How actin dynamics affect membrane nanotube mechanics

The living cell is an out-of equilibrium system that constantly remodels its architecture to ensure biological functions such as division or intracellular transport. The latter involves the formation of intermediate cylindrical membrane nanotubes. These nanotubes are then split into membrane compartments transported in other areas of the cell. Whereas mechanics of pure membrane nanotubes are now well understood, the role of the actin cytoskeleton on tube stability remains unclear.

To address this question, we develop a bottom-up approach based on model lipid membranes on which we reconstitute actin assembly with a minimal number of proteins. Two distinct methods allow us to reproduce the in vivo cylindrical membrane geometry. First, nanotubes are pulled from vesicles with optical tweezers. Second, lipid deposits are brushed and characterized by atomic force microscopy. We finally observe the formation of an actin muff that proves successful activation of actin network growth around the tube.

Both methods allow us to derive the viscoelastic properties of the muff. For example, tubes surrounded by the muff retract slowly (3.5 s) compared to pure membranes (< 0.5 s). A striking observation is that tube radius decreases in the presence of actin and may initiate further scission.