A fresh look at the type III secretion system: two-step model of effector translocation in pathogenic bacteria

General Commentary published: 18 May 2011 doi: 10.3389/fmicb.2011.00113 A fresh look at the type III secretion system: two-step model of effector translocation in pathogenic bacteria Ana Victoria C. Pilar and Brian K. Coombes* Department of Biochemistry and Biomedical Sciences, Michael G. De Groote Institute for Infectious Disease Research, McMaster University, Hamilton, ON, Canada *Correspondence: A commentary on Translocation of surface-localized effectors in type III secretion by Akopyan, K., Edgren, T., Wang-Edgren, H., Rosqvist, R., Fahlgren, A., Wolf-Watz, H., and Fallman, M. (2011). Proc. Natl. Acad. Sci. U.S.A. 108, 1639–1644. Acquisition of genetic elements such as virulence plasmids or pathogenicity islands (PI) by horizontal gene transfer can endow pathogenic bacteria with an arsenal of virulence factors that promote bacterial survival and replication within their hosts. Despite the differences in the host organisms and pathology caused by important pathogenic bacteria such as Escherichia coli, Yersinia, Salmonella, and Shigella, a common virulence mechanism exists in the form of a needle-like structure that translocates bacterial proteins into host cells to hijack the host machinery and modulate the host immune response (Ghosh, 2004). Enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) belong to a family of attaching and effacing (A/E) pathogens responsible for diarrheal diseases in humans and animals. The diseases are characterized by the effacement of the intestinal microvilli, bacterial colonization, and attachment on pedestals induced by localized actin polymerization upon contact with enterocytes and disruption of tight junctions (Dean and Kenny, 2009; Croxen and Finlay, 2010). A type III secretion system (T3SS) encoded by the locus of enterocyte effacement (LEE) secretes proteins called effectors to form A/E lesions in the host and subvert various host processes such as disruption of the host cytoskeletal network and modulation of the host innate immune signaling (Sharma et al., 2006; Ruchaud-Sparagano et al., 2007; Khan et al., 2008). Yersinia employs the plasmidencoded Ysc-Yop T3SS to deliver effectors called Yops (Yersinia outer proteins) to the host cytosol to paralyze phagocytes www.frontiersin.org and block bacterial uptake (Cornelis et al., 1989; Rosqvist et al., 1990). This tactic of evading the host immune response ensures an environment conducive for the lifestyle of Yersinia. In contrast, Salmonella possesses two PI encoding distinct T3SS called SPI-1 and SPI-2. During invasion, the SPI-1 secretion apparatus deploys effectors to the host cell milieu to promote phagocytosis (Galán, 1999) while the SPI-2 T3SS activity creates a niche for replication and survival of Salmonella within target cells (Cirillo et al., 1998). The components of the T3SS are generally conserved among Gram-negative bacteria and even heterologous effectors can be secreted in another host bacteria such as the case of the Yersinia effector YopE expressed in Salmonella enterica serovar Typhimurium (Rosqvist et al., 1995). The Yersinia injectisome consists of a membrane-spanning basal body and a hollow conduit of YscF polymers through which effectors transit for secretion (Hoiczyk and Blobel, 2001). Dedicated T3SS chaperones bind to their cognate effectors and keep them in a locally unfolded, secretion-competent state (Ghosh, 2004). The chaperone-effector interaction is thought to provide specificity for effector docking on the secretion apparatus. The N-terminal domain or the 5′ end of most secreted effectors contains the signal sequence for secretion, translocation, and chaperone binding (Sory et al., 1995; Miao and Miller, 2000). However, there is no clear consensus sequence for the signals due to the degeneracy of the sequences at the amino acid or RNA level (Ghosh, 2004). How these signals are recognized by the secretion apparatus is not well understood but differential signal recognition by the chaperones or translocon components is thought to be the basis for the hierarchical secretion of effectors (Lara-Tejero et al., 2011; Osborne and Coombes, 2011). A defined order of secretion of effectors ensures that effector functions are activated in a spatial and temporal manner. A recent study revealed a cytoplasmic complex made up of SpaO/OrgA/ OrgB that functions as a platform for sorting chaperone-effector pairs prior to secretion (Lara-Tejero et al., 2011). Differential binding of the specific chaperones to the complex leads to the sequential loading of substrates. The translocators YopB and YopD are needed to complete the translocation of effectors across the host cell membrane and deletion of these translocators results in the extracellular secretion but not translocation of effectors into the host cytosol (Håkansson et al., 1996; Neyt and Cornelis, 1999). In Yersinia, the first ∼15 amino acids at the N-terminus is sufficient for secretion but not for translocation of the effectors leading to the conclusion that the presence of YopB/YopD and a distinct translocation signal are required for proper effector translocation (Sory et al., 1995). In the absence of translocators, secretion of effectors can be induced by growing bacteria in media that mimic environmental cues for T3SS activation such as low calcium, phosphate or magnesium (Michiels et al., 1990; Yu et al., 2010). Hence, although secretion and translocation are both necessary for infection, these events have different regulatory and structural requirements. YopB/D, as well as the related translocators in E. coli (EspB/D), Shigella (IpaB/ C/D), and Salmonella (SipB/C/D), contain hydrophobic domains and are proposed to form a pore by inserting into the host cell membrane (Ghosh, 2004). However, direct evidence for effectors being transported through this pore is lacking. In the onestep microinjection model, the effectors are injected directly by the T3SS into the host cytosol. However, one issue with this model lies in the structural and functional relationship of the translocators and the injectisome. It still remains to be elucidated whether the injectisome itself actually pierces the host cell membrane or the translocators act as the terminal connection of the injectisome to the target cells by creating a membrane pore (Hoiczyk and Blobel, 2001; Cornelis, 2002). May 2011 | Volume 2 | Article 113 | 1 Pilar and Coombes A recent paper by Akopyan et al. (2011) aimed to elucidate the translocation mechanism of the T3SS. They d emonstrated that Y. pseudotuberculosis effectors localized to the bacterial surface are translocated by the T3SS. They first observed that the effectors YopE and YopH and the translocator YopD were evenly distributed on the bacterial surface prior to host cell contact (Schesser et al., 1996; Akopyan et al., 2011). Interestingly, previous studies also found secreted Ipa effectors on the surface of Shigella before injection into the host cell (Watarai et al., 1995). (...truncated)