This is the middle domain of the flagellar rotor protein FliG [1-2].

1: Crystal structure of the middle and C-terminal domains of the flagellar rotor protein FliG. Brown PN, Hill CP, Blair DF; EMBO J. 2002;21:3225-3234. PMID:12093724

2: Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching. Lee LK, Ginsburg MA, Crovace C, Donohoe M, Stock D; Nature. 2010;466:996-1000. PMID:20676082