The type III secretion system needle tip complex mediates host cell sensing and translocon insertion

Abstract

SummaryType III secretion systems (T3SSs) are essential virulence determinants of many Gram-negative bacterial pathogens. The Shigella T3SS consists of a cytoplasmic bulb, a transmembrane region and a hollow 'needle' protruding from the bacterial surface. Physical contact with host cells initiates secretion and leads to assembly of a pore, formed by IpaB and IpaC, in the host cell membrane, through which proteins that facilitate host cell invasion are translocated. As the needle is implicated in host cell sensing and secretion regulation, its tip should contain components that initiate host cell contact. Through biochemical and immunological studies of wild-type and mutant Shigella T3SS needles, we reveal tip complexes of differing compositions and functional states, which appear to represent the molecular events surrounding host cell sensing and pore formation. Our studies indicate that the interaction between IpaB and IpaD at needle tips is key to host cell sensing, orchestration of IpaC secretion and its subsequent assembly at needle tips. This allows insertion into the host cell membrane of a translocation pore that is continuous with the needle.

N-terminal residues of SipB are required for its surface localization on Salmonella enterica serovar Typhimurium

“…It has been also reported that Shigella IpaB, a homologue of Salmonella SipB, may be associated with the bacterial surface (Menard et al, 1994;Mills et al, 1988;Watarai et al, 1995;Veenendaal et al, 2007). Therefore, we first sought to confirm that SipB protein was present on the bacterial outer membrane.…”

“…It was recently reported that IpaB and IpaD, the translocon proteins of Shigella, can be localized on the bacterial cell surface (Olive et al, 2007;Veenendaal et al, 2007). However, the surface localization of PopB and PopD of Pseudomonas, BopB and BopD of Bordetella, and BipB and BipD of Burkholderia have not yet been characterized.…”

“…It was recently suggested that the cell-surface localization of Shigella IpaB, which is homologous to SipB, could be dependent on the presence of IpaD, and furthermore that an interaction between IpaB and IpaD is required for insertion of the translocation pore into the host-cell membrane (Johnson et al, 2006;Veenendaal et al, 2007). In Salmonella, sipD mutants have been shown to be deficient for invasion of epithelial cells (Kaniga et al, 1995;Collazo & Galan, 1997), even though these sipD mutants have increased secretion of Sip proteins (Kaniga et al, 1995).…”

“…Recent reports have shown that localization of Shigella IpaB to the bacterial cell surface is dependent on the presence of IpaD, a homologue of Salmonella SipD, and have further shown that interaction of IpaB and IpaD at the needle tip is necessary for their insertion into the hostcell membrane (Johnson et al, 2006;Veenendaal et al, 2007). Since no earlier study has examined whether outermembrane localization of SipB is dependent on SipD, we prepared culture supernatants, cell lysates and outermembrane fractions from a sipD deletion strain, and examined SipB localization by Western blotting.…”

“…Recently, Johnson et al (2006) have shown that surface localization of IpaB is dependent on the presence of IpaD. Veenendaal et al (2007) have also suggested that the interaction between IpaB and IpaD at the needle tip is a key to host-cell sensing, and allows insertion of the translocation pore into the host-cell membrane. Interestingly, Hayward et al (2000) have shown that Salmonella SipB can also be localized to the cell surface of bacteria during invasion of mammalian target cells, and furthermore that the protein fractionated with outer-membrane proteins.…”