Buchrieser, 2000 (The virulence plasmid pWR100 and the repertoire of proteins secreted by the type III secretion apparatus of Shigella flexneri) [Article]

Molecular Microbiology

What are the key virulence traits of Shigella bacteria?

"The virulence traits of these pathogens include their ability to enter into epithelial cells and induce apoptosis in macrophages. Expression of these functions requires the Mxi±Spa type III secretion apparatus and the secreted IpaA±D proteins, all of which are encoded by a virulence plasmid."

How is the type III secretion system regulated in wild-type Shigella?

"The S. flexneri type III secretion apparatus is weakly active during growth of bacteria in laboratory media, and its activity is induced upon contact of bacteria with epithelial cells."

What is the role of IpaB and IpaC in the type III secretion system?

"There is evidence that IpaB and IpaC insert within the cytoplasmic membrane of target cells and might form a channel allowing translocation of other proteins, such as IpaA and IpgD."

What is known about the stability requirements for IpaB, IpaC and IpgD?

"The ipgC gene encoding a cytoplasmic chaperone required for stability of IpaB and IpaC and the ipgE gene encoding a cytoplasmic chaperone required for stability of IpgD."

How many proteins are estimated to be secreted by the Mxi-Spa secretion apparatus?

"These and previous results indicate that 19 proteins are secreted by the Mxi±Spa secretion apparatus: Spa32, MxiC and MxiL, which might be involved in the assembly or regulation of the secretion apparatus, and IpaA±D, IpgB1, IpgD, OspB±G, VirA and three IpaH proteins."

What is the total size of the Shigella type III secretion system in terms of protein components?

"Including components of the secretion apparatus, secreted proteins, chaperones and regulators, the type III secretion system of S. flexneri comprises about 50 proteins."

What factors can activate the Mxi-Spa secretion apparatus besides cell contact?

"The Mxi±Spa secretion apparatus can also be activated by such external inducers as the dye Congo red, extracellular matrix components or bile salts."

How is constitutive secretion achieved in laboratory strains?

"Constitutive secretion, i.e. secretion in the absence of inducers, is observed after inactivation of the ipaB or ipaD genes."

What is the role of VirF in virulence regulation?

"Expression of both icsA and virB is controlled by VirF, a transcriptional activator of the AraC family that is encoded by the plasmid."

What is the size and importance of the entry region on the virulence plasmid?

"Genetic analysis has shown that a 31 kb fragment of the virulence plasmid is necessary and sufficient for entry of bacteria into epithelial cells."

What is the relationship between IpgB1 and IpgB2?

“IpgB1 exhibits 20% sequence identity with (i) TrcA (Tobe et al., 1999a) and LEE19 (Elliott et al., 1998), which are encoded by the chromosomal LIM and LEE loci, respectively, of enteropathogenic E. coli (EPEC) strains; (ii) TrcP, which is encoded by the adherence factor plasmid of EPEC (Tobe et al., 1999b); and (iii) IpgB2, which is also encoded by pWR100. The most closely related proteins are IpgB2 and TrcA, which exhibit 37% sequence identity. TrcA has been proposed to be a cytoplasmic chaperone required for the production of BfpA and intimin (Tobe et al., 1999b).”

What evidence suggests the entry region and osp genes have the same origin?

"Genes of the entry region and osp genes have a similar G1C content (average 34%), which is different from those of the transfer and replication regions (55% G1C), the icsA and virK regions (41% G1C) or sepA (49% G1C). This suggests that the entry region and osp genes have the same origin and were probably acquired simultaneously, even though they are not linked on the plasmid."

What's notable about chaperone requirements for secreted proteins?

"No genes encoding potential chaperones were detected in the vicinity of osp and ipaH genes, which suggests that the requirement for specific cytoplasmic chaperones concerns only a subset of proteins that are secreted by the type III secretion pathway."

What is the relationship between IcsA and VirK?

"The icsA and virK regions": "The function of VirK is not yet known; however, a functional relationship between VirK and IcsA is suggested by the phenotype of the virK mutant."

How is IcsA involved in bacterial motility and what regulates its localization?

The ability of bacteria to move within the cytoplasm of infected cells relies on the expression and proper localization of the outer membrane protein IcsA (VirG)" and "IcsP (sopA) gene encodes an outer membrane protease that is involved in cleavage of IcsA."

What is known about the regulation of the VirB protein?

"VirB is required for transcription of genes of the entry region by a mechanism that has not yet been elucidated" and "No gene encoding a protein homologous to ParA is present in the vicinity of virB or elsewhere on the virulence plasmid [...] which suggests that VirB is not involved in plasmid partitioning."

What are the roles of IpaB and IpaC in cellular signaling?

"Parts of IpaB and IpaC might also be directly involved in altering cellular signalling processes to result in internalization of bacteria and triggering of apoptosis."

How is the type III secretion system activated upon cell contact?

"The type III secretion apparatus is weakly active during growth of bacteria in laboratory media, and its activity is induced upon contact of bacteria with epithelial cells [...] IpaB and IpaC insert within the cytoplasmic membrane of target cells and might form a channel allowing translocation of other proteins, such as IpaA and IpgD."

What are the specific roles of IpgC as a chaperone?

"The ipgC gene encoding a cytoplasmic chaperone required for stability of IpaB and IpaC [...] ipgE gene encoding a cytoplasmic chaperone required for stability of IpgD."

What is the relationship between OspD proteins?

"The osp genes" section: "OspD2 (569 residues) and OspD3 (SenA; 565 residues) exhibit 38% sequence identity over their entire length, and the C-terminal region of both proteins contains six repeats of 44 residues [...] A similar repeat is present three times in the C-terminal region of OspD1 (225 residues)."

What is unique about OspF compared to other Osp proteins?

"No proteins sharing sequence similarity with Osp proteins were detected in protein sequence databases, except for OspF, which exhibits 63% sequence identity with SpvC, a protein encoded by the virulence plasmid of Salmonella typhimurium."