Quantitative Experimental Assessment of Macromolecular Crowding Effects at Membrane Surfaces

We examined how crowding of the surfaces of lipid vesicles with either grafted polyethyleneglycol (PEG) chains or bilayer-anchored protein molecules affects the binding of soluble proteins to the vesicle surface. Escherichia coli dihydrofolate reductase (DHFR, 18 kDa) or a larger fusion protein, NusA-DHFR (72 kDa), binds reversibly but with high affinity to a methotrexate-modified lipid (MTX-PE) incorporated into large unilamellar vesicles. Incorporation of phosphatidylethanolamine-PEG5000 into the vesicles strongly decreases the affinity of binding of both proteins, to a degree that varies roughly exponentially with the lateral density of the PEG chains. Covalently coupling maltose-binding protein (MBP) to the vesicle surfaces also strongly decreases the affinity of binding of NusDHFR or DHFR, to a degree that likewise varies roughly exponentially with the surface density of anchored MBP. Surface-coupled MBP strongly decreases the rate of binding of NusDHFR to MTX-PE-incorporating vesicles but does not affect the rate of NusDHFR dissociation. The large magnitudes of these effects (easily exceeding an order of magnitude for moderate degrees of surface crowding) support previous theoretical analyses and suggest that surface-crowding effects can markedly influence a variety of important aspects of protein behavior in membranes.