Activation and secretion of Serratia hemolysin.
The hemolysin of Serratia marcescens (ShlA) is secreted into the culture medium and forms small pores of a defined size in erythrocytes and in black lipid membranes. The protein is synthesized as an inactive precursor of 1608 residues which is translocated across the cytoplasmic membrane by the Sec-export system. In the absence of the outer membrane protein ShlB, the ShlA protein (designated ShlA*) stays in the periplasm and displays about 0.1% of the activity of the secreted form. Secretion 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 signal resides in the amino terminal part of ShlA. ShlA* can be activated in vitro by a cell lysate containing ShlB, the activated ShlA remains hemolytic upon removal of ShlB. The assumed covalent modification of ShlA* by ShlB occurs in the N-terminus of ShlA since an amino terminal 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 protein. In contrast to the permanent modification of ShlA* by ShlB, ShlA activity achieved by complementation with the ShlA fragments is abolished upon removal of the fragments. Apparently, the N-terminal portion of ShlA contains the information for secretion through the outer membrane and for insertion into the erythrocyte membrane. This information is 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 themselves are too short to build pores. The HpmA hemolysin of Proteus mirabilis shows extensive homology to ShlA. In vitro activation of HpmA* by ShlB and complementation by the 28 kDa ShlA fragment indicates a common activation mechanism.
Braun
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V
Mikrobiologie II, Universität Tübingen, Germany.
Ondraczek
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Hobbie
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D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000378:metabolism; D004910:Erythrocyte Membrane / Q000378:metabolism; D006460:Hemolysin Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D012706:Serratia marcescens / Q000235:genetics / Q000378:metabolism*
D001425:Bacterial Outer Membrane Proteins; D006460:Hemolysin Proteins; D011498:Protein Precursors