ACTIVATION AND SECRETION OF SERRATIA HEMOLYSIN

The hemolysin of Serratia marcescens (ShlA) is secreted into the cultu re medium and forms small pores of a defined size in erythrocytes and in black lipid membranes. The protein is synthesized as an inactive pr ecursor of 1608 residues which is translocated across the cytoplasmic membrane by the Sec-export system. In the absence of the outer membran e protein ShlB, the ShlA protein (designated ShlA) stays in the perip lasm and displays about 0.1% of the activity of the secreted form. Sec retion of ShlA with the help of ShlB is accompanied by its conversion to the hemolytic form. A ShlA derivative consisting of the N-terminal 238 residues of ShlA is secreted by ShlB, showing that the secretion s ignal resides in the amino terminal part of ShlA. ShlA can be activat ed in vitro by a cell lysate containing ShlB. the activated ShlA remai ns hemolytic upon removal of ShlB. The assumed covalent modification o f ShlA by ShlB occurs in the N-terminus of ShlA since an amino termin al fragment (M(r) 28 000) secreted by ShlB, and a trypsin fragment of ShlA (M(r) 15 000) are both able to convert ShlA to a hemolytic prote in. In contrast to the permanent modification of ShlA by ShlB, ShlA a ctivity achieved by complementation with the ShlA fragments is abolish ed upon removal of the fragments. Apparently, the N-terminal portion o f ShlA contains the information for secretion through the outer membra ne and for insertion into the erythrocyte membrane. This information i s lacking in ShlA formed in the absence of ShlB but contained in the ShlA fragments formed in the presence of ShlB. The latter bind to ShlA and direct ShlA* into the erythrocyte membrane. The fragments themse lves are too short to build pores. The HpmA hemolysin of Proteus mirab ilis shows extensive homology to ShlA. In vitro activation of HpmA by ShlB and complementation by the 28 kDa ShlA fragment indicates a comm on activation mechanism.