AFM: a nanotool in membrane biology.

Cellular membranes are vital for life. They confine cells and cytosolic compartments and are involved in virtually every cellular process. Cellular membranes form cellular contacts and focal adhesions, anchor the cytoskeleton, generate energy gradients, transform energy, transduce signals, move cells, and actively form compartments to assemble different membrane proteins into functional entities. But how do cellular membranes perform these tasks? What do the machineries of cellular membranes look like, and how are they controlled and guided? Atomic force microscopy (AFM) allows the observation of biological surfaces in their native environment at a signal-to-noise ratio superior to that of any optical microscopic technique. With a spatial resolution approaching approximately 1 nm, AFM can identify the supramolecular assemblies, characteristic structure, and functional conformation of native membrane proteins. In recent years, AFM has evolved from imaging applications to a multifunctional "laboratory on a tip" that allows observation and manipulation of the machineries of cellular membranes. In the force spectroscopy mode, AFM detects interactions between two single cells at molecular resolution. Force spectroscopy can also be used to probe the local elasticity, chemical groups, and receptor sites of live cells. Other applications locate molecular interactions driving membrane protein folding, assembly, and their switching between functional states. It is also possible to examine the energy landscape of biomolecular reactions, as well as reaction pathways, associated lifetimes, and free energy. In this review, we provide a flavor of the fascinating opportunities offered by the use of AFM as a nanobiotechnological tool in modern membrane biology.