Site‐Selective Functionalization of Flagellin by Steric Self‐Protection: A Strategy To Facilitate Flagellin as a Self‐Adjuvanting Carrier in Conjugate Vaccine

Flagellin (FliC) can act as a carrier protein in the preparation of conjugate vaccines to elicit a T‐cell‐dependent immune response and as an intrinsic adjuvant to activate the toll‐like receptor 5 (TLR5) to enhance vaccine potency. To enable the use of FliC as a self‐adjuvanting carrier, an effective method for site‐selective modification (SSM) of pertinent amino‐acid residues in the D2 and D3 domains of FliC is explored without excessive modification of the D0 and D1 domains, which are responsible for activating and binding with TLR5. In highly concentrated Na2SO4 solution, FliC monomers form flagellar filaments, in which the D0 and D1 domains are situated inside the tubular structure. Thus, the lysine residues (K219, K224, K324, and K331) in the D2 and D3 domains of flagellin are selectively modified by a diazo‐transfer reaction with imidazole‐1‐sulfonyl azide. The sites with azido modification are confirmed by MALDI‐TOF‐MS, ESI‐TOF‐MS, and LC–MS/MS analyses along with label‐free quantitation. The azido‐modified filament dissolves to give FliC monomers, which can conjugate with alkyne‐hinged saccharides by the click reaction. Transmission electron microscopy imaging, dynamic light scattering measurements, and the secreted embryonic alkaline phosphatase reporter assay indicate that the modified FliC monomers retain the ability either to bind with TLR5 or to reassemble into filaments. Overall, this study establishes a feasible method for the SSM of FliC by steric self‐protection of the D0 and D1 domains. Hidden gift: The D0 and D1 domains for activation of the toll‐like receptor 5 are effectively protected from chemical modification by steric hindrance in the assembled flagellar filament. Herein, we demonstrate a feasible site‐selective modification method that will benefit the immunogenic flagellin as a self‐adjuvanting carrier.