1.C.36.3.1
IIITCP protein complex, IpaB/IpaC/IpaD. Physical contact with host cells initiates secretion and leads to assembly of a pore, IpaB/IpaC, in the host cell membrane. The active needle tip complex of S. flexneri is composed of a tip protein, IpaD, and the two pore-forming proteins, IpaB and IpaC. The atomic structures of IpaD and a protease-stable coiled-coil fragment in the N-terminal regions of IpaB from S. flexneri and the homologous SipB from Salmonella enterica have been determined (Barta et al. 2012).  Structural comparisons revealed similarity to the coiled-coil regions of pore-forming proteins such as colicin Ia (TC# 1.C.1.1.1).  Interaction between IpaB and IpaD at the needle tip is key to host cell sensing, orchestration of IpaC secretion and its subsequent assembly at needle tips (Veenendaal et al. 2007). The N-terminus of IpaC is extracellular and the C-terminus is intracellular, and its topology has been studied (Russo et al. 2019). Residures lining the pore channel of the plasma membrane-embedded Shigella flexneri type 3 secretion translocase, IpaB, have been identified (Chen et al. 2021).