Flow transports extracellular lipid-anchored proteins across the surface of living COS-7 cells
The rapid diffusion of membrane lipids and membrane proteins in living cell plasma membranes demonstrates that the membrane is fluid. However, motion of membrane molecules is inhibited on one side by the cytoskeletal mesh, and on the other by the glycocalyx, a layer of proteoglycans with long polysaccharide chains that covers the membrane surface. A variety of biological fluid flows (including blood circulation, cilia-driven flows, and swimming motion of microorganisms) apply shear stress to cell surfaces. Cell responses to these flows govern important physiological processes such as blood pressure and immune activation. The presence of the glycocalyx is generally thought to shield cell membranes from shear stress that arises from flow. However, here we show that two different proteins, each attached by a lipid anchor to the extracellular membrane surface of living COS-7 cells, formed reversible, cell-wide concentration gradients in the direction of applied flow. Protein redistribution occurred within minutes after we applied shear stress levels commonly found in animal cardiovascular systems. The dynamic and spatial features of these gradients were consistent with passive transport by flow. Passive flow transport could be a general mechanism for spatial organization of membrane proteins. This mechanism may explain protein patterning previously observed on flow-exposed cells, and potentially forms an initial step in flow sensing.