Membrane Targeting of TIRAP is Negatively Regulated by Phosphorylation in its Phosphoinositide-Binding Motif

Abstract

Toll-like receptors (TLRs) trigger early immune system recognition and response to infection. Pathogen-activated TLRs, such as TLR2 and TLR4, dimerize and move laterally across the plasma membrane to phosphatidylinositol (4,5)-bisphosphate-enriched domains. At these sites, TLRs interact with the TIR domain-containing adaptor protein (TIRAP), triggering a signaling cascade that leads to innate immune responses. Membrane recruitment of TIRAP is mediated by its phosphoinositide-binding motif (PBM). We show that TIRAP PBM transitions from a disordered to a helical conformation in the presence of either zwitterionic micelles or monodispersed phosphoinositides. TIRAP PBM bound phosphoinositides through basic and nonpolar residues with high affinity, favoring a more ordered structure. TIRAP is phosphorylated at Thr28 within its PBM, which leads to its ubiquitination and degradation. We demonstrate that phosphorylation distorts the helical structure of TIRAP PBM, reducing phosphoinositide interactions and cell membrane targeting. Our study provides the basis for TIRAP membrane insertion and the mechanism by which it is removed from membranes to avoid sustained innate immune responses.