Crystal structures of Campylobacter jejuni CadF reveal a potential role for the arginine residue R268 in peptidoglycan recognition and pocket formation

Campylobacter jejuni is a pathogenic gram-negative bacterium that primarily causes foodborne enteritis in humans. The C. jejuni protein CadF is located in the outer membrane and is essential for the adhesion of C. jejuni to human cells. CadF contains an outer membrane-embedded domain and a periplasmic domain (CadF^PD). The transmembrane domain is well known to bind fibronectin, contributing to host cell recognition. However, the exact function of the CadF^PD has never been assessed experimentally. In this study, we present the crystal structures of the CadF^PD and demonstrate that CadF may function as a peptidoglycan-binding protein. The CadF^PD adopts a two-layer α/β structure and harbors a narrow pocket. The CadF^PD interacts with a peptidoglycan-derived peptide by accommodating m-diaminopimelic acid (m-DAP), a non-proteinogenic amino acid of the gram-negative peptidoglycan peptide, into the pocket. The positively charged R268 residue of CadF forms the base of the pocket and specifically interacts with the carboxyl group of m-DAP via salt bridges. This observation combined with the ligand-less structure of the CadF^PD mutant R268E highlights the potential role of the R268 residue in maintaining the intact pocket in the CadF^PD structure and mediating peptidoglycan recognition.