Coaction of Electrostatic and Hydrophobic Interactions: Dynamic Constraints on Disordered TrkA Juxtamembrane Domain.

In the receptor tyrosine kinase family, conformational change induced by ligand binding is transmitted across the membrane via a single transmembrane helix and a flexible juxtamembrane domain (JMD). Membrane dynamics makes it challenging to study the structural mechanism of receptor activation experimentally. In this study, we employ all-atom molecular dynamics with highly mobile membrane mimetic (HMMM) to capture the native conformation of the JMD in tropomyosin receptor kinase A (TrkA). We find that phosphatidylinositol 4,5-bisphosphate (PIP2) lipids engage in stable binding with multiple basic residues. Anionic lipids can compete with salt bridges within the peptide and alter TrkA-JMD conformation. We discover three-residue insertion into the membrane and are able to either enhance or reduce the level of insertion through computationally-designed point mutations. The vesicle-binding experiment supports computational results and indicates that hydrophobic insertion is comparable to electrostatic binding for membrane anchoring. Biochemical assays on cell lines with mutated TrkA show that enhanced TrkA-JMD insertion promotes receptor degradation but does not affect the short-term signaling capacity. Our joint work points to a scenario where lipid headgroups and tails interact with basic and hydrophobic residues on disordered domain, respectively, to restrain flexibility and potentially modulate protein function.