Forces and Flows at Cell Surfaces.

Flow is an important physiological signal and modulates a variety of cell functions. However, the molecular mechanisms that cells use to sense flow have remained surprisingly opaque. The first steps in flow sensing are likely to occur at the plasma membrane, the fluid barrier between the inside and outside of a cell. This membrane is an organized, two-dimensional molecular array that has both solid and fluid properties. The mobility of membrane proteins and lipids is constrained by complex interactions with the cytoskeletal protein network that supports the membrane. Physiologically relevant flows can only generate tiny forces on individual proteins, smaller than those from thermal noise. However, cells could overcome this problem by sensing micron-scale concentration gradients of extracellular membrane proteins. This is possible in cell plasma membranes because their particular physical state allows flow to sort membrane proteins laterally over the extracellular surface.