Bridging molecular to cellular scales for models of membrane receptor signaling
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Biochemical interactions at membranes are the starting points for cell signaling networks. But bimolecular reaction kinetics are difficult to experimentally measure on 2-dimensional membranes and are usually measured in volumetric in vitro assays. Membrane tethering produces confinement and steric effects that will significantly impact binding rates in ways that are not readily estimated from volumetric measurements. Also, there are situations when 2D reactions do not conform to simple kinetics. Here we show how highly coarse-grained molecular simulations using the SpringSaLaD software can be used to estimate membrane-tethered rate constants from experimentally determined volumetric kinetics. The approach is validated using an analytical solution for dimerization of binding sites anchored via stiff linkers. This approach can provide 2-dimensional bimolecular rate constants to parameterize cell-scale models of receptor-mediated signaling. We explore how factors such as molecular reach, steric effects, disordered domains, local concentration and diffusion affect the kinetics of binding. We find that for reaction-limited cases, the key determinant in converting 3D to 2D rate constant is the distance of the binding sites from the membrane. On the other hand, the mass action rate law may no longer be obeyed for diffusion-limited reaction on surfaces; the simulations reveal when this situation pertains. We then apply our approach to epidermal growth factor receptor (EGFR) mediated activation of the membrane-bound small GTPase Ras. The analysis reveals how prior binding of Ras to the allosteric site of SOS, a guanine nucleotide exchange factor (GEF) that is recruited to EGFR, significantly accelerates its catalytic activity.
SIGNIFICANCE STATEMENT
In cell signaling, the activation of a surface receptor leads to a cascade of intracellular biochemical events. Many of these occur near the inner plasma membrane surface. However, accurate rate parameters for these initial steps in models of signaling are rarely available because membrane-tethered reaction kinetics are difficult to experimentally measure. Here, we use a highly coarse-grained molecular simulator to model the kinetics of reactions between binding sites that are tethered to a membrane. We can fit these simulation outputs to 2-dimensional rate laws to obtain rate constants that can be used to build complex models of cell signaling. These rate constants can also be compared to understand the key biophysical features controlling the kinetics of bimolecular membrane reactions.
