Quantifying the impact of membrane microtopology on effective two-dimensional affinity.
Just as interactions of soluble proteins are affected by the solvent, membrane protein binding is influenced by the surface environment. This is particularly true for adhesion receptors because their function requires tightly apposed membranes. We sought to demonstrate, and further, to quantify the possible scale of this phenomenon by comparing the effective affinity and kinetic rates of an adhesion receptor (CD16b) placed in three distinct environments: red blood cells (RBCs), detached Chinese hamster ovary (CHO) cells, and K562 cells. Effective affinity reflects both the intrinsic receptor-ligand kinetics and the effectiveness of their presentation by the host membranes. Expression of CD16b, a low affinity Fcgamma receptor, was established by either transfection or spontaneous insertion via its glycosylphosphatidylinositol anchor. Binding to IgG-coated RBCs, measured using a micropipette method, indicated a 50-fold increase in effective affinity for receptors on RBCs over CHO and K562 cells, whereas the off rates were similar for all three. Electron microscopy confirmed that specific tight contacts were broad in RBC-RBC conjugates but sparse in CHO-RBC conjugates. We suggest that through modulation of surface roughness the cytoskeleton can greatly impact the effectiveness of adhesion molecules, even those with no cytoplasmic structures. Implications for locomotion and static adhesion are discussed.
George W. Woodruff School of Mechanical Engineering and Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0363, USA.
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Williams
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Nagarajan
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Selvaraj
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Zhu
D000818:Animals; D016466:CHO Cells; D002448:Cell Adhesion; D002462:Cell Membrane / Q000502:physiology* / Q000648:ultrastructure*; D006224:Cricetinae; D004910:Erythrocyte Membrane / Q000502:physiology / Q000648:ultrastructure; D006801:Humans; D007074:Immunoglobulin G / Q000097:blood / Q000502:physiology; D020014:K562 Cells; D017452:Receptors, IgG / Q000502:physiology*; D011994:Recombinant Proteins / Q000276:immunology; D014162:Transfection
D007074:Immunoglobulin G; D017452:Receptors, IgG; D011994:Recombinant Proteins