knob domain of R1 and R2 pyocins and similar domains
The knob domain is present as a tandemly repeated structural domain in R-type pyocins, which are high-molecular weight bacteriocins produced by some strains of Pseudomonas aeruginosa to specifically kill other strains of the same species. R-type pyocins are structurally similar to simple contractile tails, such as those of phage P2 and Mu, and they punch a hole in the bacterial envelope to efficiently kill target cells. The second knob domain may contain regions responsible for determining the killing spectrum. Knob-like domains occur in host-recognition and binding proteins of, not only pyocins, but also phages, such as in phage K1F endosialidase (not represented by this model), where it may interact with sialic acid, the cell surface molecule that is recognized during infection.