(1/900) Chloride dependence of hyperpolarization-activated chloride channel gates.
1. ClC proteins are a class of voltage-dependent Cl- channels with several members mutated in human diseases. The prototype ClC-0 Torpedo channel is a dimeric protein; each subunit forms a pore that can gate independently from the other one. A common slower gating mechanism acts on both pores simultaneously; slow gating activates ClC-0 at hyperpolarized voltages. The ClC-2 Cl- channel is also activated by hyperpolarization, as are some ClC-1 mutants (e.g. D136G) and wild-type (WT) ClC-1 at certain pH values. 2. We studied the dependence on internal Cl- ([Cl-]i) of the hyperpolarization-activated gates of several ClC channels (WT ClC-0, ClC-0 mutant P522G, ClC-1 mutant D136G and an N-terminal deletion mutant of ClC-2), by patch clamping channels expressed in Xenopus oocytes. 3. With all these channels, reducing [Cl-]i shifted activation to more negative voltages and reduced the maximal activation at most negative voltages. 4. We also investigated the external halide dependence of WT ClC-2 using two-electrode voltage-clamp recording. Reducing external Cl- ([Cl-]o) activated ClC-2 currents. Replacing [Cl-]o by the less permeant Br- reduced channel activity and accelerated deactivation. 5. Gating of the ClC-2 mutant K566Q in normal [Cl-]o resembled that of WT ClC-2 in low [Cl-]o, i.e. channels had a considerable open probability (Po) at resting membrane potential. Substituting external Cl- by Br- or I- led to a decrease in Po. 6. The [Cl-]i dependence of the hyperpolarization-activated gates of various ClC channels suggests a similar gating mechanism, and raises the possibility that the gating charge for the hyperpolarization-activated gate is provided by Cl-. 7. The external halide dependence of hyperpolarization-activated gating of ClC-2 suggests that it is mediated or modulated by anions as in other ClC channels. In contrast to the depolarization-activated fast gates of ClC-0 and ClC-1, the absence of Cl- favours channel opening. Lysine 556 may be important for the relevant binding site. (+info)
(2/900) Monoclonal antibody 3F3 against conformational epitope of Torpedo acetylcholinesterase.
AIM: To study the type of epitope of native Torpedo acetylcholinesterase (AChE) directed by its monoclonal antibody (McAb) 3F3. METHODS: Enzyme-linked immunosorbent assay (ELISA) was used for the assay of the reaction between antigen and antibody. RESULTS: McAb 3F3 immunoreacted well with the native AChE, but not with the reduced- and alkylated-AChE (RA-AChE) at all. Soman did not interfere the binding of 3F3 with AChE molecule. The synthesized 24-peptide containing the active serine residue of the AChE active center did not react with McAb 3F3. CONCLUSION: 3F3 is a monoclonal antibody against the conformational epitope of Torpedo AChE active center, but dose not occupy the active serine residue of the enzyme. (+info)
(3/900) Cloning, expression, and properties of a nonneuronal secreted acetylcholinesterase from the parasitic nematode Nippostrongylus brasiliensis.
We have isolated a full-length cDNA encoding an acetylcholinesterase secreted by the nematode parasite Nippostrongylus brasiliensis. The predicted protein is truncated in comparison with acetylcholinesterases from other organisms such that the carboxyl terminus aligns closely to the end of the catalytic domain of the vertebrate enzymes. The residues in the catalytic triad are conserved, as are the six cysteines which form the three intramolecular disulfide bonds. Three of the fourteen aromatic residues which line the active site gorge in the Torpedo enzyme are substituted by nonaromatic residues, corresponding to Tyr-70 (Thr), Trp-279 (Asn), and Phe-288 (Met). High level expression was obtained via secretion from Pichia pastoris. The purified enzyme behaved as a monomeric hydrophilic species. Although of invertebrate origin and possessing the above substitutions in the active site gorge residues, the enzyme efficiently hydrolyzed acetylthiocholine and showed minimal activity against butyrylthiocholine. It displayed excess substrate inhibition with acetylthiocholine at concentrations over 2. 5 mM and was highly sensitive to both active site and "peripheral" site inhibitors. Northern blot analysis indicated a progressive increase in mRNA for AChE B in parasites isolated from 6 days postinfection. (+info)
(4/900) Characterization of phosphotyrosine containing proteins at the cholinergic synapse.
Tyrosine phosphorylation has been associated with several aspects of the regulation of cholinergic synaptic function, including nicotinic acetylcholine receptor (AChR) desensitization as well as the synthesis and clustering of synaptic components. While some progress has been made in elucidating the molecular events initiating such signals, the downstream targets of these tyrosine kinase pathways have yet to be characterized. In this paper we have used molecular cloning techniques to identify proteins which are tyrosine phosphorylated at the cholinergic synapse. Phosphotyrosine containing proteins (PYCPs) were isolated from the electric organ of Torpedo californica by anti-phosphotyrosine immunoaffinity chromatography. Peptide sequencing and expression cloning then identified the isolated proteins. The proteins identified included heat shock protein 90, type III intermediate filament from Torpedo electric organ, alpha-fodrin, beta-tubulin, actin and rapsyn. These tyrosine phosphorylated proteins may play a role in the regulation of synaptic function by tyrosine kinases. (+info)
(5/900) Scanning a DRB3*0101 (DR52a)-restricted epitope cross-presented by DR3: overlapping natural and artificial determinants in the human acetylcholine receptor.
