SERRATIA-MARCESCENS S-LAYER PROTEIN IS SECRETED EXTRACELLULARLY VIA AN ATP-BINDING CASSETTE EXPORTER, THE LIP SYSTEM

The Serratia marcescens Lip exporter belonging to the ATP-binding cass ette (ABC) exporter is known to be involved in signal peptide-independ ent extracellular secretion of a lipase and a metalloprotease. Althoug h the genes of secretory proteins and their ABC exporters are usually all reported to be linked in several Gram-negative bacteria, neither t he lipase nor the protease gene is located close to the Lip exporter g enes, lipBCD. A gene (slaA) located upstream of the lipBCD genes was c loned, revealing that it encodes a polypeptide of 100 kDa and is parti ally similar to the Caulobacter crescentus paracrystalline cell surfac e layer (S-layer) protein. The Lip exporter-deficient mutants of S. ma rcescens failed to secrete the SlaA protein. Electron micrography demo nstrated the cell surface layer of S. marcescens. The S-layer protein was secreted to the cultured media in Escherichia coli cells carrying the Lip exporter. Three ABC exporters, Prt, Has and Hly systems, could not allow the S-layer secretion, indicating that the S. marcescens S- layer protein is strictly recognized by the Lip system. This is the fi rst report concerning secretion of an S-layer protein via its own secr etion system.