SH3 domain, which recruits binding partners such as the oligomeric membrane-fissioning GTPase dynamin. How precisely BAR/F-BAR domain-mediated membrane deformation is regulated at the cellular level is unknown. Here we present the crystal structures of full-length syndapin 1 and its F-BAR domain. Our data show that syndapin 1 F-BAR-mediated membrane deformation is subject to autoinhibition by its SH3 domain. Release from the clamped conformation is driven by association of syndapin 1 SH3 with the proline-rich domain of dynamin 1, thereby unlocking its potent membrane-bending activity. We hypothesize that this mechanism might be commonly used to regulate BAR/F-BAR domain-induced membrane deformation and to potentially couple this process to dynamin-mediated fission. Our data thus suggest a structure-based model for SH3-mediated regulation of BAR/F-BAR domain function.

SUMMARY Dynamin is a 100 kDa GTPase that organizes into helical assemblies at the base of nascent clathrin-coated vesicles. Formation of these oligomers stimulates the intrinsic GTPase activity of dynamin, which is necessary for efficient membrane fission during endocytosis. Recent evidence suggests that the transition-state of dynamin's GTP hydrolysis reaction serves as a key determinant of productive fission. Here we present the structure of a transition-state-defective dynamin mutant, K44A, trapped in a pre-fission state, at 12.5 Å resolution. This structure constricts to 3.7 nm, reaching the theoretical limit required for spontaneous membrane fission. Computational docking indicates that the ground state conformation of the dynamin polymer is sufficient to achieve this super-constricted pre-fission state and reveals how a 2-start helical symmetry promotes the most efficient packing of dynamin tetramers around the membrane neck. These data suggest a new model for the assembly and regulation of the minimal dynamin fission machine.

Fusion and fission drive all vesicular transport. Although topologically opposite, these reactions pass through the same hemi-fusion/fission intermediate1,2, characterized by a ‘stalk’ in which only the inner monolayers of the two compartments have merged to form a localized non-bilayer connection1-3. Formation of the hemi-fission intermediate requires energy input from proteins catalyzing membrane remodeling; however the relationship between protein conformational rearrangements and hemi-fusion/fission remains obscure. Here we analyzed how the GTPase cycle of dynamin, the prototypical membrane fission catalyst4-6, is directly coupled to membrane remodeling. We used intra-molecular chemical cross-linking to stabilize dynamin in its GDP•AlF4--bound transition-state. In the absence of GTP this conformer produced stable hemi-fission, but failed to progress to complete fission, even in the presence of GTP. Further analysis revealed that the pleckstrin homology domain (PHD) locked in its membrane-inserted state facilitated hemi-fission. A second mode of dynamin activity, fueled by GTP hydrolysis, couples dynamin disassembly with cooperative diminishing of the PHD wedging, thus destabilizing the hemi-fission intermediate to complete fission. Molecular simulations corroborate the bimodal character of dynamin action and indicate radial and axial forces as dominant, although not independent drivers of hemi-fission and fission transformations, respectively. Mirrored in the fusion reaction7-8, the force bimodality might constitute a general paradigm for leakage-free membrane remodeling.

Clathrin-mediated vesicle recycling in synapses is maintained by a unique set of endocytic proteins and interactions. We show that endophilin localizes in the vesicle pool at rest and in spirals at the necks of clathrin-coated pits (CCPs) during activity in lamprey synapses. Endophilin and dynamin colocalize at the base of the clathrin coat. Protein spirals composed of these proteins on lipid tubes in vitro have a pitch similar to the one observed at necks of CCPs in living synapses, and lipid tubules are thinner than those formed by dynamin alone. Tubulation efficiency and the amount of dynamin recruited to lipid tubes are dramatically increased in the presence of endophilin. Blocking the interactions of the endophilin SH3 domain in situ reduces dynamin accumulation at the neck and prevents the formation of elongated necks observed in the presence of GTPγS. Therefore, endophilin recruits dynamin to a restricted part of the CCP neck, forming a complex, which promotes budding of new synaptic vesicles.

Intersectin is a multidomain dynamin-binding protein implicated in numerous functions in the nervous system, including synapse formation and endocytosis. Here, we demonstrate that during neurotransmitter release in the central synapse, intersectin, like its binding partner dynamin, is redistributed from the synaptic vesicle pool to the periactive zone. Acute perturbation of the intersectin-dynamin interaction by microinjection of either intersectin antibodies or Src homology 3 (SH3) domains inhibited endocytosis at the fission step. Although the morphological effects induced by the different reagents were similar, antibody injections resulted in a dramatic increase in dynamin immunoreactivity around coated pits and at constricted necks, whereas synapses microinjected with the GST (glutathione S-transferase)-SH3C domain displayed reduced amounts of dynamin in the neck region. Our data suggest that intersectin controls the amount of dynamin released from the synaptic vesicle cluster to the periactive zone and that it may regulate fission of clathrin-coated intermediates.

