Interstitial friction greatly impacts membrane mechanics.

Membrane bilayers separate organelles from the cytoplasm and nucleoplasm of cells, and cells from their extracellular milieu. The mechanical compliance and tension of these membranes play a central role in their biological functions. For instance, the plasma membrane (which contains both lipid molecules and proteins and is physically connected to the underlying actin-rich cytoskeleton meshwork of the cell through linker proteins) regulates the cell’s ability to spread, deform, and migrate (1–4). Because of its physiological importance, membrane mechanics has long been an important subject of biophysical research, studied using complementary systems and models, including live cells and biomimetic systems. The exquisite versatility and control of biomimetic, in vitro systems (mainly liposomes), which make use of purified lipids and proteins, have made these systems highly attractive to determine the complex physical laws that govern membrane mechanics and identify important molecular contributors. Work with liposomes has long suggested that changes in cytoskeleton content, myosin-based activity, and cytoskeletal coupling to the membrane all significantly impact membrane stiffness and dynamics. In particular, the friction between the two leaflets of a plasma membrane was supposed to be as small as the friction of a lipid