Mitochondria, glutamate neurotoxicity and the death cascade

AbstractThis review focuses on two questions: the role of mitochondria in excitotoxic neuronal death and the connection of mitochondria with the apoptotic death cascade. The goal is to highlight the regulatory role of mitochondrial channels on the mitochondrial membrane potential, Δψ, and their involvement in determining neuronal survival or death. A hypothesis is developed centered on the notion that protein–protein interactions between members of the Bcl-2 family of death suppressor and promoter proteins lead to the selective elimination of depolarizing currents that, in turn, collapse Δψ and set in motion the irreversible pathway of cell death. The model considers the remarkable propensity of Bcl-2 family proteins to dimerize or oligomerize and thereby restrict the localization of partner molecules to mitochondrial membrane contact sites. The fundamental principle invoked here is that through a concerted set of protein–protein interactions, information is exchanged by specific heterodimers, one of the partners acting as a toxic protein and the second as its antidote. The review concludes with the elaboration of a speculative model about cellular mechanisms for the prevention of cell destruction as triggered by extracellular signals which may be conserved in its molecular design from bacteria to eukaryotes

Incorporation of membrane proteins into large single bilayer vesicles. Application to rhodopsin.

A general procedure to incorporate membrane proteins in a native state into large single bilayer vesicles is described. The results obtained with rhodopsin from vertebrate and invertebrate retinas are presented. The technique involves: (a) the direct transfer of rhodopsin-lipid complexes from native membranes into ether or pentane, and (b) the sonication of the complex in apolar solvent with aqueous buffer followed by solvent evaporation under reduced pressure. The spectral properties of rhodopsin in the large vesicles are similar to those of rhodopsin in photoreceptors; furthermore, bleached bovine rhodopsin is chemically regenerable with 9-cis retinal. These results establish the presence of photochemically functional rhodopsin in the large vesicles. Freeze-fracture replicas of the vesicles reveal that both internal and external leaflets contain numerous particles approximately 80 A in diameter, indicating that rhodopsin is symmetrically distributed within the bilayer. More than 75% of the membrane area is incorporated into vesicles larger than 0.5 micron and approximately 40% into vesicles larger than 1 micron

Design, synthesis and functional characterization of a pentameric channel protein that mimics the presumed pore structure of the nicotinic cholinergic receptor

AbstractNicotinic cholinergic receptors are membrane proteins composed of five subunits organized around a central aqueous pore. A pentameric channel protein, T5M2δ, that emulates the presumed pore-forming structure of this receptor was generated by assembling five helix-forming peptide modules at the lysine ϵ-amino groups of the 11-residue template [K∗AK∗KK∗PGK∗EK∗G], where ∗ indicates attachment sites. Helical modules represent the sequence of the M2 segment of the Torpedo californica acetylcholine receptor (AChR) δ subunit; M2 segments are considered involved in pore-lining. Purified T5M2δ migrates in SDS-PAGE with an apparent Mr~14,000, concordant with a protein of 126 residues. T5M2δ forms cation-selective channels when reconstituted in planar lipid bilayers. The single channel conductance in symmetric 0.5 M K.C1 is 40 pS. This value approximates the 45 pS single channel conductance characteristic of authentic purified Torpedo AChR, recorded under otherwise identical conditions. These results, together with conformational energy calculations, support the notion that a bundle of five amphipathic a-helices is a plausible structural motif underlying the inner bundle that forms the pore of the pentameric AChR channel

Functional reassembly of membrane proteins in planar lipid bilayers

Recent progress in membrane biology has brought us to a stage where it is possible to associate complex biological processes to identifiable membrane proteins. Technical advances in the biochemical characterization and purification of membrane proteins have contributed a wealth of structural information. The reconstitution approach has proved to be valuable in our efforts to understand the molecular mechanisms of membrane transport and energy transductio

Design of a functional calcium channel protein: Inferences about an ion channel‐forming motif derived from the primary structure of voltage‐gated calcium channels

To identify sequence‐specific motifs associated with the formation of an ionic pore, we systematically evaluated the channel‐forming activity of synthetic peptides with sequence of predicted transmembrane segments of the voltage‐gated calcium channel. The amino acid sequence of voltage‐gated, dihydropyridine (DHP)‐sensitive calcium channels suggests the presence in each of four homologous repeats (I–IV) of six segments (S1–S6) predicted to form membrane‐spanning, α‐helical structures. Only peptides representing amphipathic segments S2 or S3 form channels in lipid bilayers. To generate a functional calcium channel based on a four‐helix bundle motif, four‐helix bundle proteins representing IVS2 (T4CaIVS2) or IVS3 (T4CaIVS3) were synthesized. Both proteins form cation‐selective channels, but with distinct characteristics: the single‐channel conductance in 50 mM BaCl2 is 3 pS and 10 pS. For T4CaIVS3, the conductance saturates with increasing concentration of divalent cation. The dissociation constants for Ba2+, Ca2+, and Sr2+ are 13.6 mM, 17.7 mM, and 15.0 mM, respectively. The conductance of T4CaIVS2 does not saturate up to 150 mM salt. Whereas T4CaIVS3 is blocked by μM Ca2+ and Cd2+, T4CaIVS2 is not blocked by divalent cations. Only T4CaIVS3 is modulated by enantiomers of the DHP derivative BayK 8644, demonstrating sequence requirement for specific drug action. Thus, only T4CaIVS3 exhibits pore properties characteristic also of authentic calcium channels. The designed functional calcium channel may provide insights into fundamental mechanisms of ionic permeation and drug action, information that may in turn further our understanding of molecular determinants underlying authentic pore structures. Copyright © 1993 The Protein Societ

