GTP-stimulated membrane fission by the N-BAR protein AMPH-1.

Membrane-enclosed transport carriers sort biological molecules between stations in the cell in a dynamic process that is fundamental to the physiology of eukaryotic organisms. While much is known about the formation and release of carriers from specific intracellular membranes, the mechanism of carrier formation from the recycling endosome, a compartment central to cellular signaling, remains to be resolved. In Caenorhabditis elegans, formation of transport carriers from the recycling endosome requires the dynamin-like, Eps15-homology domain (EHD) protein, RME-1, functioning with the Bin/Amphiphysin/Rvs (N-BAR) domain protein, AMPH-1. Here we show, using a free-solution single-particle technique known as burst analysis spectroscopy (BAS), that AMPH-1 alone creates small, tubular-vesicular products from large, unilamellar vesicles by membrane fission. Membrane fission requires the amphipathic H0 helix of AMPH-1 and is slowed in the presence of RME-1. Unexpectedly, AMPH-1-induced membrane fission is stimulated in the presence of GTP. Furthermore, the GTP-stimulated membrane fission activity seen for AMPH-1 is recapitulated by the heterodimeric N-BAR amphiphysin protein from yeast, Rvs161/167p, strongly suggesting that GTP-stimulated membrane fission is a general property of this important class of N-BAR proteins.

Single particle fluorescence burst analysis of epsin induced membrane fission.

Vital cellular processes, from cell growth to synaptic transmission, rely on membrane-bounded carriers and vesicles to transport molecular cargo to and from specific intracellular compartments throughout the cell. Compartment-specific proteins are required for the final step, membrane fission, which releases the transport carrier from the intracellular compartment. The role of fission proteins, especially at intracellular locations and in non-neuronal cells, while informed by the dynamin-1 paradigm, remains to be resolved. In this study, we introduce a highly sensitive approach for the identification and analysis of membrane fission machinery, called burst analysis spectroscopy (BAS). BAS is a single particle, free-solution approach, well suited for quantitative measurements of membrane dynamics. Here, we use BAS to analyze membrane fission induced by the potent, fission-active ENTH domain of epsin. Using this method, we obtained temperature-dependent, time-resolved measurements of liposome size and concentration changes, even at sub-micromolar concentration of the epsin ENTH domain. We also uncovered, at 37°C, fission activity for the full-length epsin protein, supporting the argument that the membrane-fission activity observed with the ENTH domain represents a native function of the full-length epsin protein.

Clathrin coat disassembly by the yeast Hsc70/Ssa1p and auxilin/Swa2p proteins observed by single-particle burst analysis spectroscopy.

The role of clathrin-coated vesicles in receptor-mediated endocytosis is conserved among eukaryotes, and many of the proteins required for clathrin coat assembly and disassembly have orthologs in yeast and mammals. In yeast, dozens of proteins have been identified as regulators of the multistep reaction required for endocytosis, including those that regulate disassembly of the clathrin coat. In mammalian systems, clathrin coat disassembly has been reconstituted using neuronal clathrin baskets mixed with the purified chaperone ATPase 70-kDa heat shock cognate (Hsc70), plus a clathrin-specific co-chaperone, such as the synaptic protein auxilin. Yet, despite previous characterization of the yeast Hsc70 ortholog, Ssa1p, and the auxilin-like ortholog, Swa2p, testing mechanistic models for disassembly of nonneuronal clathrin coats has been limited by the absence of a functional reconstitution assay. Here we use single-particle burst analysis spectroscopy, in combination with fluorescence correlation spectroscopy, to follow the population dynamics of fluorescently tagged yeast clathrin baskets in the presence of purified Ssa1p and Swa2p. An advantage of this combined approach for mechanistic studies is the ability to measure, as a function of time, changes in the number and size of objects from a starting population to the reaction products. Our results indicate that Ssa1p and Swa2p cooperatively disassemble yeast clathrin baskets into fragments larger than the individual triskelia, suggesting that disassembly of clathrin-coated vesicles may proceed through a partially uncoated intermediate.

Regulation of exocytosis by the exocyst subunit Sec6 and the SM protein Sec1.

Trafficking of protein and lipid cargo through the secretory pathway in eukaryotic cells is mediated by membrane-bound vesicles. Secretory vesicle targeting and fusion require a conserved multisubunit protein complex termed the exocyst, which has been implicated in specific tethering of vesicles to sites of polarized exocytosis. The exocyst is directly involved in regulating soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) complexes and membrane fusion through interactions between the Sec6 subunit and the plasma membrane SNARE protein Sec9. Here we show another facet of Sec6 function-it directly binds Sec1, another SNARE regulator, but of the Sec1/Munc18 family. The Sec6-Sec1 interaction is exclusive of Sec6-Sec9 but compatible with Sec6-exocyst assembly. In contrast, the Sec6-exocyst interaction is incompatible with Sec6-Sec9. Therefore, upon vesicle arrival, Sec6 is proposed to release Sec9 in favor of Sec6-exocyst assembly and to simultaneously recruit Sec1 to sites of secretion for coordinated SNARE complex formation and membrane fusion.

At the junction of SNARE and SM protein function.

