A Chlamydial Plasmid-Dependent Secretion System for the Delivery of Virulence Factors to the Host Cytosol

Chlamydia are obligate intracellular Gram-negative bacteria distinguished by a unique developmental biology confined within a parasitophorous vacuole termed an inclusion. The chlamydial plasmid is a central virulence factor in the pathogenesis of infection. Plasmid gene protein 4 (Pgp4) regulates the expression of plasmid gene pro-tein 3 (Pgp3) and chromosomal glycogen synthase (GlgA), virulence factors secreted from the inclusion to the host cytosol by an unknown mechanism. Here, we identified a plasmid-dependent secretion system for the cytosolic delivery of Pgp3 and GlgA. The secretion system consisted of a segregated population of globular structures originat-ing from midcycle reticulate bodies. Globular structures contained the Pgp4-regulated proteins CT143, CT144, and CT050 in addition to Pgp3 and GlgA. Genetic replacement of Pgp4 with Pgp3 or GlgA negated the formation of globular structures, resulting in retention of Pgp3 and GlgA in chlamydial organisms. The generation of globular struc-tures and secretion of virulence factors occurred independently of type 2 and type 3 secretion systems. Globular structures were enriched with lipopolysaccharide but lacked detectable major outer membrane protein and heat shock protein 60, implicating them as outer membrane vesicles. Thus, we have discovered a novel chlamydial plasmid-de-pendent secretion system that transports virulence factor cargo from the chlamydial inclusion to the host cytosol. IMPORTANCE The Chlamydia trachomatis plasmid regulates the expression and secre-tion of immune evasion virulence factors to the host cytosol by an unknown mecha-nism. In this study, we identified a novel plasmid gene protein 4 (Pgp4)-dependent secretion system. The system consists of globular structures distinct from typical chlamydial developmental forms that export Pgp3 and GlgA to the host cytosol. Globular structures emerged at mid-chlamydial growth cycle from distinct popula-tions of reticulate bodies. The formation of globular structures occurred independ-ently of known chlamydial secretion systems. These results identify a Pgp4-depend-ent secretory system required for exporting plasmid regulated virulence factors to the host cytosol.

Automated summary

"A novel plasmid-dependent secretion system has been identified that transports proteins regulated by Pgp4 from the chlamydial inclusion to the host cytosol. This system consists of globular structures that form independently of known chlamydial secretion systems."