Microfluidic measurements of protein size and shape

Individual cells sense and respond to flow, and their responses can regulate important physiological processes such as blood pressure. The mechanism of shear flow sensing in mammalian blood vessels is not well understood; in fact, the forces applied by shear stress (on the order of femtonewtons) are too small to alter individual protein conformations. However, femtonewton-sized forces can easily accomplish lateral transport of lipid-anchored extracellular membrane proteins.

We observe that in both living cells and glass-supported lipid bilayers, proteins move downstream when flow is on, forming a  micron-scale concentration gradient. We use fluorescence microscopy to correlate hydrodynamic force with the folded shape of lipid-anchored proteins, distinguish membrane drag on different lipid anchors, and demonstrate that similar protein transport can occur on the surface of living cells. Our results support the hypothesis that lateral transport of membrane proteins may contribute to flow sensing.