Campylobacter fetus surface layer proteins are transported by a type I secretion system

Area of Science:

• Microbiology
• Bacterial Pathogenesis
• Protein Secretion

Background:

• Campylobacter fetus virulence is linked to its paracrystalline surface layer (S-layer), which provides serum resistance.
• S-layer proteins (SLPs) are key virulence factors, attaching to lipopolysaccharide in a serospecific manner.
• Antigenic variation in SLPs is achieved through DNA inversion of promoter elements.

Purpose of the Study:

• To elucidate the secretion mechanism of Campylobacter fetus S-layer proteins (SLPs).
• To identify the genes responsible for SLP secretion in C. fetus.
• To characterize the potential secretion system involved in SLP transport.

Main Methods:

• Cloning and sequencing of the invertible DNA region containing sapA homologs.
• Bioinformatic analysis of gene products for homology to known secretion systems.
• Construction and analysis of a C. fetus sapD mutant.
• Heterologous expression of C. fetus genes in E. coli.

Main Results:

• A 5.6-kb operon (sapCDEF) was identified within the invertible region, transcribed opposite to sapA.
• SapD, SapE, and SapF showed homology to components of type I protein secretion systems.
• A sapD mutant failed to produce or secrete SLPs.
• Heterologous expression confirmed sapCDEF genes mediate SLP secretion.

Conclusions:

• Campylobacter fetus SLPs are secreted via a type I secretion system.
• The sapCDEF genes encode the machinery for SLP transport to the cell surface.
• This represents a novel mechanism for bacterial surface protein secretion.