Abstract
Lactosylceramide (LacCer) in the plasma membranes of immune cells is an important lipid for signaling in innate immunity through the formation of LacCer-rich domains together with cholesterol (Cho). However, the properties of the LacCer domains formed in multicomponent membranes remain unclear. In this study, we examined the properties of the LacCer domains formed in Cho-containing 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) membranes by deuterium solid-state NMR and fluorescence lifetimes. The potent affinity of LacCer-LacCer (homophilic interaction) is known to induce a thermally stable gel phase in the unitary LacCer bilayer. In LacCer/Cho binary membranes, Cho gradually destabilized the LacCer gel phase to form the liquid-ordered phase by its potent order effect. In the LacCer/POPC binary systems without Cho, the (2)H NMR spectra of 10',10'-d(2)-LacCer and 18',18',18'-d(3)-LacCer probes revealed that LacCer was poorly miscible with POPC in the membranes and formed stable gel phases without being distributed in the liquid crystalline domain. The lamellar structure of the LacCer/POPC membrane was gradually disrupted at around 60°C, whereas the addition of Cho increased the thermal stability of the lamellarity. Furthermore, the area of the LacCer gel phase and its chain order were decreased in the LacCer/POPC/Cho ternary membranes, whereas the liquid-ordered domain, which was observed in the LacCer/Cho binary membrane, was not observed. Cho surrounding the LacCer gel domain liberated LacCer and facilitated forming the submicron to nano-scale small domains in the liquid crystalline domain of the LacCer/POPC/Cho membranes, as revealed by the fluorescence lifetimes of trans-parinaric acid and trans-parinaric acid-LacCer. Our findings on the membrane properties of the LacCer domains, particularly in the presence of Cho, would help elucidate the properties of the LacCer domains in biological membranes.