A Directed TM→JM Coupling in Receptor Tyrosine Kinase Dimers, Set by Activating Mutations and the Membrane Environment.

The direction of conformational coupling in a membrane protein, that is, which domain drives which, has been inaccessible to experimental measurement. We recover this directivity from molecular dynamics (MD) of transmembrane-juxtamembrane (TM-JM) dimers of receptor tyrosine kinases EGFR and FGFR3. Coupling is detected with a Bayesian network framework (CASCADE); its direction is measured with PERI (Phase-plane Estimation of Rotational Irreversibility) as the net phase-plane circulation, validated on synthetic data, and resolved at 0.1 ns. Direction is summarized as the TM→JM directed-mass fraction f+ (0.5 = balanced) via a hierarchical Bayesian model. The activating TM mutants EGFR L658Q and FGFR3 A391E are TM→JM (posterior probabilities: 0.95 and 0.99); fluid wild-type EGFR leans the same way (0.93), in agreement with its experimentally reported constitutive activity in fluid but not ordered bilayers; the ligand-dependent ordered wild-type is balanced (0.45); and an activating mutation raises the TM→JM bias above the ordered wild-type with probability 0.94. The directivity thus tracks the measured activity state of the receptor, distinguishing signaling-competent from ligand-dependent RTK dimers by a property not apparent from structure alone.