This domain is found in the central portion bacterial flagellin FliC. The domain contains a structural motif called a beta-folium fold [1]. Although no specific function is assigned to this domain its deletion leads to a reduction in filament stability [2].

1: Structure of the bacterial flagellar protofilament and implications for a switch for supercoiling. Samatey FA, Imada K, Nagashima S, Vonderviszt F, Kumasaka T, Yamamoto M, Namba K; Nature. 2001;410:331-337. PMID:11268201

2: Flagellar filament structure and cell motility of Salmonella typhimurium mutants lacking part of the outer domain of flagellin. Yoshioka K, Aizawa S, Yamaguchi S; J Bacteriol. 1995;177:1090-1093. PMID:7860589