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Biomedical subjects

Thomas J Melia

Publications and source records attributed to Thomas J Melia.

9 recordsLinked to original sources
PubMed2007

Selective activation of cognate SNAREpins by Sec1/Munc18 proteins.

Sec1/Munc18 (SM) proteins are required for every step of intracellular membrane fusion, but their molecular mechanism of action has been unclear. In this work, we demonstrate a fundamental role of the SM protein: to act as a stimulatory subunit of its cognate SNARE fusion machinery. In a reconstituted system, mammalian SNARE pairs assemble between bilayers to drive a basal fusion reaction. Munc18-1/nSec1, a synaptic SM protein required for neurotransmitter release, strongly accelerates this reaction through direct contact with both t- and v-SNAREs. Munc18-1 accelerates fusion only for the cognate SNAREs for exocytosis, therefore enhancing fusion specificity.

Amino Acid Motifs↗
PubMed2006

Lipidic antagonists to SNARE-mediated fusion.

SNARE proteins mediate the fusion of lipid bilayers by the directed assembly of coiled-coil domains arising from apposing membranes. We have utilized inverted cone-shaped lipids, antagonists of the necessary membrane deformation during fusion to characterize the extent and range of SNARE assembly up to the moment of stalk formation between bilayers. The inverted cone-shaped lipid family of acyl-CoAs specifically inhibits the completion of fusion in an acyl-chain length-dependent manner. Removal of acyl-CoA from the membrane relieves the inhibition and initiates a burst of membrane fusion with rates exceeding any point in the control curves lacking acyl-CoA. This burst indicates the accumulation of semi-assembled fusion complexes. These preformed complexes are resistant to cleavage by botulinum toxin B and thus appear to have progressed beyond the "loosely zippered" state of docked synaptic vesicles. Surprisingly, application of the soluble domain of VAMP2, which blocks SNARE assembly by competing for binding on the available t-SNAREs, blocks recovery from the acyl-CoA inhibition. Thus, complexes formed in the presence of a lipidic antagonist to fusion are incompletely assembled, suggesting that the formation of tightly assembled SNARE pairs requires progression all the way through to membrane fusion. In this regard, physiologically docked exocytic vesicles may be anchored by a highly dynamic and potentially even reversible SNAREpin.

Acyl Coenzyme A↗
PubMed2006

A clamping mechanism involved in SNARE-dependent exocytosis.

During neurotransmitter release at the synapse, influx of calcium ions stimulates the release of neurotransmitter. However, the mechanism by which synaptic vesicle fusion is coupled to calcium has been unclear, despite the identification of both the core fusion machinery [soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)] and the principal calcium sensor (synaptotagmin). Here, we describe what may represent a basic principle of the coupling mechanism: a reversible clamping protein (complexin) that can freeze the SNAREpin, an assembled fusion-competent intermediate en route to fusion. When calcium binds to the calcium sensor synaptotagmin, the clamp would then be released. SNARE proteins, and key regulators like synaptotagmin and complexin, can be ectopically expressed on the cell surface. Cells expressing such "flipped" synaptic SNAREs fuse constitutively, but when we coexpressed complexin, fusion was blocked. Adding back calcium triggered fusion from this intermediate in the presence of synaptotagmin.

Adaptor Proteins, Vesicular Transport↗
PubMed2005

SNAREs can promote complete fusion and hemifusion as alternative outcomes.

Using a cell fusion assay, we show here that in addition to complete fusion SNAREs also promote hemifusion as an alternative outcome. Approximately 65% of events resulted in full fusion, and the remaining 35% in hemifusion; of those, approximately two thirds were permanent and approximately one third were reversible. We predict that this relatively close balance among outcomes could be tipped by binding of regulatory proteins to the SNAREs, allowing for dynamic physiological regulation between full fusion and reversible kiss-and-run-like events.

3T3 Cells↗
PubMed2004

How a G protein binds a membrane.

Heterotrimeric G proteins interact with receptors and effectors at the membrane-cytoplasm interface. Structures of soluble forms have not revealed how they interact with membranes. We have used electron crystallography to determine the structure in ice of a helical array of the photoreceptor G protein, transducin, bound to the surface of a tubular lipid bilayer. The protein binds to the membrane with a very small area of contact, restricted to two points, between the surface of the protein and the surface of the lipids. Fitting the x-ray structure into the membrane-bound structure reveals one membrane contact near the lipidated Ggamma C terminus and Galpha N terminus, and another near the Galpha C terminus. The narrowness of the tethers to the lipid bilayer provides flexibility for the protein to adopt multiple orientations on the membrane, and leaves most of the G protein surface area available for protein-protein interactions.

Animals↗
PubMed2004

Imaging single membrane fusion events mediated by SNARE proteins.

Using total internal reflection fluorescence microscopy, we have developed an assay to monitor individual fusion events between proteoliposomes containing vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and a supported planar bilayer containing cognate target SNAREs. Approach, docking, and fusion of individual vesicles to the target membrane were quantified by delivery and subsequent lateral spread of fluorescent phospholipids from the vesicle membrane into the target bilayer. Fusion probability was increased by raising divalent cations (Ca2+ and Mg2+). Fusion of individual vesicles initiated in <100 ms after the rise of Ca2+ and membrane mixing was complete in 300 ms. Removal of the N-terminal H(abc) domain of syntaxin 1A increased fusion probability >30-fold compared to the full-length protein, but even in the absence of the H(abc) domain, vesicle fusion was still enhanced in response to Ca2+ increase. Our observations establish that the SNARE core complex is sufficient to fuse two opposing membrane bilayers at a speed commensurate with most membrane fusion processes in cells. This real-time analysis of single vesicle fusion opens the door to mechanistic studies of how SNARE and accessory proteins regulate fusion processes in vivo.

Base Sequence↗
PubMed2003

Fusion of cells by flipped SNAREs.

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) hypothesis suggests that pairs of proteins known as vesicle (v-) SNAREs and target membrane (t-) SNAREs interact specifically to control and mediate intracellular membrane fusion events. Here, cells expressing the interacting domains of v- and t-SNAREs on the cell surface were found to fuse spontaneously, demonstrating that SNAREs are sufficient to fuse biological membranes.

Animals↗
PubMed2002

Regulation of membrane fusion by the membrane-proximal coil of the t-SNARE during zippering of SNAREpins.

We utilize structurally targeted peptides to identify a "tC fusion switch" inherent to the coil domains of the neuronal t-SNARE that pairs with the cognate v-SNARE. The tC fusion switch is located in the membrane-proximal portion of the t-SNARE and controls the rate at which the helical bundle that forms the SNAREpin can zip up to drive bilayer fusion. When the fusion switch is "off" (the intrinsic state of the t-SNARE), zippering of the helices from their membrane-distal ends is impeded and fusion is slow. When the tC fusion switch is "on," fusion is much faster. The tC fusion switch can be thrown by a peptide that corresponds to the membrane-proximal half of the cognate v-SNARE, and binds reversibly to the cognate region of the t-SNARE. This structures the coil in the membrane-proximal domain of the t-SNARE and accelerates fusion, implying that the intrinsically unstable coil in that region is a natural impediment to the completion of zippering, and thus, fusion. Proteins that stabilize or destabilize one or the other state of the tC fusion switch would exert fine temporal control over the rate of fusion after SNAREs have already partly zippered up.

Amino Acid Sequence↗