Membrane resistance to Triton X-100 explored by real-time atomic force microscopy

Lateral segregation of lipids and proteins in biological membranes leads to the formation of detergent-resistant domains, also called "rafts". Understanding the mechanisms governing the biomembrane's resistance to solubilization by detergents is crucial in biochemical research. Here, we used real-time atomic force microscopy (AFM) imaging to visualize the behavior of a model supported lipid bilayer in the presence of different Triton X-100 (TX-100) concentrations. Mixed dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC) supported bilayers were prepared by vesicle fusion. Real-time AFM imaging revealed that, at concentrations below the critical micelle concentration (CMC), TX-100 did not solubilize the bilayer, but the DPPC domains were eroded in a time-dependent manner. This effect was attributed to the DPPC molecular packing disorganization by the detergent starting from the DOPC/DPPC interface. Just above the CMC, the detergent led to a complete solubilization of the DOPC matrix, leaving the DPPC domains unaltered. At higher TX-100 concentrations, the DOPC was also immediately removed just after detergent addition, and the DPPC domains remaining on the mica surface appeared to be more swollen and were gradually solubilized. This progressive solubilization of the DPPC remaining phase did not start at the edge of the domains but from holes appearing and expanding at the center of DPPC patches. The swelling of the DPPC domains was directly correlated with TX-100 concentration above the CMC and with detergent intercalation between DPPC molecules. We are convinced that this approach will provide a key system to elucidate the physical mechanisms of membrane solubilization by nonionic detergents.