Solubilization of Giant Vesicles Composed of Erythrocyte Lipid Extracts and of Ternary Lipid Mixtures by Triton X-100

Abstract

Detergents are widely used as solubilizing agents of biological membranes. Detergent resistant membranes, called DRMs, are frequently obtained and share similarities in composition and properties with the so-called lipid rafts and with the liquid-ordered phase (Lo) of model membranes. Here we use optical and fluorescence microscopy to study the solubilization process of giant unilamellar vesicles (GUVs) made of erythrocyte lipid extracts (erythro-GUVs) by the commonly used detergent Triton X-100. For comparison, ternary lipid mixtures of POPC, sphingomyelin (SM) and cholesterol are also investigated. Erythro-GUVs do not exhibit macroscopic phase separation, as seen by the homogenous distribution of a fluorescence probe that preferentially partition into the fluid (Ld) phase. However, contact with Triton X-100 immediately causes formation of micrometer-sized bright domains, typical of Lo/Ld phase separation. Then, at Triton X-100 concentrations above its cmc, the fluid part is first detached from the original vesicle and then completely solubilized by Triton X-100. The insoluble vesicle left is only faintly fluorescent and appears to be in the Lo phase. Typically, one third of the erythro-GUV surface area is solubilized. Solubilization of POPC:SM:chol GUVs in a certain composition range follow the same sequence of events. Our results show that Triton X-100 promotes formation of macroscopic domains in both biological and biomimetic membranes, and that the partial solubilization occurs after phase separation, suggesting that the composition of the insoluble membrane fraction is modulated by the choice of the detergent used. Financial support: CNPq, FAPESP and INCT-FCx.