The 40 years since the seminal papers of Hodgkin and Huxley appeared have been extraordinarily productive in terms of understanding the molecular basis for electrical activity. The Hodgkin-Huxley proposal that electrical excitability should be understood in terms of voltage-dependent changes in discrete sites has been resoundingly verified. Indeed, the Hodgkin-Huxley framework is remarkable in that its essential elements have remained largely intact as molecular understanding has advanced. This robustness is, at least in part, a result of the fact that Hodgkin and Huxley developed a mathematical model, based on simple physical arguments, that was sufficiently comprehensive to describe the kinetics of the voltage-clamped currents and yet simple enough to be predictive. The predictive features were demonstrated early by the reconstruction of both space-clamped and propagated action potentials on a desk-top calculator (293) and, later, when the sites of Hodgkin and Huxley developed into being well-characterized molecular structures. Voltage- and ligand-dependent ion-selective channels are now the established framework within which cellular electrophysiology is being pursued. Moreover, electrophysiological measurements of membrane and single-channel currents have become essential tools to examine molecular questions pertaining to channel structure and activity. The last 10 years have witnessed spectacular activity, which has resulted from two developments, the giga-seal patch clamp (249) and the elucidation of primary sequences of a number of channel-forming proteins (494), along with the first outlines of their low-resolution three-dimensional structures (651). The stage is now set for 1) applying a variety of convergent techniques to decipher molecular structural details at high resolution, and 2) seeking to understand the complex dynamic functions, gating, and ion selectivity at the molecular level. The early successes are likely to be in understanding the molecular determinants of ion conductance and selectivity, initially in terms of quantitative descriptions of how a sequence modification can alter a channel's permeability characteristics. Channel gating is a far more elusive target because it involves molecular rearrangements, which are poorly understood at any level of description and which may be modified by the channel's environment. The general mechanisms of ion permeation and gating will differ among different classes of ion channels, but a molecular understanding of either phenomenon must eventually be based on an understanding of intermolecular forces, which are invariant among all channel types.(ABSTRACT TRUNCATED AT 400 WORDS)
Neuronal exocytotic membrane fusion occurs on a fast time scale and is dependent on interactions between the vesicle SNARE synaptobrevin-2 and the plasma membrane SNAREs syntaxin-1a and SNAP-25. Reproducing fast fusion rates as observed in cells by reconstitution in vitro has been hindered by the spontaneous assembly of a 2:1 syntaxin-1a:SNAP-25 complex on target membranes that kinetically inhibits the binding of synaptobrevin-2. Previously, an artificial SNARE acceptor complex consisting of 1:1:1 syntaxin-1a(residues 183-288):SNAP-25:syb(residues 49-96) was found to greatly accelerate the rates of lipid mixing of reconstituted target and vesicle SNARE proteoliposomes. Here we present two new procedures to assemble membrane-bound 1:1 SNARE acceptor complexes that produce fast and efficient fusion without the need of the syb(49-96) peptide. In the first procedure, syntaxin-1a is purified in strictly monomeric form and subsequently assembled with SNAP-25 in detergent with the correct 1:1 stoichiometry. In the second procedure, monomeric syntaxin-1a and dodecylated (d-) SNAP-25 are separately reconstituted into proteoliposomes and subsequently assembled in the plane of merged lipid bilayers. Examining single particle fusion between synaptobrevin-2 proteoliposomes and planar supported bilayers containing the two different SNARE acceptor complexes revealed similar fast rates of fusion. Changing the stoichiometry of syntaxin-1a and d-SNAP-25 in the target bilayer had significant effects on docking, but little effect on the rates of synaptobrevin-2 proteoliposome fusion.
No abstract
1. Free glutamic acid, aspartic acid, glutamic acid from glutamine and, in some instances, the glutamic acid from glutathione and the aspartic acid from N-acetyl-aspartic acid were isolated from the brains of sheep and assayed for radioactivity after intravenous injection of [2-(14)C]glucose, [1-(14)C]acetate, [1-(14)C]butyrate or [2-(14)C]propionate. These brain components were also isolated and analysed from rats that had been given [2-(14)C]propionate. The results indicate that, as in rat brain, glucose is by far the best precursor of the free amino acids of sheep brain. 2. Degradation of the glutamate of brain yielded labelling patterns consistent with the proposal that the major route of pyruvate metabolism in brain is via acetyl-CoA, and that the short-chain fatty acids enter the brain without prior metabolism by other tissue and are metabolized in brain via the tricarboxylic acid cycle. 3. When labelled glucose was used as a precursor, glutamate always had a higher specific activity than glutamine; when labelled fatty acids were used, the reverse was true. These findings add support and complexity to the concept of the metabolic ;compartmentation' of the free amino acids of brain. 4. The results from experiments with labelled propionate strongly suggest that brain metabolizes propionate via succinate and that this metabolic route may be a limited but important source of dicarboxylic acids in the brain.
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