A study published recently in the Journal of Biological Chemistry by Alejandro Heuck, biochemistry and molecular biology, with Yuzhou Tang and colleagues, offers new insights on the interaction and assembly mechanisms of channel-forming proteins PopB and PopD in the disease-causing bacterium, Pseudomonas aeruginosa. Heuck has for many years studied how toxic proteins secreted by pathogens make holes in cell membranes.
As he and co-authors explain, many pathogenic bacteria such as Pseudomonas cause life-threatening human diseases, including pneumonia, food poisoning and bubonic plague, by injecting toxins into host cells to establish the infection. They do this using a syringe-like nano-machine known as the Type III Secretion (T3S) system. Toxins gain access to the cell through a channel formed into the target membrane. The channel, named the T3S translocon, is formed by two proteins, PopB and PopD, secreted by the bacteria using the same T3S system. PopB and PopD are named traslocators.
The mechanism of translocator secretion during bacterial infection was known, but “how the membrane-associated translocators transition to a functional translocon has remained elusive,” the authors point out. Forming this channel is critical for the bacteria to cause infection. The researchers therefore hypothesized that blocking the channel assembly could interfere with the infection and lead to new drug treatments, which could be especially important to people with compromised immune systems, say Heuck, Tang and colleagues.
Combining a series of biophysical and cell-based assays, they show that the interaction between PopB and PopD play a key early role in guiding channel assembly. Further, they show that the interaction of the two is required to properly insert PopD into the target cell membrane and assemble functional channels.