Shape instability of a biomembrane driven by a local softening of the underlying actin cortex

We present a theory showing that local shape instabilities of composite bio logical membranes, consisting of a lipid bilayer and an underlying actin co rtex, can be triggered by a local softening, of the membrane-associated cyt oskeleton. A membrane containing such cortical defects can form blisters or invaginations, depending on external conditions. The theoretical predictio ns agree with observations provided by two sets of experiments: (i) microsc opic observations of shape changes of giant vesicles with underlying shells of a thin actin network show the formation of local blisters and (ii) micr opipet aspiration experiments of Dictyostelium discoideum cells in which we observed the formation of blisters in the aspirated cell part. In the latt er experiments, the existence of a hole in the underlying cortex is confirm ed by observation of the entrance of cell organelles into the blister. Our model may also be applied to the formation of lobopodia, fast-growing cell protrusions that play an important role in the locomotion and spreading of biological cells.