Fast rebinding increases dwell time of Src homology 2 (SH2)-containing proteins near the plasma membrane

Abstract

Receptor tyrosine kinases (RTKs) control a host of biological functions by phosphorylating tyrosine residues of intracellular proteins upon extracellular ligand binding. The phosphotyrosines (p-Tyr) then recruit a subset of ∼100 Src homology 2 (SH2) domain-containing proteins to the cell membrane. The in vivo kinetics of this process are not well understood. Here we use total internal reflection (TIR) microscopy and single-molecule imaging to monitor interactions between SH2 modules and p-Tyr sites near the cell membrane. We found that the dwell time of SH2 modules within the TIR illumination field is significantly longer than predictions based on chemical dissociation rate constants, suggesting that SH2 modules quickly rebind to nearby p-Tyr sites after dissociation. We also found that, consistent with the rebinding model, the effective diffusion constant is negatively correlated with the respective dwell time for different SH2 domains and the dwell time is positively correlated with the local density of RTK phosphorylation. These results suggest a mechanism whereby signal output can be regulated through the spatial organization of multiple binding sites, which will prompt reevaluation of many aspects of RTK signaling, such as signaling specificity, mechanisms of spatial control, and noise suppression.