Scanning curves and kinetics of the acetylcholine/acetylcholine receptor hysteresis

The initiation of the muscle action potential

The specific functional properties of the nicotinic acetylcholine receptors (AChR) and the particular oligomeric membrane organization of AChR are suggested to be the basis for the steep electrical depolarisation, required for the initiation of the postsynaptic action potentials causing muscle contraction and discharge of electric organs. The dimer (M(r) approximately 580,000) and the monomer (M(r) approximately 290,000) of the detergent-solubilized, affinity-purified AChR of Torpedo californica electrocytes exhibit different channel conductances and larger oligochannels. Patch clamp data of the dimer, reconstituted in large lipid vesicles, show that the dimer is a double-channel protein causing single-channel events of conductance G(D) = 84 +/- 6 pS at 0.11 M K+ and 0.1 mM Ca2+ at 293 K (20 degrees C). At the same ionic conditions the vesicle-reconstituted monomer, if prevented from aggregation, exhibits a channel conductance, G(M) = 42 +/- 3 pS, which is only half of that of the dimer. The dimer conductivity event thus reflects the synchronous switching of its two constituent monomeric parts. The K(+)-conductance of the reconstituted Torpedo dimer is the same, and shows the same inhibitory Ca(2+)-dependence, as that of the Torpedo AChR expressed in Xenopus laevis oocytes (Imoto et al., Nature, 324, 670-674, 1986). In terms of Ca(2+)-binding, reducing K(+)-transport, the equilibrium constant is KCa = 0.48 mM at 0.11 m K+, 20 degrees C; G0([Ca]-->0) = 98 +/- 6 pS and G infinity ([Ca]-->infinity) = 27 +/- 6 pS. The ratio G0/G infinity and an estimate of the lateral surface area of the channel vestibule yields about 16 negatively charged groups in an average distance of 1.8 nm. These negative charges cause an accumulation of K+ ions in the channel vestibule by a factor of about 4. Our results and the comparison with the oocyte data reveal that it is also the dimer which is the physiological opening-closing unit of the AChR in the oocyte membrane. The larger macrochannel events are multiples of the dimer or of the monomer conductances. The occurrence of such oligochannels from AChR protein oligomers could guarantee the steep electrical depolarisation necessary to generate the action potential by the Na(+)-channel system.

Structural heterogeneity of membrane receptors and GTP-binding proteins and its functional consequences for signal transduction

Recent information obtained, mainly by recombinant cDNA technology, on structural heterogeneity of hormone and transmitter receptors, of GTP-binding proteins (G-proteins) and, especially, of G-protein-linked receptors is reviewed and the implications of structural heterogeneity for diversity of hormone and transmitter actions is discussed. For the future, three-dimensional structural analysis of membrane proteins participating in signal transmission and transduction pathways is needed in order to understand the molecular basis of allosteric regulatory mechanisms governing the interactions between these proteins including hysteretic properties and cell-cybernetic aspects.