This family consists of several invasion plasmid antigen IpaD proteins found in Shigella [1] as well as homologues such as SipD from Salmonella and BipD from Burkholderia [2]. Gram-negative bacteria use type III secretion systems (T3SSs) as protein transport devices for injecting virulence effector proteins into eukaryotic cells during infection. T3SSs consist of a needle complex (NC) and a tip complex (TC). In Shigella the TC is composed of two proteins, each essential to host cell sensing: IpaD and IpaB. IpaD is hydrophilic and required for tip recruitment of the hydrophobic proteins IpaB and IpaC, which later form the pore in host cell membranes [3].

1: Characterization of the interaction of IpaB and IpaD, proteins required for entry of Shigella flexneri into epithelial cells, with a lipid membrane. De Geyter C, Wattiez R, Sansonetti P, Falmagne P, Ruysschaert JM, Parsot C, Cabiaux V; Eur J Biochem 2000;267:5769-5776. PMID:10971588

2: Identification of the bile salt binding site on IpaD from Shigella flexneri and the influence of ligand binding on IpaD structure. Barta ML, Guragain M, Adam P, Dickenson NE, Patil M, Geisbrecht BV, Picking WL, Picking WD; Proteins. 2012;80:935-945. PMID:22423359

3: Three-dimensional electron microscopy reconstruction and cysteine-mediated crosslinking provide a model of the type III secretion system needle tip complex. Cheung M, Shen DK, Makino F, Kato T, Roehrich AD, Martinez-Argudo I, Walker ML, Murillo I, Liu X, Pain M, Brown J, Frazer G, Mantell J, Mina P, Todd T, Sessions RB, Namba K, Blocker AJ; Mol Microbiol. 2015;95:31-50. PMID:25353930