Team:Warsaw/Project/endosome

Escape from the phagosome

Theoretical basis

Although the induction of phagocytosis and entrance into the eukaryotic cell seems to be simple, this is not the final step of bacteria invasion. Majority of the enteropathogenic bacteria are incapable of dividing within the phagosome, but subsequently to leaving the phagosome they can proliferate rapidly within the cytoplasm. Escape from the phagosome is possible due to hemolysin, which causes membrane permeabilisation and phagosome disruption [16]. It was previously demonstrated that transfection with hemolysin-encoding gene is sufficient for enabling the escape from phagosomal vesicle of both Bacillus subtilis and Escherichia coli[1].
Process of escape from the phagosome is usually strictly regulated. Interesting example is the PhoP/PhoQ regulon, existing for example in Salmonella enterica ser. typhimurium LT2. It's a two-component regulatory system which reply to intramacrophage conditions like low pH or low metal ions concentration, leading to activation of PhoP-dependent genes, responsible for virulence of bacteria, like the MgtC gene.

Endosome detection operon

Fig 3. Overview of endosomal detection operon. It is composed of cro – antirepressor from phage λ, tetR – repressor of tetracycline resistance operon, cI – repressor from phage λ and cfp – Cyan Fluorescent Protein.

Low endosomal pH and low concentration of metal ions lead to decrease the likelihood of transmembrane receptor kinase phoQ binding its ligands – divalent cations. This enables the phosphorylation of phoP by phoQ. Phosphorylated phoP activates the gene expression from MgtC promoter leading to production of proteins encoded by endosome detection operon. cI protein inactivates the invasion operon. TetR protein activates cytoplasmic operon. Not all genes encoded by cytoplasmic operon are expressed in endosome, due to the presence of cro box sequence, which binds cro protein thus stopping the trancription Cro protein is encoded by endosome detection operon. After bacterium escapes into cytoplasm the endosome detection operon genes are no longer expressed, since phoP is not phosphorylated in cytoplasmic conditions. The process can be monitored using CFP fluorescence protein.