Type III secretion system, SctE/BipB (T3SS_SctE/BipB)
Type: Family

Secretion of virulence factors in Gram-negative bacteria involves transportation of the protein across two membranes to reach the cell exterior . There have been four secretion systems described in animal enteropathogens, such as Salmonella and Yersinia, with further sequence similarities in plant pathogens like Ralstonia and Erwinia.

The type III secretion system is of great interest, as it is used to transport virulence factors from the pathogen directly into the host cell and is only triggered when the bacterium comes into close contact with the host. The protein subunits of the system are very similar to those of bacterial flagellar biosynthesis. However, while the latter forms a ring structure to allow secretion of flagellin and is an integral part of the flagellum itself , type III subunits in the outer membrane translocate secreted proteins through a channel-like structure.

Exotoxins secreted by the type III system do not possess a secretion signal, and are considered unique for this reason . Certain proteobactria, including Salmonella and Shigella spp., secrete invasin protein B, which is required for internalisation of the bacterium within the host cell . Induction of apoptosis is then carried out, by the binding of IL-1 converting enzyme to the exotoxin.

This entry represents invasin protein B from various proteobacteria, including:

SipB (also known as SPI-1 type 3 secretion system translocon protein SctE) from Salmonella sp., which is required for entry into the host cell through presentation or delivery of SipC at the host cell plasma membrane. Along with SipC, SipB is necessary for the transfer of other effector proteins into the host cell. It induces macrophage apoptosis either by binding and activating the proapoptotic enzyme caspase-1 (caspase-1 dependent), resulting in the release of interleukin-1 beta active form, or by disrupting mitochondria and inducing autophagy (caspase-1 independent). The former is dependent of its membrane-fusion activity. The sipBC complex, in association with its chaperone sicA, is regulated by binding of invE .

IpaB (also known as SctE) from Shigella sp., which forms a pore with IpaC and is inserted into the host cell membrane through the Mxi/Spa apparatus during cell contact . This pore probably allows the translocation of IpaA. IpaB has also been found to be necessary and sufficient to activate macrophage apoptosis by binding to interleukin-1 beta converting enzyme (ICE). It has also been shown to be important, along with IpaD, to block or regulate secretion through the Mxi/Spa translocon in the presence or absence of the secretion signal, respectively . IpaB is secreted through the specialised type-III secretion system Mxi/Spa and is inserted into the host cell membrane. It is also secreted into the host cell cytoplasm after the escape of bacteria from phagosome, where it colocalizes with ICE. Interaction of IpaB/SctE with host membrane lipids promotes recruitment of IpaC/SctB at the needle tip concomitant with translocon insertion into the host membrane and type III secretion induction.

BipB from Burkholderia sp., which plays a role in the bacterium-induced formation of multinucleated giant cell (MNGC), which is formed after host cell fusion, as well as in the intercellular spreading of bacteria and in the induction of apoptosis in macrophages. BipB may act in concert with other effector proteins to induce fusion of host cell membranes .