The Hypervariable D3 Domain of Salmonella Flagellin Is an Autonomous Folding Unit

The hypervariable D3 domain of Salmonella flagellin, composed of the 190-285 segment, is the major determinant of flagellar antigenicity. D3 was cloned and overexpressed in E. coli. Although previous studies concluded that D3 is stabilized by interactions with the D2 domain, our calorimetric experiments have revealed that isolated D3 has a stable tertiary structure which is highly resistant against proteolytic digestion. Repeated heating experiments demonstrated that unfolding of D3 is reversible. Its small size and stable structure makes D3 a promising protein scaffold for the development of artificial binding proteins by directed evolution.