Binding constants of membrane-anchored receptors and ligands depend strongly on the nanoscale roughness of membranes

Abstract

Cell adhesion and the adhesion of vesicles to the membranes of cells or organelles are pivotal for immune responses, tissue formation, and cell signaling. The adhesion processes depend sensitively on the binding constant of the membrane-anchored receptor and ligand proteins that mediate adhesion, but this constant is difficult to measure in experiments. We have investigated the binding of membrane-anchored receptor and ligand proteins with molecular dynamics simulations. We find that the binding constant of the anchored proteins strongly decreases with the membrane roughness caused by thermally excited membrane shape fluctuations on nanoscales. We present a theory that explains the roughness dependence of the binding constant for the anchored proteins from membrane confinement and that relates this constant to the binding constant of soluble proteins without membrane anchors. Because the binding constant of soluble proteins is readily accessible in experiments, our results provide a useful route to compute the binding constant of membrane-anchored receptor and ligand proteins. A central problem in cell adhesion is to quantify the binding affinity of the membrane-anchored receptor and ligand proteins that cause adhesion (1-4). The distinction of "self" and "foreign" in cell-mediated immune responses, for example, depends on subtle affinity differences between receptor and ligand proteins anchored on the surfaces of apposing cells (5). The binding affinity of anchored receptor and ligand proteins, which are restricted to the two-dimensional (2D) membrane environment, is typically described by the binding equilibrium constant K 2D of the proteins. Because K 2D is difficult to measure in experiments, it is often estimated from the binding constant K 3D of soluble variants of the receptors and ligands that lack the membrane anchors and are free to diffuse in three dimensions (3D). Standard approaches are based on the relation K 2D = K 3D =l c suggested by Bell et al. (6), where l c is a characteristic length that reflects the different units of area and volume for K 2D and K 3D , respectively. However, different methods to measure the binding equilibrium constant of membraneanchored proteins have led to values of K 2D and associated values of l c that differ by several orders of magnitude (7). In contrast to the standard approaches, the simulation data and theory presented here indicate that the relation between K 2D and K 3D involves three different length scales, and that the most important of these length scales is the membrane roughness resulting from shape fluctuations on nanoscales. Because the membrane roughness depends on the concentration of the receptor-ligand bonds that constrain the shape fluctuations, our results help to understand differences in K 2D values from different experiments.In this article, we report simulations of biomembrane adhesion with a molecular model of lipids and proteins (Fig. 1A). We systematically vary the size of the membranes and the numbers of receptors and ligands and determine...

E-cadherin junction formation involves an active kinetic nucleation process

“…Intermembrane E-cadherin adhesion does not appear to follow simple laws of mass action. That is, the rate of intermembrane bond formation does not exhibit proportionality to receptor concentration as is seen in other systems, even when considering the geometry and mechanics of the intermembrane junction (31)(32)(33). Rather, E-cadherin junction formation requires active nucleation before extensive adhesive junctions form.…”

Remarkably low affinity of CD4/peptide-major histocompatibility complex class II protein interactions

“…However, it is also possible that CD4 function and signaling are rather more dependent on prior TCR/ pMHC II engagement, because this facilitates CD4 recruitment. For example, the 2D K d of the CD4/pMHC II interaction could be lower due to suppressed membrane fluctuations and/or optimal positioning of pMHC II for CD4 binding (34) (Fig. S9).…”

Model for Probing Membrane-Cortex Adhesion by Micropipette Aspiration and Fluctuation Spectroscopy

“…The adhesion of a flexible membrane on a substrate by means of discrete linkers has been extensively studied in the past (18)(19)(20)(21)(22)(23), mostly using computer simulations. It is a highly nontrivial problem due to the multiplicity of energy scales (membrane rigidity and tension, linker stiffness, and binding energy) and timescales (membrane and cytosol fluidity, linker's diffusion, and binding kinetics).…”

“…This assumption allows us to disregard membrane fluctuations between attachment points, and yields a simple analytical form for the unbinding transition. However, it does not capture binding cooperativity occurring due to the smoothing of membrane fluctuations near attachment points (19)(20)(21)(22)(23). Two relevant dimensionless quantities characterize the mechanics of the linkers: the kinetic ratio, c, and the ratio of the force on the membrane to an intrinsic force scale of the linkers, a, with ch k 0 off k on and ah sd r 0 k B T :…”