Near-critical tuning of cooperativity revealed by spontaneous switching in a protein signalling array

Abstract

Dynamic properties of allosteric complexes are crucial for cellular information processing. However, direct observations of allosteric switches have been limited to compact molecular assemblies. Here, we report in vivo FRET measurements of spontaneous discrete-level fluctuations in the activity of the Escherichia coli chemosensory array -- an extensive membrane-associated assembly comprising thousands of molecules. Finite-size scaling analysis of the temporal statistics by a two-dimensional conformational spread model revealed nearest-neighbor coupling strengths within 3% of the Ising second-order phase transition, indicating that chemosensory arrays are poised at criticality. Our analysis yields estimates for the intrinsic timescale of conformational changes (~ 10 ms) of allosteric units, and identifies near-critical tuning as a design principle for balancing the inherent tradeoff between response amplitude and response speed in higher-order signaling assemblies. One-setence summaryIn vivo measurements of protein signaling array fluctuations reveal an allosteric system poised at criticality.