Physical basis for the interaction between Drosophila ROS1 and the GPCR BOSS

Abstract/SummaryDrosophila ROS1 (dROS1, Sevenless) is a receptor tyrosine kinase (RTK) essential for the differentiation of Drosophila R7 photoreceptor cells1, 2. Activation of dROS1 is mediated by binding to the extracellular region (ECR) of the GPCR (G protein coupled receptor) BOSS (Bride Of Sevenless) on adjacent cells1, 3, 4. Genetic evidence together with in vitro activity assays confirmed the activation of dROS1 by BOSS and identified subsequent downstream signaling pathways including SOS (Son of Sevenless)1, 5. However, the physical basis for how dROS1 interacts with the GPCR BOSS has long remained unknown. Here we provide the first structure, using Cryo-Electron Microscopy (CryoEM), of dROS1s extracellular region, which mediates ligand binding. We show that the N-terminal region of dROS1 adopts a folded-over conformation harboring a novel structural domain. We further narrowed down the interacting binding epitopes on both dROS1 and BOSS. This includes a beta-strand in dROS1s third Fibronectin type III (FNIII) domain and the C-terminal portion of BOSS ECR. Our mutagenesis studies, coupled with AlphaFold complex predictions, support a binding interaction mediated by a hydrophobic interaction and beta-strand augmentation between these regions. Our findings provide a fundamental understanding of the regulatory function of dROS1 and further provide mechanistic insight into the human ortholog and oncogene ROS1.