Cell-size, giant
liposomes have been formed by submitting a mixture of
asolectin lipid vesicles and native membranes from Torpedo, highly enriched in
acetylcholine receptor (AcChR), to a partial
dehydration/
rehydration cycle [Criado, M., & Keller, B. U. (1987) FEBS Lett. 224, 172-176]. Giant
liposomes can be prepared in bulk quantities, in the absence of potentially damaging
detergents or organic
solvents, and their formation is mediated by membrane fusion phenomena. In fact, fluorescence microscopy and freeze-fracture data indicate that
protein and
lipid components of the initial membranes and
lipid vesicles are homogenously distributed in the resulting
liposomes. Giant
liposomes containing AcChR have been used as a model to evaluate whether this system can be used to monitor the activity of
ionic channels by using high-resolution, patch-clamp techniques. Excised
liposome patches in an "inside-out" configuration have been used in this work. We find that the most frequent pattern of electrical activity in response to the presence of
acetylcholine in the patch pipet corresponds to a
cation-specific channel exhibiting a dominant conductance level and a sublevel of approximately 78 and 25 pS, respectively. Such channel activity exhibits the pharmacological specificity,
ion channel activation, ion selectivity, and desensitization properties expected from native Torpedo AcChR. Thus, it appears that the giant
liposome technique offers a distinct advantage over other reconstitution procedures in that it provides a unique opportunity to undertake simultaneous biochemical, morphological, and electrophysiological studies of the incorporated
ionic channel proteins.