THE ANTIGENIC DOMAIN OF FLAGELLIN FROM S-PARATYPHI SHARES A STRUCTURAL FOLD WITH SUBTILISIN

Bacterial flagellin has two domains: the polymerizing domain consistin g of N- and C-terminal regions which are partly disordered in the mono meric state; and the central antigenic domain with compact globular st ructure. The polymerizing domain is highly conserved in flagellins fro m different species but the antigenic domain is diverse in sequence an d size. Whereas the former has direct functional significance for bact erial motility, the latter has not been identified as having a specifi c function except for defining the distinct serotype of the bacterium. The sequence alignment of flagellin from S. paratyphi with proteins o f known three-dimensional structure reveals significant homology of th e central 265 residue stretch with the bacterial serine protease, subt ilisin. This homology is evident also in the comparison of the predict ed secondary structure of flagellin with the observed secondary struct ural features in subtilisin. The deletions/insertions arising due to o ptimal alignment of the two proteins occur on the surface loops in the structure. Thus, a domain of S. paratyphi flagellin and subtilisin ap pear to have similar structural folds.