Highly purified colicin E3 contains immunity protein.

Colicin E3, even when highly purified, still contains about one molar equivalent of a second protein, "E3 immunity protein." The two proteins are bound together in a complex that can be dissociated only under strongly denaturing conditions, such as electrophoresis in sodium dodecyl sulfate-polyacrylamide gles or by gel filtration in 6 M guanidine.hydrochloride. Colicin and immunity protein were separated by preparative electrophoresis in sodium dodecyl sulfate-polyacrylamide gels. The immunity protein prepared in this way was shown to be functionally and immunologically identical to immunity protein purified from colicinogenic cells by other methods. Colicin E3 that had been freed of immunity protein was much more active than complexed colicin in inhibiting protein synthesis in vitro.