Coexisting stripe- and patch-shaped domains in giant unilamellar vesicles

We report a new type of gel-liquid phase segregation in giant unilamellar vesicles (GUVs) of mixed lipids. Coexisting patch- and stripe-shaped gel domains in GUV bilayers composed of DOPC/DPPC or DLPC/DPPC are observed by confocal fluorescence microscopy. The lipids in stripe domains are shown to be tilted according to the DiIC18 fluorescence intensity dependence on the excitation polarization. The patch domains are found to be mainly composed of DPPC-d(62) according to the coherent anti-Stokes Raman scattering (CARS) images of DOPC/DPPC-d(62) bilayers. When cooling GUVs from above the miscibility temperature, the patch domains start to appear between the chain melting and the pretransition temperature of DPPC. In GUVs containing a high molar percentage of DPPC, the stripe domains form below the pretransition temperature. Our observations suggest that the patch and stripe domains are in the P-beta' and L-beta' gel phases, respectively. According to the thermoelastic properties of GUVs described by Needham and Evans [(1988) Biochemistry 27, 8261-8269], the P-beta' and L-beta' phases are formed at relatively low and high membrane tensions, respectively. GUVs with high DPPC percentage have high membrane surface tension and thus mainly exhibit L-beta' domains, while GUVs with low DPPC percentage have low membrane surface tension and form P-beta' domains accordingly. Adding negatively charged lipid to the lipid mixtures or applying an osmotic pressure to GUVs using sucrose solutions releases the surface tension and leads to the disappearance of the L-beta' gel phase. The relationship between the observed domains in free-standing GUV bilayers and those in supported bilayers is discussed.