Sec1/Munc18 (SM) proteins bind to and function with soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) at each vesicle fusion site in the cell. The purpose for these interactions is becoming clearer, as what had been interpreted as functional divergence between SM proteins acting at different vesicle trafficking steps, or in specialized cells, is giving way to more recent evidence for common functions among all SM proteins. What is emerging is a picture of SM proteins acting not merely as SNARE regulators, but also as central components of the membrane fusion apparatus. The available data suggest sequential models that describe how the soluble SM protein might first regulate SNARE complex assembly and then cooperate with SNAREs to stimulate membrane fusion.

AMPH-1/Amphiphysin/Bin1 functions with RME-1/Ehd1 in endocytic recycling.

RME-1/EHD1 (receptor mediated endocytosis/Eps15 homology-domain containing 1) family proteins are key residents of the recycling endosome, which are required for endosome-to-plasma membrane transport in Caenorhabditis elegans and mammals. Recent studies suggest similarities between the RME-1/EHD proteins and the Dynamin GTPase superfamily of mechanochemical pinchases, which promote membrane fission. Here we show that endogenous C. elegans AMPH-1, the only C. elegans member of the Amphiphysin/BIN1 family of BAR (Bin1-Amphiphysin-Rvs161p/167p)-domain-containing proteins, colocalizes with RME-1 on recycling endosomes in vivo, that amph-1-deletion mutants are defective in recycling endosome morphology and function, and that binding of AMPH-1 Asn-Pro-Phe(Asp/Glu) sequences to the RME-1 EH-domain promotes the recycling of transmembrane cargo. We also show a requirement for human BIN1 (also known as Amphiphysin 2) in EHD1-regulated endocytic recycling. In vitro, we find that purified recombinant AMPH-1-RME-1 complexes produce short, coated membrane tubules that are qualitatively distinct from those produced by either protein alone. Our results indicate that AMPH-1 and RME-1 cooperatively regulate endocytic recycling, probably through functions required for the production of cargo carriers that exit the recycling endosome for the cell surface.

Yeast Sec1p functions before and after vesicle docking.

Sec1/Munc18 (SM) proteins bind cognate soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes and stimulate vesicle membrane fusion. Before fusion, vesicles are docked to specific target membranes. Regulation of vesicle docking is attributed to some but not all SM proteins, suggesting specialization of this earlier function. Yeast Sec1p seems to function only after vesicles are docked and SNARE complexes are assembled. Here, we show that yeast Sec1p is required before and after SNARE complex assembly, in support of general requirements for SM proteins in both vesicle docking and fusion. Two classes of sec1 mutants were isolated. Class A mutants are tightly blocked in cell growth and secretion at a step before SNARE complex assembly. Class B mutants have a SNARE complex binding defect, with a range in severity of cell growth and secretion defects. Mapping the mutations onto an SM protein structure implicates a peripheral bundle of helices for the early, docking function and a deep groove, opposite the syntaxin-binding cleft on nSec1/Munc-18, for the interaction between Sec1p and the exocytic SNARE complex.

Tag team action at the synapse.

Communication between neurons relies on chemical synapses and the release of neurotransmitters into the synaptic cleft. Neurotransmitter release is an exquisitely regulated membrane fusion event that requires the linking of an electrical nerve stimulus to Ca(2+) influx, which leads to the fusion of neurotransmitter-filled vesicles with the cell membrane. The timing of neurotransmitter release is controlled through the regulation of the soluble N-ethylmaleimide sensitive factor attachment receptor (SNARE) proteins-the core of the membrane fusion machinery. Assembly of the fusion-competent SNARE complex is regulated by several neuronal proteins, including complexin and the Ca(2+)-sensor synaptotagmin. Both complexin and synaptotagmin bind directly to SNAREs, but their mechanism of action has so far remained unclear. Recent studies revealed that synaptotagmin-Ca(2+) and complexin collaborate to regulate membrane fusion. These compelling new results provide a molecular mechanistic insight into the functions of both proteins: complexin 'clamps' the SNARE complex in a pre-fusion intermediate, which is then released by the action of Ca(2+)-bound synaptotagmin to trigger rapid fusion.

Specific SNARE complex binding mode of the Sec1/Munc-18 protein, Sec1p.

The Sec1/Munc-18 (SM) family of proteins is required for vesicle fusion in eukaryotic cells and has been linked to the membrane-fusion proteins known as soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). SM proteins may activate the target-membrane SNARE, syntaxin, for assembly into the fusogenic SNARE complex. In support of an activation role, SM proteins bind directly to their cognate syntaxins. An exception is the yeast Sec1p, which does not bind the yeast plasma-membrane syntaxin, Sso1p. This exception could be explained if the SM interaction motif were blocked by the highly stable closed conformation of Sso1p. We tested the possibility of a latent binding motif using sso1 mutants in yeast and reconstituted the Sec1p binding specificity observed in vivo with purified proteins in vitro. Our results indicate there is no latent binding motif in Sso1p. Instead, Sec1p binds specifically to the ternary SNARE complex, with no detectable binding to the binary t-SNARE complex or any of the three individual SNAREs in their uncomplexed forms. We propose that vesicle fusion requires a specific interaction between the SM protein and the ternary SNARE complex.