Measuring flow-mediated protein drift across stationary supported lipid bilayers.

Fluid flow near biological membranes influences cell functions such as development, motility, and environmental sensing. Flow can laterally transport extracellular membrane proteins located at the cell-fluid interface. To determine whether this transport contributes to flow signaling in cells, quantitative knowledge of the forces acting on membrane proteins is required. Here, we demonstrate a method for measuring flow-mediated lateral transport of lipid-anchored proteins. We rupture giant unilamellar vesicles to form discrete patches of supported membrane inside rectangular microchannels and then allow proteins to bind to the upper surface of the membrane. While applying flow, we observe the formation of protein concentration gradients that span the membrane patch. By observing how these gradients dynamically respond to changes in applied shear stress, we determine the flow mobility of the lipid-anchored protein. We use simplified model membranes and proteins to demonstrate our method's sensitivity and reproducibility. Our intention was to design a quantitative, reliable method and analysis for protein mobility that we will use to compare flow transport for a variety of proteins, lipid anchors, and membranes in model systems and on living cells.

Systematic measurements of interleaflet friction in supported bilayers.

When lipid membranes curve or are subjected to strong shear forces, the two apposed leaflets of the bilayer slide past each other. The drag that one leaflet creates on the other is quantified by the coefficient of interleaflet friction, b. Existing measurements of this coefficient range over several orders of magnitude, so we used a recently developed microfluidic technique to measure it systematically in supported lipid membranes. Fluid shear stress was used to force the top leaflet of a supported membrane to slide over the stationary lower leaflet. Here, we show that this technique yields a reproducible measurement of the friction coefficient and is sensitive enough to detect differences in friction between membranes made from saturated and unsaturated lipids. Adding cholesterol to saturated and unsaturated membranes increased interleaflet friction significantly. We also discovered that fluid shear stress can reversibly induce gel phase in supported lipid bilayers that are close to the gel-transition temperature.

Divide and conquer: How phase separation contributes to lateral transport and organization of membrane proteins and lipids.

Biological membranes are fluid, dynamic and heterogeneous, with the dual tasks of defining cell compartments and facilitating communication between them. Within membranes, lipid phase separation can alter local composition, dynamics, and allosteric regulation of membrane proteins. The interplay between lipid-lipid, lipid-protein and protein-protein interactions gives flexibility to membrane lateral organization. In this review we examine how lipid phase separation impacts lateral transport of lipids and proteins within membranes. First, we discuss the role of liquid-liquid coexistence in the organization of model biomembranes, and how such demixing can redistribute lipids and proteins into different regions. Next, the role of curvature in membrane patterning via its influence on lipid composition and protein spatial distribution in both model and biological systems is examined. Then, we discuss how critical fluctuations can organize membrane proteins. Finally, we review how external forces can be used to control the organization of lipids and proteins within biomembranes; with examples covering how ATP driven protein adsorption, electrophoresis, and hydrodynamic flow can transport and redistribute lipids and proteins laterally within membranes.