Conformational dynamics of an intrinsically disordered receptor enable signal transduction
Signal transduction is initiated when activated cell-surface receptors physically engage intracellular signaling proteins. Although structural and biochemical studies have established the molecular architecture of receptor tyrosine kinase (RTK) signaling, the earliest dynamic events that couple receptor activation to intracellular signal propagation have remained inaccessible because existing approaches provide static structural snapshots or ensemble-averaged measurements rather than direct observation of molecular motion. Here, using high-speed atomic force microscopy (HS-AFM), we directly visualize these initial molecular events at the single-molecule level in real time. Using fibroblast growth factor receptor 1 (FGFR1) as a model, we show that receptor autophosphorylation releases its intrinsically disordered juxtamembrane region, enabling extensive conformational dynamics of the kinase domain that permit productive engagement of the signaling adaptor FRS2, a key initiator of downstream RTK signaling. In contrast, a kinase-inactive mutation or the clinically approved FGFR inhibitor futibatinib restricts receptor motion, stabilizes a compact receptor configuration, and markedly reduces adaptor engagement. Molecular dynamics simulations suggest that the disruption of inhibitory interactions between the juxtamembrane region and the kinase domain provides a structural explanation for the observed receptor dynamics. Together, our findings uncover a previously inaccessible physical mechanism underlying the initiation of RTK signaling and establish HS-AFM as a powerful platform for visualizing receptor dynamics, providing a framework for mechanistic studies of signal transduction and for the development and mechanistic evaluation of compounds that modulate receptor signaling.