SEPA, THE MAJOR EXTRACELLULAR PROTEIN OF SHIGELLA-FLEXNERI - AUTONOMOUS SECRETION AND INVOLVEMENT IN TISSUE INVASION

In addition to Ipa proteins and IcsA, which are involved in entry into epithelial cells and intercellular spread, respectively, Shigella sec retes a 110 kDa protein, designated SepA. We report the identification , cloning, and nucleotide sequence determination of the sepA gene, ana lysis of SepA secretion, and construction and characterization of a se pA mutant. The sepA gene is carried by the virulence plasmid and codes for a 150 kDa precursor. Upon secretion, which does not involve acces sory proteins encoded by the virulence plasmid, the precursor is conve rted to a mature protein of 110 kDa by two cleavages removing an N-ter minal signal sequence and a C-terminal fragment. Extensive similaritie s were detected between the sequence of the first 500 residues of matu re SepA and the N-terminal region of IgA1 proteases from Neisseria gon orrhoeae and Haemophilus influenzae, the Tsh haemagglutinin of an avia n pathogenic Escherichia coli, and the Hap protein involved in adhesio n and penetration of H. influenzae. The C-terminal domain of the SepA precursor, which is not present in the secreted protein, exhibits sequ ence similarity with pertactin of Bordetella pertussis and the ring-fo rming protein of Helicobacter mustelae. Construction and phenotypic ch aracterization of a sepA mutant indicated that SepA is required neithe r for entry into cultured epithelial cells nor for intercellular disse mination. However, in the rabbit ligated ileal loop model, the sepA mu tant exhibited an attenuated virulence, which suggests that SepA might play a role in tissue invasion.