1.C.36 The Bacterial Type III-Target Cell Pore (IIITCP) Family

The Type III secretory pathway (IIISP, TC #3.A.6) allows bacteria to pass proteins directly from the bacterial cytoplasm into the host animal or plant cell cytoplasm. In order for this to occur, the bacteria construct a complex secretory apparatus allowing a direct pathway for the passage of proteins. One bacterial protein of each type IIISP system is secreted directly into the host cell membrane where it oligomerizes to form a transmembrane translocation pore at the site of bacterial contact. These homologous proteins comprise the IIITCP family. They are very divergent in sequence but have a uniform topology with two conserved transmembrane spanners (TMSs) and one conserved coiled coil region. The best characterized members of the IIITCP family include EspD for enteropathogenic E. coli, YopB for Yersinia species, PopB and PepB for Pseudomonas species, IpaB for Shigella species and SipB for Salmonella species. The three clusters of proteins indicated in the table (1, EspD; 2, YopB, PopB and PepB; and 3, IpaA and SipB) are very divergent in sequence so that PSI-BLAST reveals only limited sequence similarity between clusters. Proteins of the third cluster, including IpaB and SipB, are also much larger than the proteins in clusters 1 and 2 due in part to a 150 amino acyl residue N-terminal extension.

YopB and YopD of Yersinia species form a complex in the bacterial cytoplasm with their chaperone, SycD, suggesting that they are secreted as a heterocomplex. They may act together in forming the pore structure in the host cell membrane. YopB contains two central hydrophobic domains and is predicted to be an integral membrane protein, while YopD has a single central hydrophobic domain. In Bordetella bronchiseptica, BopB and BopD are also complexed to form a pore in the host cell plasma membrane (Nogawa et al., 2004). It seems likely that these two proteins are required for pore formation in all of these homologous (but very distantly related) systems.

YopB and YopD of Yersinia enterocolitica have been reported to both be required for pore formation in macrophage plasma membranes. The estimated inner pore diameter is about 2.0 nm. There is disagreement as to the requirement for YopD; some suggest that YopB alone forms the pore while others suggest that the YopB-YopD complex comprise the structural components of the pore. Studies in artificial membranes suggest that YopB alone forms channels with no ionic selectivity and that YopD alters the channel (Tardy et al., 1999). Thus, YopB is probably the channel while YopD is an accessory protein. The P. aeruginosa homologues, PopB/PopD with the PcrV protein form channels (2.8-3.5 nm inner diameter) both in red blood cells and in macrophage. In the former, they induce haemolysis. By electron microscopy PopB and PopD form 80 Å wide rings that encircle a ~40 Å cavity (Schoehn et al., 2003).

The TTSS has three main structural parts: a base, a needle, and a translocon, which work together to ensure the polarized movement of Yops directly from the bacterial cytosol into the host cell cytosol. The tip of the TTSS needle interacts with the translocon. Mutations in the needle protein, YscF (87 aas; TC# 3.A.6.1.1) separate its function in secretion from its role in translocation (Davis and Mecsas, 2006). Two yscF mutants formed needles and secreted Yops normally displayed distinct translocation defects. One showed diminished pore formation, suggesting incomplete pore insertion and/or assembly. The second formed pores but showed nonpolar translocation, suggesting unstable needle-translocon interactions.

The Salmonella SipB protein has been studied topologically in model bilayers (McGhie et al., 2002). The protein can exist in soluble and membrane-inserted forms, a property typical of bacterial exotoxins. SipB inserts into the host cell membrane and has heterotypic membrane fusion activity in vitro. The membrane integrated protein (593 aas) is predominantly α-helical as is true for the soluble form. Two hydrophobic α-helices (residues 320-353 and 409-427) integrate into the membrane obliquely, allowing residues 354-408 to cross the bilayer while a C-terminal region associates with the bilayer surface. The homologous IpAB of Shigella assembles into a homotrimer via an N-terminal coiled-coil domain (Hume et al., 2003). SipB and IpaB integrate with the same membrane topology.

The proteins in this family appear to be related to the putative pore-forming bacterial proteins in TC sub-family 1.C.127.2 which appear to be distantly related to pore-forming lipoproteins of animals in TC sub-family 1.C.127.1 However, the relationships between these subfamilies and 1.C.36 is not fully established. 

The generalized transport reaction that is facilitated by IIITCP family members is:

proteins (bacterial cytoplasm) → proteins (host cell cytoplasm).