Cloning and characterization of two bistructural S-layer-RTX proteins fromCampylobacter retus

Campylobacter rectus is an important periodontal pathogen in humans. A surf ace-layer (S-layer) protein and a cytotoxic activity have been characterize d and are thought to be its major virulence factors. The cytotoxic activity was suggested to be due to a pore-forming protein toxin belonging to the R TX (repeats in the structural toxins) family. In the present work, two clos ely related genes, csxA and csxB (for C. rectus S-layer and RTX protein) we re cloned from C. rectus and characterized. The Csx proteins appear to be b ifunctional and possess two structurally different domains. The N-terminal part shows similarity with S-layer protein, especially SapA and SapB of C, fetus and Crs of C. rectus. The C-terminal part comprising most of CsxA and CsxB is a domain with 48 and 59 glycine-rich canonical nonapeptide repeats , respectively, arranged in three blocks. Purified recombinant Csx peptides bind Ca2+. These are characteristic traits of RTX toxin proteins. The S-la yer and RTX domains of Csx are separated by a proline-rich stretch of 48 am ino acids. All C. rectus isolates studied contained copies of either the cs xA or csxB gene or both; csx genes were absent from all other Campylobacter and Helicobacter species examined. Serum of a patient with acute gingiviti s showed a strong reaction to recombinant Csx protein on immunoblots.