The dynamics of melittin-induced membrane permeability.

The transport of co-encapsulated solutes through the melittin-induced pores in the membrane of giant phospholipid vesicles was studied, and the characteristics of the pore formation process were modeled. Molecules of two different sizes (dextran and the smaller, fluorescent marker Alexa Fluor) were encapsulated inside the vesicles. The chosen individual vesicles were then transferred by micromanipulation from the stock suspension to the environment with the melittin (MLT). The vesicles were observed optically with a phase-contrast microscope and by monitoring the fluorescence signal. Such an experimental setup enabled an analysis of a single vesicle's response to the MLT on the basis of simultaneous, separate measurements of the outflow of both types of encapsulated molecules through the MLT-induced pores in the membrane. The mechanisms of the MLT's action were suggested in a model for MLT pore formation, with oligomeric pores continuously assembling and dissociating in the membrane. Based on the model, the results of the experiments were explained as a consequence of the membrane's permeability dynamics, with a continuously changing distribution of pores in the membrane with regard to their size and number. The relatively stable "average MLT pore" characteristics can be deduced from the proposed model.