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The hemolytic activity of Serratia marcescens is determined by two polypeptides, termed ShlA and ShlB. ShlA is synthesized as an inactive precursor (ShlA*) and secreted with the help of ShlB, which is located in the outer membrane. In this study, it is shown that a cell lysate containing ShlB as well as partially purified ShlB converted ShlA* to the active ShlA hemolysin. ShlA remained active after removal of ShlB by column chromatography. In contrast to the stable modification of ShlA* by ShlB, a reversible activation was achieved by adding to ShlA* an N-terminal fragment of ShlA (ShlA16), consisting of 269 amino acid residues of ShlA and 18 residues of the vector. The nonhemolytic ShlA16 complemented ShlA* only when it was synthesized in an ShlB-producing cell. A deletion derivative of ShlA*, lacking residues 4 to 117, was complemented by ShlA16 but not activated by ShlB. Activation of ShlA* by ShlB at 4 degrees C proceeded at a much slower rate than complementation by ShlA16. It is concluded that ShlA* is modified by ShlB. ShlA16 modified by ShlB complements the missing modification of ShlA* in trans. Modification by ShlB occurs in the N-terminal part of ShlA*, which is also the reaction in vivo which results in active ShlA hemolysin in the culture supernatant. The HpmA hemolysin of Proteus mirabilis, which is very similar to ShlA, was also activated in vitro by ShlB and complemented by ShlA16.

In vitro activation of the Serratia marcescens hemolysin through modification and complementation