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The chlamydial periplasmic stress response serine protease cHtrA is secreted into host cell cytosol.

Wu X, Lei L, Gong S, Chen D, Flores R, Zhong G - BMC Microbiol. (2011)

Bottom Line: The periplasmic cHtrA protein appeared to be actively secreted into host cell cytosol since no other chlamydial periplasmic proteins were detected in the host cell cytoplasm.Most chlamydial species secreted cHtrA into host cell cytosol and the secretion was not inhibitable by a type III secretion inhibitor.Since it is hypothesized that chlamydial organisms possess a proteolysis strategy to manipulate host cell signaling pathways, secretion of the serine protease cHtrA into host cell cytosol suggests that the periplasmic cHtrA may also play an important role in chlamydial interactions with host cells.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Microbiology and Immunology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA.

ABSTRACT

Background: The periplasmic High Temperature Requirement protein A (HtrA) plays important roles in bacterial protein folding and stress responses. However, the role of chlamydial HtrA (cHtrA) in chlamydial pathogenesis is not clear.

Results: The cHtrA was detected both inside and outside the chlamydial inclusions. The detection was specific since both polyclonal and monoclonal anti-cHtrA antibodies revealed similar intracellular labeling patterns that were only removed by absorption with cHtrA but not control fusion proteins. In a Western blot assay, the anti-cHtrA antibodies detected the endogenous cHtrA in Chlamydia-infected cells without cross-reacting with any other chlamydial or host cell antigens. Fractionation of the infected cells revealed cHtrA in the host cell cytosol fraction. The periplasmic cHtrA protein appeared to be actively secreted into host cell cytosol since no other chlamydial periplasmic proteins were detected in the host cell cytoplasm. Most chlamydial species secreted cHtrA into host cell cytosol and the secretion was not inhibitable by a type III secretion inhibitor.

Conclusion: Since it is hypothesized that chlamydial organisms possess a proteolysis strategy to manipulate host cell signaling pathways, secretion of the serine protease cHtrA into host cell cytosol suggests that the periplasmic cHtrA may also play an important role in chlamydial interactions with host cells.

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A proposed model for C. trachomatis secretion of effectors into host cell cytosol. When an infectious and metabolically inactive elementary body (EB) attaches to an epithelial cell, preexisting effectors such as TARP and CT694 can be injected into host cell cytosol via a single step type 3 secretion system (T3SS) for facilitating EB invasion. Once the internalized EB is differentiated into a non-infectious but metabolically active reticulate body (RB), newly synthesized chlamydial proteins can be secreted into host cell cytosol via either the single step T3SS (for example, secretion of CT847) or multi-step pathways. The C. trachomatis-secreted proteins (CtSPs) with an N-terminal signal sequence (termed Sec-CtSPs) such as cHtrA & CPAF may be translocated into periplasm via a SecY-dependent pathway while those without any N-terminal signal sequences (Nonsec-CtSPs) may be translocated into the periplasmic space via a novel translocon or a leaking T3SS pathway. The periplasmically localized CtSPs may exit the chlamydial organisms via an outer membrane vesicle (OMV) budding mechanism. The CtSP-laden vesicles in the inclusion lumen can enter host cell cytosol via vesicle fusion with or passing through the inclusion membrane. That's why CT621 can be visualized in granules in the lumen of inclusion and its secretion can also be inhibited by C1, a small molecule inhibitor known to target bacterial T3SS.
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Figure 8: A proposed model for C. trachomatis secretion of effectors into host cell cytosol. When an infectious and metabolically inactive elementary body (EB) attaches to an epithelial cell, preexisting effectors such as TARP and CT694 can be injected into host cell cytosol via a single step type 3 secretion system (T3SS) for facilitating EB invasion. Once the internalized EB is differentiated into a non-infectious but metabolically active reticulate body (RB), newly synthesized chlamydial proteins can be secreted into host cell cytosol via either the single step T3SS (for example, secretion of CT847) or multi-step pathways. The C. trachomatis-secreted proteins (CtSPs) with an N-terminal signal sequence (termed Sec-CtSPs) such as cHtrA & CPAF may be translocated into periplasm via a SecY-dependent pathway while those without any N-terminal signal sequences (Nonsec-CtSPs) may be translocated into the periplasmic space via a novel translocon or a leaking T3SS pathway. The periplasmically localized CtSPs may exit the chlamydial organisms via an outer membrane vesicle (OMV) budding mechanism. The CtSP-laden vesicles in the inclusion lumen can enter host cell cytosol via vesicle fusion with or passing through the inclusion membrane. That's why CT621 can be visualized in granules in the lumen of inclusion and its secretion can also be inhibited by C1, a small molecule inhibitor known to target bacterial T3SS.

