Intrinsically disordered regions of Src and Abl kinases contribute to membrane binding.

Abstract

Membrane targeting regulates protein localization and function. Abl and Src kinase feature an N-terminal myristoylation site followed by an intrinsically disordered region (IDR) of positive net charge, suggesting these regions contribute to membrane targeting. We investigated the interplay of myristoylation and IDR binding to phosphatidylinositol-4,5-bisphosphate (PIP2)-containing membranes using atomistic equilibrium and non-equilibrium molecular dynamics (MD) simulations. After having established Charmm36m-TIP4P-fb as a force field combination of choice for the challenging IDR-membrane systems of interest, we assessed the binding propensities of Abl and Src's IDRs to lipid bilayers, and observed strong binding only when PIP2 was present, as also quantified by significant increases in detachment forces and unbinding free energies when including 10 mol% PIP2. Interactions were mediated by saltbridges between the IDR's basic residues and PIP2 headgroups, and largely unaffected by cholesterol. On this basis, we designed Src and Abl mutants with deficient PIP2 binding, and confirmed impaired membrane binding using liposomal-binding assays and atomic force microscopy experiments. Simulations and experiments of further variants corroborated that both a myristoylation site and the charged IDR are required for firm PIP2-membrane interactions. Our results establish myristoylated IDRs in kinases - and potentially beyond - as important regulators of PIP2-specific membrane targeting. STATEMENT OF SIGNIFICANCE: Abl and Src kinase are key cell signalling proteins that feature myristoylated long disordered regions, but their mechanism of membrane-anchoring remains unclear. Both feature a long disordered region with many basic residues, akin to PIP2-recognizing basic patches in PH or FERM domains, such as in Focal Adhesion Kinase, which is activated when pulled away from a PIP2 membrane. Using a combined simulation and experimental approach, we here show for Src and Abl kinase that membrane targeting and high-stability binding to PIP2-containing membranes can be achieved, providing that myristoylation and basic residues along the basic disordered region can synergistically stabilize the protein-membrane interaction. Myristoylated disordered regions might play similar roles in other proteins.