Subcellular location and unique secretion of the hemolysin of Serratia marcescens

It is shown that Serratia marcescens exports a hemolysin to the cell surface and secretes it to the extracellular space. Escherichia coli containing the cloned hemolysin genes shlA and shlB exported and secreted the S. marcescens hemolysin. A nonhemolytic secretion-incompetent precursor of the hemolysin, designated ShlA*, was synthesized in a shlB deletion mutant and accumulated in the periplasmic space of E. coli. Immunogold-labeled ultrathin sections revealed ShlA* bound to the outer face of the cytoplasmic membrane and to the inner face of the outer membrane. A number of mutants carrying 3' deletions in the shlA gene secreted truncated polypeptides, the smallest of which contained only 261 of the 1578 amino acids of the mature ShlA hemolysin, showing that the information for export to the cell surface of E. coli and secretion into the culture medium is located in the NH2-terminal segment of the hemolysin. We propose a secretion pathway in which ShlA and ShlB are exported across the cytoplasmic membrane via a signal sequence-dependent mechanism. ShlB is integrated into the outer membrane. ShlA is translocated across the outer membrane with the help of ShlB. During the latter export process or at the cell surface, ShlA acquires the hemolytically active conformation and is released to the extracellular space. The hemolysin secretion pathway appears to be different from any other secretion system hitherto reported and involves only a single specific export protein.