7 research outputs found
SANS, FRET, and ESR Reveal <6nm Domains in Brain Sphingomyelin-Containing Membrane Models
Comparison of Three Ternary Lipid Bilayer Mixtures: FRET and ESR Reveal Nanodomains
Phase diagrams of ternary lipid mixtures containing cholesterol have provided valuable insight into cell membrane behaviors, especially by describing regions of coexisting liquid-disordered (Ld) and liquid-ordered (Lo) phases. Fluorescence microscopy imaging of giant unilamellar vesicles has greatly assisted the determination of phase behavior in these systems. However, the requirement for optically resolved Ld + Lo domains can lead to the incorrect inference that in lipid-only mixtures, Ld + Lo domain coexistence generally shows macroscopic domains. Here we show this inference is incorrect for the low melting temperature phosphatidylcholines abundant in mammalian plasma membranes. By use of high compositional resolution Förster resonance energy transfer measurements, together with electron spin resonance data and spectral simulation, we find that ternary mixtures of DSPC and cholesterol together with either POPC or SOPC, do indeed have regions of Ld + Lo coexistence. However, phase domains are much smaller than the optical resolution limit, likely on the order of the Förster distance for energy transfer (R0, ∼2–8 nm)
Bilayer Thickness Mismatch Controls Domain Size in Model Membranes
The observation of lateral phase separation in lipid bilayers has
received considerable attention, especially in connection to lipid
raft phenomena in cells. It is widely accepted that rafts play a central
role in cellular processes, notably signal transduction. While micrometer-sized
domains are observed with some model membrane mixtures, rafts much
smaller than 100 nmbeyond the reach of optical microscopyare
now thought to exist, both in vitro and in vivo. We have used small-angle
neutron scattering, a probe free technique, to measure the size of
nanoscopic membrane domains in unilamellar vesicles with unprecedented
accuracy. These experiments were performed using a four-component
model system containing fixed proportions of cholesterol and the saturated
phospholipid 1,2-distearoyl-<i>sn</i>-glycero-3-phosphocholine
(DSPC), mixed with varying amounts of the unsaturated phospholipids
1-palmitoyl-2-oleoyl-<i>sn</i>-glycero-3-phosphocholine
(POPC) and 1,2-dioleoyl-<i>sn</i>-glycero-3-phosphocholine
(DOPC). We find that liquid domain size increases with the extent
of acyl chain unsaturation (DOPC:POPC ratio). Furthermore, we find
a direct correlation between domain size and the mismatch in bilayer
thickness of the coexisting liquid-ordered and liquid-disordered phases,
suggesting a dominant role for line tension in controlling domain
size. While this result is expected from line tension theories, we
provide the first experimental verification in free-floating bilayers.
Importantly, we also find that changes in bilayer thickness, which
accompany changes in the degree of lipid chain unsaturation, are entirely
confined to the disordered phase. Together, these results suggest
how the size of functional domains in homeothermic cells may be regulated
through changes in lipid composition