Clathrin-mediated synaptic vesicle (SV) recycling involves the spatiotemporally controlled assembly of clathrin coat components at phosphatidylinositiol (4, 5)-bisphosphate [PI(4,5)P 2 ]-enriched membrane sites within the periactive zone. Such spatiotemporal control is needed to coordinate SV cargo sorting with clathrin/AP2 recruitment and to restrain membrane fission and synaptojanin-mediated uncoating until membrane deformation and clathrin coat assembly are completed. The molecular events underlying these control mechanisms are unknown. Here we show that the endocytic SH3 domaincontaining accessory protein intersectin 1 scaffolds the endocytic process by directly associating with the clathrin adaptor AP2. Acute perturbation of the intersectin 1-AP2 interaction in lamprey synapses in situ inhibits the onset of SV recycling. Structurally, complex formation can be attributed to the direct association of hydrophobic peptides within the intersectin 1 SH3A-B linker region with the "side sites" of the AP2 α-and β-appendage domains. AP2 appendage association of the SH3A-B linker region inhibits binding of the inositol phosphatase synaptojanin 1 to intersectin 1. These data identify the intersectin-AP2 complex as an important regulator of clathrinmediated SV recycling in synapses.endocytosis | synapse | scaffolding proteins | appendage | synaptojanin S ynaptic vesicles (SVs), following their activity-dependent exocytic fusion with the presynaptic plasma membrane, are recycled by compensatory endocytosis at the periactive zone (1-3), largely via clathrin-mediated reinternalization of fully fused SV membrane (4). Clathrin-coated pit (CCP) formation (5) proceeds through the assembly of endocytic proteins at phosphatidylinositiol (4, 5)-bisphosphate [PI(4,5)P 2 ]-enriched membrane sites (6, 7). A key factor in the assembly pathway is the heterotetrameric adaptor complex AP2, whose α-and β2-appendage domains act as major recruitment platforms for accessory proteins (6, 7), regulating distinct steps within the pathway. Despite our extensive knowledge regarding the endocytic interactome, we know comparably little about the structural components within the periactive zone that scaffold the endocytic process, thereby allowing the high fidelity of SV recycling. Such spatiotemporal control is needed to coordinate SV cargo protein sorting with coat recruitment (8) and to restrain membrane fission and uncoating until membrane deformation and CCP assembly are completed. Moreover, stabilizing scaffolds may aid coupling of SV exo-and endocytosis (1, 3). The Drosophila multidomain protein Dap160, an ortholog of mammalian intersectin, has been postulated to act as an endocytic scaffold of the periactive zone (9-11), although its precise role in SV recycling in mammalian nerve terminals remains largely unclear (12).Here we show that intersectin 1 scaffolds the endocytic process by directly associating with AP2. Acute perturbation of intersectin-AP2 complex formation blocks the onset of SV recycling. Moreover, association of the SH3A-B l...

Objective-The proatherogenic effect of IL-18 is shown to be dependent on IFN-␥ production. It is believed that activated T cells play a proatherogenic role through secretion of IFN-␥. However, recent studies in vitro have shown that macrophages, NK cells, and even vascular smooth muscle cells may also secrete IFN-␥ after stimulation by cytokines like IL-18. We therefore investigated whether cells other than activated T cells can play a proatherogenic role via IFN-␥ secretion under the stimulation of IL-18 in vivo. Methods and Results-SCID/apoE knockout mice were injected intraperitoneally with either IL-18 or phosphate-buffered saline 3 times per week for 7 weeks. Our results show that administration of IL-18 leads to 3-fold larger lesions and 2-fold higher circulating IFN-␥ despite the absence of T cells. In addition, increased IFN-␥, accompanied by elevation of the scavenger receptor/chemokine CXCL16, was observed in both lesions and spleens.

Integration of the reverse-transcribed viral DNA into host chromosomes is a critical step in the life-cycle of retroviruses, including an oncogenic delta(δ)-retrovirus human T-cell leukemia virus type-1 (HTLV-1). Retroviral integrase forms a higher order nucleoprotein assembly (intasome) to catalyze the integration reaction, in which the roles of host factors remain poorly understood. Here, we use cryo-electron microscopy to visualize the HTLV-1 intasome at 3.7-Å resolution. The structure together with functional analyses reveal that the B56γ (B’γ) subunit of an essential host enzyme, protein phosphatase 2 A (PP2A), is repurposed as an integral component of the intasome to mediate HTLV-1 integration. Our studies reveal a key host-virus interaction underlying the replication of an important human pathogen and highlight divergent integration strategies of retroviruses.