Mentions: HtrA is a highly conserved serine protease present in the ER of eukaryotic and periplasmic space of bacterial cells. However, there has been no report on its secretion outside of eukaryotic or bacterial cells. Secretion of cHtrA out of chlamydial organisms may represent a unique feature Chlamydia has evolved during its interactions with host cells. A sec-dependent pathway may play an important role in exporting cHtrA into host cell cytosol since the N-terminal leader peptide of cHtrA is functional and the secretion is not inhibitable by a type III secretion inhibitor. However, The sec-dependent pathway can only translocate cHtrA into the periplasmic region. It is still unknown how the periplasmic cHtrA passes through the outer membrane to enter the chlamydial inclusion lumen and further into host cell cytosol. The same challenge also applies to the secretion of CPAF. A sec-dependent pathway is necessary for CPAF secretion [62]. Similarly, how the periplasmic CPAF crosses the outer membrane remains unclear. Since CPAF was detected in granules in the lumen of inclusions during the early stage of chlamydial intracellular growth, an outer membrane vesicular budding model has been proposed for CPAF secretion into host cell cytosol [62], which may also be suitable for the secretion of cHtrA (Figure 8). Evidence for supporting this hypothesis comes from the observation that cHtrA-laden granules/vesicles that are free of chlamydial organisms were readily detected in the chlamydial inclusions. Although it remains to be determined how exactly cHtrA or CPAF is secreted out of the organisms and into host cell cytosol, as more effector molecules are identified, more tools will be available for figuring out the secretion pathways Chlamydia has evolved for exporting virulence factors.


The chlamydial periplasmic stress response serine protease cHtrA is secreted into host cell cytosol.

Wu X, Lei L, Gong S, Chen D, Flores R, Zhong G - BMC Microbiol. (2011)

