High Resolution Structure of BipD: An Invasion Protein Associated with the Type III Secretion System of Burkholderia Pseudomallei

Burkoldheria pseudomallei is a Gram-negative bacterium that possesses a protein secretion system similar to those found in Salmonella and Shigella. Recent work has indicated that the protein encoded by the BipD gene of B. pseudomallei is an important secreted virulence factor. BipD is similar in sequence to IpaD from Shigella and SipD from Salmonella and is therefore likely to be a translocator protein in the type-III secretion system of B. pseudomallei. The crystal structure of BipD has been solved at a resolution of 2.1 Å revealing the detailed tertiary fold of the molecule. The overall structure is appreciably extended and consists of a bundle of antiparallel α-helical segments with two small β-sheet regions. The longest helices of the molecule form a four-helix bundle and most of the remaining secondary structure elements (three helices and two three-stranded β-sheets) are formed by the region linking the last two helices of the four-helix bundle. The structure suggests that the biologically active form of the molecule may be a dimer formed by contacts involving the C-terminal α-helix, which is the most strongly conserved part of the protein. Comparison of the structure of BipD with immunological and other data for IpaD indicates that the C-terminal α-helix is also involved in contacts with other proteins that form the translocon.