TAM receptors, comprising Tyro3, Axl, and Mer, are a unique subfamily of receptor tyrosine kinases (RTKs) with critical roles in immune regulation and cellular homeostasis through apoptotic cell clearance. Their activation is mediated through the ligands Gas6 and Protein S, which require association with the exposed membrane lipid phosphatidylserine (PtdSer) on apoptotic cells for full receptor activation. Unlike most RTKs, TAM receptors cannot be fully activated by ligand binding alone, and lipid engagement contributes to receptor-specific differences in activation strength. Yet, the molecular details of TAM receptor activation and how lipid engagement contributes to this process remain poorly defined. Although fragments of Tyro3, Axl, Mer, and Gas6 have been structurally determined, no full-length receptor-ligand complex structures exist, and how lipid engagement influences these assemblies is unknown. Our work focuses on defining the structural and functional changes that occur when TAM receptors interact with their ligands, both in the presence and absence of lipid. Preliminary modeling of TAM receptor extracellular domains bound to Gas6 or Protein S, using the minimal Axl:Gas6 structure as a reference reveals conserved residues predicted to stabilize receptor-ligand interactions. Guided by these models and variants identified in the COSMIC database, we are testing the effects of targeted mutations on receptor activation using phosphorylation assays, followed by binding studies to assess changes in affinity. Importantly, we obtained a preliminary 3D reconstruction of the Axl:Gas6 complex at high resolution, demonstrating that this high-affinity pair is a strong candidate for further cryo-EM analysis with and without lipid mimetics. Ongoing work combining cryo-EM and crystallography aims to generate the first complete framework of TAM receptor-ligand-lipid complexes, which will shed light on how lipid interactions influence aberrant receptor activation and signaling in diseases such as cancer and autoimmunity.
Arango et al. (Sun,) studied this question.