To be or not to be in membrane domains: transbilayer asymmetry and sphingomyelin-dependent preferential partitioning of the acetylcholine receptor

Abstract

The preferential partitioning of the nicotinic acetylcholine receptor (AChR) in liquid-ordered (Lo) domains, heterogeneous membrane domains commonly known as rafts,is thought to be a part of its clustering mechanism. Previous studies from our group have shown that AChR lacks preference for Lo domains when reconstituted in sphingomyelin (SM), cholesterol (Chol) and POPC (1:1:1) model systems. Here we study the effect on the possible Lo-preferential partitioning of purified AChR reconstituted in two model systems (POPC:Chol, 1:1 and POPC:Chol:SM, 1:1:1) under: a) induced transbilayer asymmetry, by addition of brain sphingomyelin (bSM) to the external hemilayer; and b) the presence of different pure SM species in the model membrane (bSM, 16:0-SM, 18:0-SM or 24:1-SM). AChR distribution was evaluated by fluorescence resonance energy transfer efficiency between the AChR intrinsic fluorescence and Laurdan or dehydroergosterol fluorescence, and also by determining the presence of AChR in detergent- resistant and detergent-soluble domains (1% Triton X-100, 4°C). Both studies show that the induction of transbilayer asymmetry or the presence of 16:0-SM or 18:0-SM, as opposed to bSM or 24:1-SM, leads to a preferential partitioning of AChR in Lo domains. Thus, the localization of AChR in Lo domains strongly depends on the characteristics of the host lipid membrane.