Molecular Template for a Voltage Sensor in a Novel K+ Channel. III. Functional Reconstitution of a Sensorless Pore Module from a Prokaryotic Kv Channel

KvLm is a prokaryotic voltage-gated K+ (Kv) channel from Listeria monocytogenes. The sequence of the voltage-sensing module (transmembrane segments S1-S4) of KvLm is atypical in that it contains only three of the eight conserved charged residues known to be deterministic for voltage sensing in eukaryotic Kv's. In contrast, the pore module (PM), including the S4-S5 linker and cytoplasmic tail (linker-S5-P-S6-C-terminus) of KvLm, is highly conserved. Here, the full-length (FL)-KvLm and the KvLm-PM only proteins were expressed, purified, and reconstituted into giant liposomes. The properties of the reconstituted FL-KvLm mirror well the characteristics of the heterologously expressed channel in Escherichia coli spheroplasts: a right-shifted voltage of activation, micromolar tetrabutylammonium-blocking affinity, and a single-channel conductance comparable to that of eukaryotic Kv's. Conversely, ionic currents through the PM recapitulate both the conductance and blocking properties of the FL-KvLm, yet the KvLm-PM exhibits only rudimentary voltage dependence. Given that the KvLm-PM displays many of the conduction properties of FL-KvLm and of other eukaryotic Kv's, including strict ion selectivity, we conclude that self-assembly of the PM subunits in lipid bilayers, in the absence of the voltage-sensing module, generates a conductive oligomer akin to that of the native KvLm, and that the structural independence of voltage sensing and PMs observed in eukaryotic Kv channels was initially implemented by nature in the design of prokaryotic Kv channels. Collectively, the results indicate that this robust functional module will prove valuable as a molecular template for coupling new sensors and to elucidate PM residue–specific contributions to Kv conduction properties

Structural stabilization of botulinum neurotoxins by tyrosine phosphorylation

AbstractTyrosine phosphorylation of botulinum neurotoxins augments their proteolytic activity and thermal stability, suggesting a substantial modification of the global protein conformation. We used Fourier-transform infrared (FTIR) spectroscopy to study changes of secondary structure and thermostability of tyrosine phosphorylated botulinum neurotoxins A (BoNT A) and E (BoNT E). Changes in the conformationally-sensitive amide I band upon phosphorylation indicated an increase of the α-helical content with a concomitant decrease of less ordered structures such as turns and random coils, and without changes in β-sheet content. These changes in secondary structure were accompanied by an increase in the residual amide II absorbance band remaining upon H-D exchange, consistent with a tighter packing of the phosphorylated proteins. FTIR and differential scanning calorimetry (DSC) analyses of the denaturation process show that phosphorylated neurotoxins denature at temperatures higher than those required by non-phosphorylated species. These findings indicate that tyrosine phosphorylation induced a transition to higher order and that the more compact structure presumably imparts to the phosphorylated neurotoxins the higher catalytic activity and thermostability

Modulation of the conductance of a 2,2′-bipyridine-functionalized peptidic ion channel by Ni2+

An α-helical amphipathic peptide with the sequence H2N-(LSSLLSL)3-CONH2 was obtained by solid phase synthesis and a 2,2′-bipyridine was coupled to its N-terminus, which allows complexation of Ni2+. Complexation of the 2,2′-bipyridine residues was proven by UV/Vis spectroscopy. The peptide helices were inserted into lipid bilayers (nano black lipid membranes, nano-BLMs) that suspend the pores of porous alumina substrates with a pore diameter of 60 nm by applying a potential difference. From single channel recordings, we were able to distinguish four distinct conductance states, which we attribute to an increasing number of peptide helices participating in the conducting helix bundle. Addition of Ni2+ in micromolar concentrations altered the conductance behaviour of the formed ion channels in nano-BLMs considerably. The first two conductance states appear much more prominent demonstrating that the complexation of bipyridine by Ni2+ results in a considerable confinement of the observed multiple conductance states. However, the conductance levels were independent of the presence of Ni2+. Moreover, from a detailed analysis of the open lifetimes of the channels, we conclude that the complexation of Ni2+ diminishes the frequency of channel events with larger open times