The Many Faces of IpaB

Abstract

The type III secretion system (T3SS) is Shigella's most important virulence factor. The T3SS apparatus (T3SA) is comprised of an envelope-spanning basal body and an external needle topped by a tip complex protein called IpaD. This nanomachine is used to deliver effector proteins into host cells to promote pathogen entry. A key component of the matured T3SS needle tip complex is the translocator protein IpaB. IpaB can exist in multiple states when prepared as a recombinant protein, however, it has also been described as having additional roles in Shigella pathogenesis. This mini-review will briefly describe some of the features of IpaB as a T3SS needle tip protein, as a pore-forming translocator protein and as an effector protein. Reflection on the potential importance of the different in vitro states of IpaB on its function and importance in serotype-independent vaccines is also provided.

Keywords: IpaB; Shigella; invasion plasmid antigen; pathogenesis; type III secretion.

Figure 1

A schematic of the type III secretion apparatus (T3SA) is shown. For Shigella, the cytoplasmic sorting platform contains multiple copies of Spa33, MxiN, and possibly MxiK. The platform is proposed to house the Spa47 ATPase and the Spa13 stalk that links it to the major export protein MxiA. Within the inner membrane are MxiJ and MxiG, the latter of which may promote interaction with the proteins of the sorting platform (e.g., Spa33). Located within the outer membrane and spanning the periplasm, respectively, are the OM ring protein MxiD and the inner rod protein MxiI. External to the outer membrane and spanning the O antigen layer of the LPS is the needle which is composed of many copies of MxiH which are assembled in a helical manner to provide a conduit from the basal body to the extracellular milieu. Capping the needle and controlling type III secretion in response to extracellular signals is the tip complex (TC). It is proposed that the TC of newly formed needles is comprised of five copies of IpaD, however, a TC containing four copies of IpaD and one of IpaB has been suggested. Following maturation (as caused by exposure to bile salts, for example), IpaB is recruited to the T3SA needle tip and it is from this position that it contributes to control of type III secretion by sensing contact with a host cell either through the recognition of receptor proteins or insertion into the host cell membrane. In the absence of IpaC, IpaB is still capable of inserting into target cell membranes to form what may be described as the pre-translocon pore. In the presence of IpaC, IpaB penetration of the host membrane promotes full secretion induction and formation of the translocon pore through which effector proteins pass.

Figure 2

While a high-resolution structure of full-length IpaB (580 amino acids) is not yet available, an understanding of its functional organization is taking shape. Two major chaperone-binding domains (CBD's) have been located between residues 11 and 76 near the IpaB N terminus. Additional regions that may be involved in mediating IpaB interactions with itself and possibly other proteins are located near the C terminus. The one region for which there is a high resolution structure lies between residues 120 and 224 which forms a highly elongated coiled-coil. A remarkably similar structure spans residues 80–226 in SipB, the Salmonella homolog of IpaB, and similar structures are predicted within the homologs EspD (Enteropathogenic E. coli) and YopB (Yersinia spp.). This coiled-coil may have a role in anchoring IpaB to IpaD at the T3SA needle tip which would allow IpaB to position its hydrophobic domain for contact with a host cell, not unlike what occurs for membrane-interacting colicins. A large hydrophobic domain lies mostly within the C-terminal half of IpaB and it contains a predicted α-helical hairpin similar to what has been observed in some membrane-interacting colicins. The region involved in IpaB's ability to associate with and activate caspase 1 is reported to also lie within this region (between residue 316 and 401).