The antigenic domain of flagellin from S. paratyphi shares a structural fold with subtilisin.

Bacterial flagellin has two domains: the polymerizing domain consisting of N- and C-terminal regions which are partly disordered in the monomeric state; and the central antigenic domain with compact globular structure. The polymerizing domain is highly conserved in flagellins from different species but the antigenic domain is diverse in sequence and size. Whereas the former has direct functional significance for bacterial motility, the latter has not been identified as having a specific function except for defining the distinct serotype of the bacterium. The sequence alignment of flagellin from S. paratyphi with proteins of known three-dimensional structure reveals significant homology of the central 265 residue stretch with the bacterial serine protease, subtilisin. This homology is evident also in the comparison of the predicted secondary structure of flagellin with the observed secondary structural features in subtilisin. The deletions/insertions arising due to optimal alignment of the two proteins occur on the surface loops in the structure. Thus, a domain of S. paratyphi flagellin and subtilisin appear to have similar structural folds.