A proposed model for C. trachomatis secretion of effectors into host cell cytosol. When an infectious and metabolically inactive elementary body (EB) attaches to an epithelial cell, preexisting effectors such as TARP and CT694 can be injected into host cell cytosol via a single step type 3 secretion system (T3SS) for facilitating EB invasion. Once the internalized EB is differentiated into a non-infectious but metabolically active reticulate body (RB), newly synthesized chlamydial proteins can be secreted into host cell cytosol via either the single step T3SS (for example, secretion of CT847) or multi-step pathways. The C. trachomatis-secreted proteins (CtSPs) with an N-terminal signal sequence (termed Sec-CtSPs) such as cHtrA & CPAF may be translocated into periplasm via a SecY-dependent pathway while those without any N-terminal signal sequences (Nonsec-CtSPs) may be translocated into the periplasmic space via a novel translocon or a leaking T3SS pathway. The periplasmically localized CtSPs may exit the chlamydial organisms via an outer membrane vesicle (OMV) budding mechanism. The CtSP-laden vesicles in the inclusion lumen can enter host cell cytosol via vesicle fusion with or passing through the inclusion membrane. That's why CT621 can be visualized in granules in the lumen of inclusion and its secretion can also be inhibited by C1, a small molecule inhibitor known to target bacterial T3SS.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Figure 8: A proposed model for C. trachomatis secretion of effectors into host cell cytosol. When an infectious and metabolically inactive elementary body (EB) attaches to an epithelial cell, preexisting effectors such as TARP and CT694 can be injected into host cell cytosol via a single step type 3 secretion system (T3SS) for facilitating EB invasion. Once the internalized EB is differentiated into a non-infectious but metabolically active reticulate body (RB), newly synthesized chlamydial proteins can be secreted into host cell cytosol via either the single step T3SS (for example, secretion of CT847) or multi-step pathways. The C. trachomatis-secreted proteins (CtSPs) with an N-terminal signal sequence (termed Sec-CtSPs) such as cHtrA & CPAF may be translocated into periplasm via a SecY-dependent pathway while those without any N-terminal signal sequences (Nonsec-CtSPs) may be translocated into the periplasmic space via a novel translocon or a leaking T3SS pathway. The periplasmically localized CtSPs may exit the chlamydial organisms via an outer membrane vesicle (OMV) budding mechanism. The CtSP-laden vesicles in the inclusion lumen can enter host cell cytosol via vesicle fusion with or passing through the inclusion membrane. That's why CT621 can be visualized in granules in the lumen of inclusion and its secretion can also be inhibited by C1, a small molecule inhibitor known to target bacterial T3SS.
Mentions: HtrA is a highly conserved serine protease present in the ER of eukaryotic and periplasmic space of bacterial cells. However, there has been no report on its secretion outside of eukaryotic or bacterial cells. Secretion of cHtrA out of chlamydial organisms may represent a unique feature Chlamydia has evolved during its interactions with host cells. A sec-dependent pathway may play an important role in exporting cHtrA into host cell cytosol since the N-terminal leader peptide of cHtrA is functional and the secretion is not inhibitable by a type III secretion inhibitor. However, The sec-dependent pathway can only translocate cHtrA into the periplasmic region. It is still unknown how the periplasmic cHtrA passes through the outer membrane to enter the chlamydial inclusion lumen and further into host cell cytosol. The same challenge also applies to the secretion of CPAF. A sec-dependent pathway is necessary for CPAF secretion [62]. Similarly, how the periplasmic CPAF crosses the outer membrane remains unclear. Since CPAF was detected in granules in the lumen of inclusions during the early stage of chlamydial intracellular growth, an outer membrane vesicular budding model has been proposed for CPAF secretion into host cell cytosol [62], which may also be suitable for the secretion of cHtrA (Figure 8). Evidence for supporting this hypothesis comes from the observation that cHtrA-laden granules/vesicles that are free of chlamydial organisms were readily detected in the chlamydial inclusions. Although it remains to be determined how exactly cHtrA or CPAF is secreted out of the organisms and into host cell cytosol, as more effector molecules are identified, more tools will be available for figuring out the secretion pathways Chlamydia has evolved for exporting virulence factors.

Bottom Line: The periplasmic cHtrA protein appeared to be actively secreted into host cell cytosol since no other chlamydial periplasmic proteins were detected in the host cell cytoplasm.Most chlamydial species secreted cHtrA into host cell cytosol and the secretion was not inhibitable by a type III secretion inhibitor.Since it is hypothesized that chlamydial organisms possess a proteolysis strategy to manipulate host cell signaling pathways, secretion of the serine protease cHtrA into host cell cytosol suggests that the periplasmic cHtrA may also play an important role in chlamydial interactions with host cells.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Microbiology and Immunology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA.

ABSTRACT

Background: The periplasmic High Temperature Requirement protein A (HtrA) plays important roles in bacterial protein folding and stress responses. However, the role of chlamydial HtrA (cHtrA) in chlamydial pathogenesis is not clear.

Results: The cHtrA was detected both inside and outside the chlamydial inclusions. The detection was specific since both polyclonal and monoclonal anti-cHtrA antibodies revealed similar intracellular labeling patterns that were only removed by absorption with cHtrA but not control fusion proteins. In a Western blot assay, the anti-cHtrA antibodies detected the endogenous cHtrA in Chlamydia-infected cells without cross-reacting with any other chlamydial or host cell antigens. Fractionation of the infected cells revealed cHtrA in the host cell cytosol fraction. The periplasmic cHtrA protein appeared to be actively secreted into host cell cytosol since no other chlamydial periplasmic proteins were detected in the host cell cytoplasm. Most chlamydial species secreted cHtrA into host cell cytosol and the secretion was not inhibitable by a type III secretion inhibitor.

Conclusion: Since it is hypothesized that chlamydial organisms possess a proteolysis strategy to manipulate host cell signaling pathways, secretion of the serine protease cHtrA into host cell cytosol suggests that the periplasmic cHtrA may also play an important role in chlamydial interactions with host cells.

Show MeSH
Related in: MedlinePlus