Self-chaperoning of the Type III Secretion System Needle Tip Proteins IpaD and BipD

Abstract

Bacteria expressing type III secretion systems (T3SS) have been responsible for the deaths of millions worldwide, acting as key virulence elements in diseases ranging from plague to typhoid fever. The T3SS is composed of a basal body, which traverses both bacterial membranes, and an external needle through which effector proteins are secreted. We report multiple crystal structures of two proteins that sit at the tip of the needle and are essential for virulence: IpaD from Shigella flexneri and BipD from Burkholderia pseudomallei. The structures reveal that the N-terminal domains of the molecules are intramolecular chaperones that prevent premature oligomerization, as well as sharing structural homology with proteins involved in eukaryotic actin rearrangement. Crystal packing has allowed us to construct a model for the tip complex that is supported by mutations designed using the structure.

What's the point of the type III secretion system needle?

“…[35][36][37][38]. Additionally, the Shigella adaptor protein (IpaD) was recently found to be required for association of the most highly conserved translocon component within the tip complex (11,13,39). Thus, the adaptor appears to mediate the interaction between the needle and the translocon and may also be involved in sensing the host cell (discussed below).…”

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

“…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.…”

“…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).…”

“…The IpaB and IpaC proteins in Shigella flexneri, BipB and BipD in Burkholderia pseudomallei, YopB and YopD in Yersinia enterocolitica, EspB and EspD in enteropathogenic Escherichia coli (EPEC), BopB and BopD in Bordetella bronchiseptica, and PopB and PopD in Pseudomonas aeruginosa are thought to be homologous components of the translocation apparatus formed by SipB and SipC in Salmonella. All of these secreted proteins have been shown to be delivered to the host-cell membrane, where they form pore complexes (Daniell et al, 2001;Davis et al, 1998;Hayward et al, 2000;Ide et al, 2001;Kuwae et al, 2003;Neyt & Cornelis, 1999;Nogawa et al, 2004;Scherer et al, 2000;Suparak et al, 2005;Faudry et al, 2006;Johnson et al 2006). In addition to its role as a translocon, SipB can also reportedly be phagocytized by macrophages and subsequently translocated via SPI-1 T3SS into the cytoplasm of the macrophage, where it induces apoptosis of the host cell by associating with the proapoptotic protease caspase-1 (Hersh et al, 1999).…”

“…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.…”