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Category: Bacterial Pathogenesis
Protein Secretion and Chlamydia Pathogenesis, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555817329/9781555816742_Chap09-1.gif /docserver/preview/fulltext/10.1128/9781555817329/9781555816742_Chap09-2.gifAbstract:
While significant progress has been made in unraveling the contributions of protein secretion to chlamydial pathogenesis, many pivotal open questions remain. This chapter endeavors to explore current knowledge regarding chlamydial protein secretion as well as point outs substantial gaps in understanding. The type III secretion (T3S) pathway is emphasized, given that this area has received considerable attention and fruitful research has provided significant insight regarding protein secretion mechanism. However, it is important to emphasize that T3S does not represent the entire arsenal for chlamydial protein secretion and other equally important secretion mechanisms contribute to chlamydial survival. Protein secretion and translocation are accomplished by an elegantly complex secretory apparatus collectively referred to as the "injectisome". In the working model, the assembly of the core apparatus begins with membrane insertion of the outer membrane (OM) secretin CdsC followed by addition of inner membrane (IM) components CdsD and CdsJ. Secretion chaperones also play a prominent role in facilitating and potentially regulating T3S. Even without genetics, protein secretion affords opportunities to push the envelope of Chlamydia research. A recent genome-wide expression study of chlamydial proteins in Saccharomyces cerevisiae revealed preliminary evidence for antihost function. The study included C. trachomatis gene products annotated as hypothetical with respect to function. The results must be taken with a grain of salt since it is likely that many of the tested proteins are not actually secreted proteins.
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Secretion pathways employed for proteins secretion in Chlamydia spp. (A) The T2S system mediates export of proteins (T2E) across the chlamydial IM via the Sec protein machinery. Once in the periplasm (PP), signal peptidases can cleave secretion signals (S), and the protein is subsequently secreted across the chlamydial OM through the GspD secretin. (B) T5S substrates (T5E) also gain access to the periplasm via the IM Sec machinery, but domains within the secreted protein mediate direct insertion into the OM. Subsequent cleavage of the passenger domain would release T5S substrates into the inclusion lumen. (C) Translocation of type II and type V secreted proteins could be achieved via formation of OMVs that subsequently fuse with host plasma membrane (PM) or inclusion membranes (InM) to release effector proteins into the host cytosol. (D) The T3S system mediates single-step secretion and translocation of effector proteins directly into the cytosol of an infected cell. doi:10.1128/9781555817329.ch9.f1
Secretion pathways employed for proteins secretion in Chlamydia spp. (A) The T2S system mediates export of proteins (T2E) across the chlamydial IM via the Sec protein machinery. Once in the periplasm (PP), signal peptidases can cleave secretion signals (S), and the protein is subsequently secreted across the chlamydial OM through the GspD secretin. (B) T5S substrates (T5E) also gain access to the periplasm via the IM Sec machinery, but domains within the secreted protein mediate direct insertion into the OM. Subsequent cleavage of the passenger domain would release T5S substrates into the inclusion lumen. (C) Translocation of type II and type V secreted proteins could be achieved via formation of OMVs that subsequently fuse with host plasma membrane (PM) or inclusion membranes (InM) to release effector proteins into the host cytosol. (D) The T3S system mediates single-step secretion and translocation of effector proteins directly into the cytosol of an infected cell. doi:10.1128/9781555817329.ch9.f1
A working model for the assembly and composition of the chlamydial T3SS. Stepwise addition of proteins is indicated with newly added components shown in dark grey and previously assembled components in light grey (shown with Cds or Cop letter designation only). Schematic representations of flagellar proteins are omitted for clarity but include FlhA/CT060 (CdsV paralog), FliI/CT717 (CdsN paralog), FliH/CT718 (CdsL paralog), and FliF/CT719 (CdsJ paralog). The completed injectisome spans the bacterial IM, periplasm (PP), and OM and includes TC, NC, and basal apparatus complexes. Secreted translocon (Tr) components are shown localized to the host plasma membrane (PM) or inclusion membrane (InM). Correct stoichiometry of multimeric proteins is not indicated. This figure was adapted from previously published images ( Betts-Hampikian and Fields, 2010 ). doi:10.1128/9781555817329.ch9.f2
A working model for the assembly and composition of the chlamydial T3SS. Stepwise addition of proteins is indicated with newly added components shown in dark grey and previously assembled components in light grey (shown with Cds or Cop letter designation only). Schematic representations of flagellar proteins are omitted for clarity but include FlhA/CT060 (CdsV paralog), FliI/CT717 (CdsN paralog), FliH/CT718 (CdsL paralog), and FliF/CT719 (CdsJ paralog). The completed injectisome spans the bacterial IM, periplasm (PP), and OM and includes TC, NC, and basal apparatus complexes. Secreted translocon (Tr) components are shown localized to the host plasma membrane (PM) or inclusion membrane (InM). Correct stoichiometry of multimeric proteins is not indicated. This figure was adapted from previously published images ( Betts-Hampikian and Fields, 2010 ). doi:10.1128/9781555817329.ch9.f2