Adhesive switching of membranes: Experiment and theory

We report on a study of a model bioadhesion system: giant vesicles in conta ct with a supported lipid bilayer. Embedded in both membranes are very low concentrations of hemophilic recognition molecules (contact site A receptor s) competing with higher concentrations of repeller molecules: polyethylene glycol (PEG) lipids. These repellers mimic the inhibiting effect of the ce ll glycocalyx on adhesion. The effective adhesive interaction between the t wo membranes is probed by interferometric analysis of thermal fluctuations. We find two competing states of adhesion: initial weak adhesion is followe d by slower aggregation of the adhesion molecules into small, tightly bound clusters that coexist with the regions of weak adhesion. We interpret our results in terms of a double-well intermembrane potential, and we present a theoretical analysis of the intermembrane interaction in the presence of m obile repeller molecules at a fixed chemical potential that shows that the interaction potential indeed should have just such a double-well shape. At a fixed repeller concentration we recover a conventional purely repulsive p otential. We discuss the implications of our findings in terms of a general amplification mechanism of the action of sparse adhesion molecules by a no nspecific double-well potential. We also discuss the important role of the Helfrich undulation force for the proposed scenario.