A molecular model for an acetylcholine binding site: ion channel and the bilayer helices of the acetylcholine receptor assigned using single group rotation theory and electrostatic interactions

Thermal stability of Torpedo californica acetylcholine receptor in a cholesterol lipid environment

Controlled heating of acetylcholine receptor (AChR) vesicles inactivates the alpha-bungarotoxin (alpha-Bgtx) binding sites with a T50 (temperature at which 50% of the initial capacity to bind alpha-Bgtx remains) of 60 +/- 0.2 degrees C. The same value was obtained for receptor reconstituted in lipid vesicles from Torpedo electroplax where the % mol composition of cholesterol to phospholipid was 30. However, when the reconstitution was carried out in dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidic acid (DOPA) vesicles (3:1 molar ratio), T50 of the curves decreased to 56 +/- 0.2 degrees C and no carbamylcholine stimulated 22Na+ flux was detected. Inclusion of cholesterol in the DOPC-DOPA vesicles increased the toxin binding site stability. The maximal T50 of the toxin binding curves was 63 +/- 0.1 degrees C when the % mol cholesterol/mol DOPC:DOPA in the vesicles was 33. Under these conditions AChR was able to translocate ions, a property that was lost upon heating at 46 degrees C. Preincubation of AChR in the presence of d-tubocurarine, tetracaine or procaine did not affect T50 values of toxin binding. However, a slight increment in thermal stability was found when the receptor was preincubated in the presence of carbamylcholine. The results show that cholesterol requirements for protecting against thermal inactivation of toxin binding and ion gating properties are different and the carbamylcholine-bound receptor may have a different conformation.

Water movement during channel opening and closing

By applying the osmotic stress of a nonpenetrating polymer, we have measured the change delta nu in polymer-inaccessible internal water volume of a voltage-gated ionic channel. The voltage-dependent anion channel (VDAC) from mitochondrial outer membranes shows a delta nu comparable in magnitude to the full channel volume estimated from solute penetrability, single-channel conductance, or image reconstruction. It thus appears that channel "gating" involves significant structure reorganization and water movement rather than the minimal changes caused by a local constriction or blockade. Hydration of the inner channel surface may be an important factor in channel gating as is the hydration of molecular surfaces in controlling macromolecular interaction in solution.

Revised assignments for the beta-, gamma- and delta-subunits of the acetylcholine receptor structural model

Revised assignments for the bilayer helices of the beta-, gamma- and delta-subunits of the acetylcholine receptor are presented. A new feature of the model is extensive charge matching between the polar groups of the ion channel elements of different subunits.