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 cassette (ABC) exporter is known to be involved in signal peptide‐independent extracellular secretion of a lipase and a metalloprotease. Although the genes of secretory proteins and their ABC exporters are usually all reported to be linked in several Gram‐negative bacteria, neither the lipase nor the protease gene is located close to the Lip exporter genes, lipBCD. A gene (slaA) located upstream of the lipBCD genes was cloned, revealing that it encodes a polypeptide of 100 kDa and is partially similar to the Caulobacter crescentus paracrystalline cell surface layer (S‐layer) protein. The Lip exporter‐deficient mutants of S. marcescens failed to secrete the SlaA protein. Electron micrography demonstrated 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 first report concerning secretion of an S‐layer protein via its own secretion system.