Fig 5. The proposed reaction mechanism for FliB.

(A) The mechanism assumes two roles of SAM binding to [4Fe-4S] in FliB: One SAM is converted into an ACP radical and facilitates the methyl transfer from the other SAM molecule to the flagellin substrate. (B) The postulated process illustrates that the flagellin subunit bound by its chaperon FliS is delivered to the FliB oligomer near the cytoplasmic membrane. The Salmonella FliB complex methylates accessible lysine residues in flagellin domains D2 and D3 before protomer secretion and flagellar assembly.