Chlamydia trachomatis Oligopeptide Transporter Performs Dual Functions of Oligopeptide Transport and Peptidoglycan Recycling

Abstract

Peptidoglycan, the sugar-amino acid polymer that composes the bacterial cell wall, requires a significant expenditure of energy to synthesize and is highly immunogenic. To minimize the loss of an energetically expensive metabolite and avoid host detection, bacteria often recycle their peptidoglycan, transporting its components back into the cytoplasm, where they can be used for subsequent rounds of new synthesis. The peptidoglycan-recycling substrate binding protein (SBP) MppA, which is responsible for recycling peptidoglycan fragments in Escherichia coli, has not been annotated for most intracellular pathogens. One such pathogen, Chlamydia trachomatis, has a limited capacity to synthesize amino acids de novo and therefore must obtain oligopeptides from its host cell for growth. Bioinformatics analysis suggests that the putative C. trachomatis oligopeptide transporter OppABCDF (OppABCDFCt) encodes multiple SBPs (OppA1Ct, OppA2Ct, and OppA3Ct). Intracellular pathogens often encode multiple SBPs, while only one, OppA, is encoded in the E. coli opp operon. We hypothesized that the putative OppABCDF transporter of C. trachomatis functions in both oligopeptide transport and peptidoglycan recycling. We coexpressed the putative SBP genes (oppA1Ct, oppA2Ct, oppA3Ct) along with oppBCDFCt in an E. coli mutant lacking the Opp transporter and determined that all three chlamydial OppA subunits supported oligopeptide transport. We also demonstrated the in vivo functionality of the chlamydial Opp transporter in C. trachomatis. Importantly, we found that one chlamydial SBP, OppA3Ct, possessed dual substrate recognition properties and is capable of transporting peptidoglycan fragments (tri-diaminopimelic acid) in E. coli and in C. trachomatis. These findings suggest that Chlamydia evolved an oligopeptide transporter to facilitate the acquisition of oligopeptides for growth while simultaneously reducing the accumulation of immunostimulatory peptidoglycan fragments in the host cell cytosol. The latter property reflects bacterial pathoadaptation that dampens the host innate immune response to Chlamydia infection.

An NlpC/P60 protein catalyzes a key step in peptidoglycan recycling at the intersection of energy recovery, cell division and immune evasion in the intracellular pathogenChlamydia trachomatis

“…The chlamydial homolog lacks membrane-spanning or lipoprotein-associated domains and a typical signal sequence for transport across the cytoplasmic membrane, suggesting a cytosolic localization of the enzyme. This may be supported by the identification of oligopeptide transporter system OppABCDF in C. trachomatis [30] which potentially transports PGN degradation-derived peptides into the chlamydial cell cytoplasm for further recycling as described for free-living bacteria.…”

“…Our data help to answer the unsolved question how chlamydiae may impede NOD1 factor signaling. We hypothesize that peptides released by AmiA are transported from the periplasm to the cytoplasm through peptide permease OppABCDF [30] Functioning as a junction between pathogenicity and two central cellular processes, OppABCDF and YkfC might be essential for survival in obligate intracellular pathogens such as chlamydiae. In contrast to the peptide transporter, YkfC has a catalytic function making it a better candidate for antichlamydial drug development.…”

Chlamydia trachomatis Encodes a Dynamic, Ring-Forming Bactofilin Critical for Maintaining Cell Size and Shape

“…To investigate the subcellular localization of BacA CT , we constructed C-terminal hexahistidine (6xH) and mCherry BacA CT fusion constructs for expression in the C. trachomatis strain L2 434/Bu. Constructs were cloned into shuttle plasmids under the control of an anhydrous tetracycline inducible (aTc) promoter and transformed into C. trachomatis , as described previously (17, 24). Cell division genes are precisely regulated in bacteria, and their overexpression often causes fitness defects, including in Chlamydia (25, 26).…”

Expression Level of the mip, pmp18D, and ompA Genes in Chlamydia abortus Isolated from Aborted Ewes

“…Chlamydiae possess a cell wall and are bordered by an outer membrane (OM) and a cytoplasmatic inner membrane (IM) ( Elwell et al 2016 ). The EB outer membrane contains phospholipids, lipids, lipooligosaccharides (LOS), and proteins, while the cell wall contains a small amount of peptidoglycan ( Singh et al 2020 ). A part of the Chlamydia cell wall is insoluble in sarkosyl, and this fraction is known as the Chlamydia outer membrane complex (COMC).…”