Structural insights into the activation of the chicken ROS1 receptor by the NEL/NICOL ligand complex

Abstract

Abstract The receptor tyrosine kinase ROS1 plays essential roles in cell growth and sperm maturation, yet its activation mechanism has remained poorly understood. Here, we report high-resolution cryo-electron microscopy (cryo-EM) structures of chicken ROS1 in its ligand-free form, in complex with its ligand NEL, and with the ligand/co-ligand complex NEL/NICOL. Unliganded ROS1 adopts an arc-shaped conformation. The interaction between NEL and ROS1 is mediated by the VWC2 domain of NEL and the β1 domain of ROS1. Binding of NICOL to the coiled-coil domain of NEL stabilizes NEL into a batwing-shaped asymmetric dimer, which can recruit only one ROS1 molecule due to steric hindrance. Structural analyses and biochemical results suggest that the 2:1 NEL/NICOL complexes further oligomerize through LamG–VWC4 domain interactions, facilitating the clustering of multiple ROS1 for its activation. Functional assays confirm that both NICOL and the multimerization of NEL/NICOL complexes are required for robust ROS1 signaling. Our findings establish NICOL as a critical co-ligand for ROS1 and suggest a distinct ligand-driven oligomerization mechanism for ROS1 activation.