Fig. (4).

Pore formation by PopB and PopD in P. aeruginosa. (A) PcrH binds to PopB and PopD in the bacterial cytosol and thereby prevents their aggregation and/or activation. (B) Upon the ‘in vivo switch’, which may involve recognition of the T3SS secretion or transport through the needle, PopB and PopD form metastable oligomers. (C) PopB and PopD might associate into a homomeric and/or heteromeric structures that recognize microdomain lipid rafts on the plasma membrane of target cells and form a ring-like structure. (D) They form a channel on the target membrane that allows transport of other T3SS proteins. The molecules are shown as pentamers only for schematic purposes.