Hydrodynamics vs. membrane drag determines the mobility of bioengineered proteins during flow

Biological membranes and the lipid-anchored proteins embedded in them experience shear forces, particularly in an environment such as the cardiovascular system. For example, Glypican-1, a GPI-anchored protein involved in flow sensing by cells, moves in the flow and direction to create a concentration gradient. The mobility of proteins and hence such formed concentration gradient profile can be affected by several factors such as protein size, shape, the viscosity of the surrounding media, protein-membrane anchor, and the composition of the membrane itself. To decouple the factors contributing to protein mobility, we developed a method to measure the flow mobility of proteins on a model system composed of lipid anchored proteins on solid-supported lipid bilayers (SSLBs). We combine microfluidics and confocal microscopy to monitor the protein gradients formed under controlled shear stress and hence measure the mobility of the protein. The protein mobility is determined by the balance of two forces experienced by the protein: the viscous drag force exerted by the membrane in which proteins are embedded and the hydrodynamic shear force exerted by the flow. The hydrodynamic shear force experienced by the protein under given stress is altered by protein size and shape whereas the viscous drag force exerted by the membranes depends on membrane composition and protein-membrane anchor. Here we independently vary these forces by creating a series of monomeric proteins and lipid mixtures. We successfully purified single and double-GFP-tagged monomeric Streptavidin protein and measured their mobilities in membranes with different lipid compositions. We obtain the parameters we can use to predict flow responses of protein in general.