A recurring epitope in the human acetylcholine receptor (AChR) alpha subunit (alpha146-160) is presented to specific T cells from myasthenia gravis patients by HLA-DRB3*0101-"DR52a"-or by DR4. Here we first map residues critical for DR52a in this epitope by serial Ala substitution. For two somewhat similar T cells, this confirms the recently deduced importance of hydrophobic "anchor" residues at peptide p1 and p9; also of Asp at p4, which complements this allele's distinctive Arg74 in DRbeta. Surprisingly, despite the 9 sequence differences in DRbeta between DR52a and DR3, merely reducing the bulk of the peptide's p1 anchor residue (Trp149-->Phe) allowed maximal cross-presentation to both T cells by DR3 (which has Val86 instead of Gly). The shared K71G73R74N77 motif in the alpha helices of DR52a and DR3 thus outweighs the five differences in the floor of the peptide-binding groove. A second issue is that T cells selected in vitro with synthetic AChR peptides rarely respond to longer Ag preparations, whereas those raised with recombinant subunits consistently recognize epitopes processed naturally even from whole AChR. Here we compared one T cell of each kind, which both respond to many overlapping alpha140-160 region peptides (in proliferation assays). Even though both use Vbeta2 to recognize peptides bound to the same HLA-DR52a in the same register, the peptide-selected line nevertheless proved to depend on a recurring synthetic artifact-a widely underestimated problem. Unlike these contaminant-responsive T cells, those that are truly specific for natural AChR epitopes appear less heterogeneous and therefore more suitable targets for selective immunotherapy. (+info)
(6/900) Anionic subsite of active center of Torpedo acetylcholinesterase constructs a part of its conformational epitope.
AIM: To study the structure-activity relationship of Torpedo acetylcholinesterase (AChE) and explore whether the anionic subsite of the active center is a constituent of the conformational epitope of enzyme. METHODS: Using ELISA and enzyme inhibition test to examine the effect of 1-methyl-2-hydroxyiminomethylpyridium chloride (2-PAM), an anionic subsite probe of AChE, on the immunoreactivity between Torpedo AChE and its monoclonal antibody (McAb) 3F3. RESULTS: McAb 3F3 did not react with 2-PAM-AChE complex. 2-PAM decreased the inhibitory rate of McAb 3F3 on AChE in a concentration-dependent fashion, but did not dissociate the McAb 3F3-AChE complex. CONCLUSION: Anionic subsite of the active center of Torpedo AChE constructs a part of its conformational epitope. (+info)
(7/900) Effect of mutations within the peripheral anionic site on the stability of acetylcholinesterase.
Torpedo acetylcholinesterase is irreversibly inactivated by modifying a buried free cysteine, Cys231, with sulfhydryl reagents. The stability of the enzyme, as monitored by measuring the rate of inactivation, was reduced by mutating a leucine, Leu282, to a smaller amino acid residue. Leu282 is located within the "peripheral" anionic site, at the entrance to the active-site gorge. Thus, loss of activity was due to the increased reactivity of Cys231. This was paralleled by an increased susceptibility to thermal denaturation, which was shown to be due to a large decrease in the activation enthalpy. Similar results were obtained when either of two other residues in contact with Leu282 in Torpedo acetylcholinesterase, Trp279 and Ser291, was replaced by an amino acid with a smaller side chain. We studied the effects of various ligands specific for either the active or peripheral sites on both thermal inactivation and on inactivation by 4,4'-dithiodipyridine. The wild-type and mutated enzymes could be either protected or sensitized. In some cases, opposite effects of the same ligand were observed for chemical modification and thermal denaturation. The mutated residues are within a conserved loop, W279-S291, at the top of the active-site gorge, that contributes to the peripheral anionic site. Theoretical analysis showed that Torpedo acetylcholinesterase consists of two structural domains, each comprising one contiguous polypeptide segment. The W279-S291 loop, located in the first domain, makes multiple contacts with the second domain across the active-site gorge. We postulate that the mutations to residues with smaller side chains destabilize the conserved loop, thus disrupting cross-gorge interactions and, ultimately, the entire structure. (+info)
(8/900) Experimental autoimmune myasthenia gravis may occur in the context of a polarized Th1- or Th2-type immune response in rats.
Experimental autoimmune myasthenia gravis (EAMG) is a T cell-dependent, Ab-mediated autoimmune disease induced in rats by a single immunization with acetylcholine receptor (AChR). Although polarized Th1 responses have been shown to be crucial for the development of mouse EAMG, the role of Th cell subsets in rat EAMG is not well established. In the present work we show that while the incidence and severity of EAMG are similar in Lewis (LEW) and Brown-Norway (BN) rats, strong differences are revealed in the immune response generated. Ag-specific lymph node cells from LEW rats produced higher amounts of IL-2 and IFN-gamma than BN lymph node cells, but expressed less IL-4 mRNA. IgG1 and IgG2b anti-AChR isotype predominated in BN and LEW rats, respectively, confirming the dichotomy of the immune response observed between the two strains. Furthermore, although IL-12 administration or IFN-gamma neutralization strongly influenced the Th1/Th2 balance in BN rats, it did not affect the disease outcome. These data demonstrate that a Th1-dominated immune response is not necessarily associated with disease severity in EAMG, not only in rats with disparate MHC haplotype but also in the same rat strain, and suggest that in a situation where complement-fixing Ab can be generated as a consequence of either Th1- or Th2-mediated T cell help, deviation of the immune response will not be an adequate strategy to prevent this Ab-mediated autoimmune disease. (+info)