MRGPRX2 D184 engagement by food and pollen allergens: a computational hypothesis for IgE-independent mast-cell activation

Background: Some food and pollen allergic reactions, including anaphylaxis disproportionate to IgE titres, are not fully explained by IgE-mediated mechanisms. MRGPRX2, a polyspecific sensor of cationic ligands, mediates IgE-independent mast-cell degranulation and requires a salt bridge to aspartate D184. Whether protein allergens can adopt D184-engaging poses is unknown. Methods: Eleven peach, cedar pollen, peanut, birch and yam allergens were docked against the experimental MRGPRX2 cryo-EM structure (PDB 7VV6) with HADDOCK3 under three protocols. Three agonists calibrated a D184 salt-bridge consistency threshold, four size-matched non-allergen proteins probed the determinants of engagement, and in silico mutagenesis tested D184 dependence. Results: Five allergens met a HIGH-consistency threshold (≥2/3): Pru p 7, Cry j 7, Cry j 1, Pru p 3 and Ara h 6; Ara h 2 was MODERATE, and mature-chain Cry j 2 did not engage D184. Engagement was charge-driven yet determined by a single accessible residue rather than net charge: HADDOCK score tracked the electrostatic term (ρ = 0.84) but not molecular weight (ρ = 0.36), the net-acidic Ara h 6 engaged through ARG4 while the net-neutral ubiquitin did not, and D184A abolished the salt bridge. Accordingly, size-matched non-allergen proteins presenting a surface lysine/arginine (ribonuclease A, carbonic anhydrase II) also formed genuine D184 salt bridges, whereas the acidic maltose-binding protein did not. Conclusions: Allergens presenting an accessible surface lysine or arginine can form the MRGPRX2 D184 salt bridge in agonist-like geometry, but engagement is governed by local cationic-residue accessibility rather than net charge or allergen identity — non-allergen proteins engage equally — so docking alone establishes neither specificity nor binding affinity, receptor activation or clinical relevance. We propose a charge-based, exposure-gated hypothesis for experimental and clinical validation.