Domain registration in raft-mimicking lipid mixtures studied using polymer-tethered lipid bilayers

The degree of domain registration in a liquid-ordered/liquid-disordered phase-separating lipid mixture consisting of 1-stearoyl-2-oleoyl-sn-3-phosphocholine, egg sphingomyelin, and cholesterol (molar mixing ratio of 1:1:1) was studied using three different planar lipid bilayer architectures distinguished by their bilayer-substrate distance d using epifluorescence microscopy. The bilayer systems, which were built layer by layer using Langmuir-Blodgett/Schaefer film depositions, included a solid-supported bilayer (d similar to 15 angstrom) and two polymer-supported bilayers with d similar to 30 angstrom and d similar to 58 angstrom, respectively. Complete domain registration between Langmuir-Blodgett and Schaefer monolayer domains was observed for d similar to 58 angstrom but not in the cases when d similar to 15 angstrom and d similar to 30 angstrom. Building the bilayer layer by layer guaranteed that any preexisting domains were not in registration initially; our data show that the domain registration observed was not caused by lipid flip-flop or by lateral rearrangement of preexisting large-scale domains. Instead, additional studies on bilayer systems with asymmetric lipid composition indicate that preexisting domains in the Langmuir-Blodgett monolayer induce the formation of completely registered domains in the opposite Schaefer monolayer. This study provides insight into possible biophysical mechanisms of transbilayer domain coupling. Our findings support the concept that the formation of transbilayer signaling platforms based on registered raft domains may occur without the active involvement of membrane-spanning proteins.