Monolayer coupling in sphingomyelin bilayer systems

Abstract

THE transmission of information across biological membranes is of obvious importance. In the past, discussions of possible mechanisms for transmembrane linkage, in the absence of transport or permeation, have centred on the role of the integral membrane proteins, which function as receptors1. When the lipids have been discussed in this context, the speculations have centred on their influence on the proteins, by way of a ‘viscotropic’ effect2, induced by changes in ionic strength3 or temperature; that is, it has generally been implicitly assumed that the two lipid monolayers act independently of each other4. A detailed nuclear magnetic resonance (NMR) study of the thermal behaviour of small single-walled vesicles composed of synthetic phosphatidylcholines has shown that this is indeed the case for these systems5. In this report, we present evidence that vesicles composed of another class of phospholipid, sphingomyelin, do exhibit coupling between the two monolayers, and thus could be involved in transmembrane communication in biological systems.