Biochemische Aspekte der cholinergen Reizung

Photoaffinity labeling of functional states of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor was subjected to photoaffinity labeling in different conformational and functional states. The photolabel used was the ion-channel blocker [3H]-TPMP+. A procedure is described for isolating labeled delta-polypeptide chains from the receptor complex by preparative SDS-polyacrylamide gel electrophoresis. The photolabel was localized in the primary structure of the delta-chain. The site of labeling was found to be identical when photoaffinity labeling was performed in the resting, desensitized, or antagonist state, respectively.

Messenger RNA from insect nervous tissue induces expression of neuronal acetylcholine receptors in Xenopus oocytes

mRNA isolated from the nervous tissue of young insects microinjected in Xenopus oocytes induced the expression of alpha-toxin binding sites which were inserted into the surface membrane. Immunoprecipitation experiments revealed that the coded receptor polypeptides were processed to the authentic size, and ion flux studies demonstrated that functional nicotinic acetylcholine receptors were produced and inserted into the oocyte membrane.

Channel properties of an insect neuronal acetylcholine receptor protein reconstituted in planar lipid bilayers

A pentameric membrane protein composed of four types of polypeptide has been identified as the minimal structural unit responsible for the electrogenic action of acetylcholine on electrocytes and muscle cells. Because many populations of central and peripheral neurons also have nicotinic acetylcholine receptors (AChRs), considerable effort has recently gone into identifying the neuronal receptor. The central nervous tissue of insects contains very high concentrations of nicotinic AChRs, and we have recently purified an alpha-toxin binding protein, a putative AChR, from neuronal membranes of locusts. It is a component of high relative molecular mass, clearly composed of identical subunits, a structure predicted for an ancestral AChR protein. To verify that the purified polypeptides not only represent ligand binding sites but that they are indeed functional receptors, we have now reconstituted the isolated protein in a planar lipid bilayer. We show that in this system cholinergic agonists activate functional ion channels, that have properties comparable to those exhibited by the peripheral AChRs in vertebrates; thus, for the first time a functional acetylcholine receptor channel has been identified in nerve cells.

A stopped-flow apparatus for photoaffinity labeling studies in the milliseconds time range. Application in investigations of the nicotinic acetylcholine receptor

A photoaffinity labeling method is described to label a protein covalently in various transient covalent states. The method uses a combination of an especially adapted stopped-flow apparatus with a Q-switched Nd: YAG DCR-2A laser (wavelength 266 nm, i.e. four-fold the primary frequency, pulse duration 4 ns, pulse energy 15 mJ). The construction of the mixing cell, the triggering device and the set-up for determining the dead time of the stopped-flow apparatus is described. The dead time is 2.4 ms. In combination with a specific photolabel the method has been used for labeling functional states (resting, activated, desensitized, antagonist-blocked) of the nicotinic acetylcholine receptor from Torpedo marmorata electric tissue.

Covalent labeling of functional states of the acetylcholine receptor. Effects of antagonists on the receptor conformation

Photoaffinity labeling of membrane-bound nicotinic acetylcholine receptor from Torpedo marmorata electric tissue with the ion-channel blocker [3H]TPMP+ reveals various functional states of the receptor protein if labeling is performed with ms time resolution. In the resting and in the activated state most of the label is incorporated into the alpha-polypeptide chains of the receptor complex. When equilibrated with agonists and antagonists, predominantly the delta-polypeptide chain (and to a lesser extent the beta-chain) reacts with the photolabel. Reactivity of the delta-chain increases after exposure to cholinergic effectors with a half-life slower than the kinetics of receptor activation or rapid desensitization. Agonists and antagonists stimulate photolabelling of the delta-chain with different kinetics. For acetylcholine, carbamoylcholine and suberyldicholine the half-life of the reactivity increases is 400 - 500 ms; for the antagonists hexamethonium, d-tubocurarine and flaxedil it is about 10 s. The latter slow kinetics are also observed when the receptor is preequilibrated with agonists or antagonists prior to mixing with [3H]TPMP+ and starting the photoreaction. We conclude that time-resolved photoaffinity labeling can convalently mark protein structures involved in receptor functions. Of special interest is the observation that antagonists also induce a conformational change in the receptor protein.