Anionic Lipids Take Charge: Juxtamembrane Domain Interactions with Cellular Membrane

Abstract

Nearly half of the human integral membrane proteome consists of single-pass transmembrane domains. Of these, receptor tyrosine kinases (RTKs), a class with twenty families of cell-surface receptors, are key regulators of vital cellular processes and targets of drug development efforts. The fibroblast growth factor receptors (FGFRs) a family of RTKs that influences cell growth, proliferation, differentiation, is also activated in a number of cancers. FGFRs dynamic architecture, that contains extracellular, single-pass transmembrane (TM), juxtamembrane (JM), and kinase domains, is tightly intertwined with cellular membrane and is a challenging target for structural studies. Recent experimental studies revealed that highly charged FGFR3 JM domains are involved in stabilization of unliganded FGFR3 dimers, which is achieved in part through interactions with charged lipids of the inner leaflet of the cellular membrane. The mechanism of JM-lipid interaction is unknown. We use all-atom molecular dynamics with highly mobile membrane mimetic (HMMM) model to capture spontaneous JM-lipid encounter complexes. To investigate the role of the anionic lipids we performed extensive simulations of multiple replicas of pure phosphatidylcholine (PC), binary mixtures of phosphatidylserine (PS) and PC, and tertiary mixtures of PC, PS, and phosphatidylinositol 4,5-bisphosphate (PIP2) lipid bilayers. Simulations revealed formation of stable anionic lipid conformations coordinated by charged JM residues. Lipid type-specific geometries lead to characteristic modes of JM-lipid interactions revealing the role of the charged lipid headgroups in establishing specific interactions. Knowledge of these dynamic structures will facilitate our understanding of mechanisms of RTK activation.