Pyocin

S-type (soluble) pyocins are binary protein complexes that compose of a cytotoxic protein and an immunity protein that protects the producing strain from cytotoxic effects. The amino-terminal domain of the protein takes part in receptor binding as the carboxy-terminal domain is responsible for cytotoxic effect. Most S-type pyocins act by degrading DNA and RNA but some exhibit their cytotoxicity by forming pores to cell surface or by lipid degradation. Several S-type pyocins have been found so far: S1, S2, AP41, S3, S4, S5, S6.[2]

Pyocin G is an example of a novel S1-type nuclease pyocin. It binds to hemin uptake receptor Hur on target cell surface and translocates to the cytoplasm where it degrades DNA. Pyocin G uses inner membrane proteins TonB1 and FtsH for translocation. Pyocin G is highly active against P.aeruginosa clinical isolates in vitro as well as in vivo and could be active in P.aeruginosa infections also in humans[3]

In silico methods are revealing also new types of S-pyocins when large databases of sequenced DNA from Pseudomonas-genus are being screened for new pyocin coding sequences.[2]

R- and F-type pyocins have been mainly investigated in P.aeruginosa. These two types differ by their structure; they are both composed of a sheath and a hollow tube forming a long helicoidal hexameric structure attached to a baseplate. There are multiple tail fibers that allow the particle to bind to the target cell. However, the R-pyocins are a large, rigid contractile tail-like structure whereas the F-pyocins are small flexible, non-contractile tail-like structures.[2]

To date five subgroups of R-type pyocins have been discovered: R1 to R5. F-type pyocins discovered so far are pyocin 28, 430f, F1, F2, and F3.[2]