A directed screen for chlamydial proteins secreted by a type III mechanism identifies a translocated protein and numerous other new candidates

Abstract

SummaryChlamydiae are strict intracellular parasites that induce their internalization upon contact with the host cell and grow inside an intracellular compartment called an inclusion. They possess a type III secretion (TTS) apparatus, which allows for the translocation of specific proteins in the host cell cytosol. In particular, chlamydial proteins of the Inc family are secreted to the inclusion membrane by a TTS mechanism; other TTS substrates are mostly unknown. Using a secretion assay based on the recognition of TTS signals in Shigella flexneri , we searched for TTS signals in the proteins of unknown function, conserved between three different chlamydial species, Chlamydia pneumoniae , C. trachomatis and C. caviae . We identified 24 new candidate proteins which did not belong to the Inc family. Four of these proteins were also secreted as full-length proteins by a TTS mechanism in S. flexneri , indicating that their translocation does not require other chlamydial proteins. One of these proteins was detected in the cytosol of infected cells using specific antibodies, directly demonstrating that it is translocated in the host cell during bacterial proliferation. More generally, this work represents the first directed search for TTS effectors not based on genetic information or sequence similarity. It reveals the abundance of proteins secreted in the host cell by chlamydiae.

Bioinformatic and Biochemical Evidence for the Identification of the Type III Secretion System Needle Protein ofChlamydia trachomatis

“…Moreover, the other genes of this locus are arranged in operons (24) and predicted to encode important members of the T3SS apparatus, including the ATPase, chaperones, and membrane components (16). There is evidence that CT668, CT670, and CT671 are secreted proteins (47,51), and yet given their gene order they could represent mobile components of the T3SS apparatus. Indeed, CT670 contains an YscO-like domain raising the possibility that CT670 and CT671 correspond to the secreted Yersinia apparatus components YscO and YscP, respectively.…”

A small-molecule inhibitor of type III secretion inhibits different stages of the infectious cycle of Chlamydia trachomatis

“…In addition to the family of Inc-proteins, other chlamydial proteins secreted into the inclusion membrane have been identified (14,15). Moreover, proteins targeted into the host-cell cytosol have been described (16)(17)(18).…”

Identification and characterization of a type III secretion system inChlamydophila psittaci

“…Moreover, these investigators showed proteins from Cp. pneumoniae and C. trachomatis, all of which had in common the presence of a large hydrophobic domain, to be secreted by a S. flexneri or Yersinia enterocolitica type III secretion machinery [9,35,36].…”