A novel protein releases the S-layer protein from the Campylobacter rectus cell surface

Campylobacter rectus is a gram negative periodontal pathogen that expresses a 150 kd surface-layer (S-layer) protein called CrsA. This abundant protein forms a homogeneous array covering the surface of C. rectus and is thought to affect the interactions of the bacterium with its host. Objectives: and Methods: Since the S-layer protein must be transported and properly assembled on the cell surface to function as a virulence determinant, we are using molecular genetic approaches to determine the mechanism of C. rectus S-layer protein localization. Results: and Conclusions: Toward this end, the genes adjacent to the crsA gene were cloned and sequenced. The results showed that an operon upstream of the C. rectus crsA gene encodes a protein, CrsD, homologous to transport proteins in other bacteria. A crsD^- mutant was constructed and analyzed; the crsD^- mutant cells made normal levels of S-layer protein, but it was not on the cell surface. This proves that CrsA is transported to the cell surface by a type I transport pathway. Interestingly, the crsD operon contains another gene, crsC, encoding a protein with no homologues of known function. Analysis of the phenotype of a non-polar crsC^- mutant demonstrated that CrsC is required for the release of S-layer protein from the cell surface. Wild type C. rectus cells release some CrsA protein into the growth supernatant but no CrsA is in the growth medium in crsC^- mutant cells. CrsC does not appear to alter the CrsA protein to cause its release. Thus, to determine the mechanism of action of CrsC, we are investigating how the S-layer is attached to and released from the bacterial surface. We are also looking into the intriguing possibility that CrsA has a different function when it is not on the cell surface. This work was supported by NIH Grant DE13100.