Importance of conserved residues of the serine protease autotransporter beta-domain in passenger domain processing and beta-barrel assembly.

Serine protease autotransporters of the family Enterobacteriaceae (SPATE) comprise a family of virulence proteins secreted by enteric Gram-negative bacteria via the autotransporter secretion pathway. A SPATE polypeptide contains a C-terminal translocator domain that inserts into the bacterial outer membrane as a beta-barrel structure and mediates secretion of the passenger domain to the extracellular environment. In the present study, we examined the role of conserved residues located in the SPATE beta-barrel-forming region in passenger domain secretion. Thirty-nine fully conserved residues in Tsh were mutated by single-residue substitution, and defects in their secretion phenotypes were assessed by cell fractionation and immunochemistry. A total of 22 single mutants exhibited abnormal phenotypes in different cellular compartments. Most mutants affecting secretion are charged residues with side chains pointing into the beta-barrel interior. Seven mutants showed notable abnormalities in processing (constructs with the E1231A, E1249A, and R1374A mutations) and beta-barrel assembly or insertion into the outer membrane (constructs with the G1158Y, F1360A, Y1375A, and F1377A mutations). The phenotypes of the beta-barrel assembly/insertion mutants and the presence of a processed Tsh passenger domain in the periplasm support the possibility that the translocator domain must undergo extensive folding prior to insertion into the outer membrane. Results from double-mutation experiments further demonstrate that F1360 and F1377 affect beta-barrel insertion/assembly at different times. In light of these new data, a more refined model for the mechanism of SPATE secretion is presented.

Toxins and secretion systems of Photorhabdus luminescens.

Photorhabdus luminescens is a nematode-symbiotic, gram negative, bioluminescent bacterium, belonging to the family of Enterobacteriaceae. Recent studies show the importance of this bacterium as an alternative source of insecticides, as well as an emerging human pathogen. Various toxins have been identified and characterized in this bacterium. These toxins are classified into four major groups: the toxin complexes (Tcs), the Photorhabdus insect related (Pir) proteins, the "makes caterpillars floppy" (Mcf) toxins and the Photorhabdus virulence cassettes (PVC); the mechanisms however of toxin secretion are not fully elucidated. Using bioinformatics analysis and comparison against the components of known secretion systems, multiple copies of components of all known secretion systems, except the ones composing a type IV secretion system, were identified throughout the entire genome of the bacterium. This indicates that Photorhabdus luminescens has all the necessary means for the secretion of virulence factors, thus it is capable of establishing a microbial infection.