Mass spectrometric characterization of the Campylobacter jejuni adherence factor CadF reveals post-translational processing that removes immunogenicity while retaining fibronectin binding

Campylobacter jejuni is a major gastrointestinal pathogen that colonizes host mucosa via interactions with extracellular matrix proteins, such as fibronectin (Fn). Fn-binding is mediated by a 37kDa outer membrane protein termed Campylobacter adherence Factor (CadF). The outer membrane protein profile of a recent gastrointestinal C. jejuni clinical isolate (JHH1) was analysed using 2-DE and MS. Several spots were identified as products of the cadF gene. These included mass and pI variants of 34 and 30 kDa, as well as 24 kDa (CadF₂₄) and 22kDa (CadF₂₂) mass variants. CadF variants were fully characterized by MALDI-TOF MS and MALDI-MS/MS. These data confirmed that CadF forms re-folding variants resulting in spots with lower mass and varying pI that are identical at the amino acid sequence level and are not modified posttranslationally. CadF₂₂ and CadF₂₄, however, were characterized as N-terminal, membrane-associated polypeptides resulting from cleavage between serine¹⁹⁵ and leucine¹⁹⁶, and glycine²⁰¹ and phenylalanine²⁰², respectively. These variants were more abundant in the virulent (O) isolate of C. jejuni NCTC11168 when compared with the avirulent (genome sequenced) isolate. Hexahistidine fusion constructs of full-length CadF (34 kDa), CadF₂₄, and the deleted C-terminal OmpA domain (14 kDa; CadF₁₄) were created in Escherichia coli. Recombinant CadF variants were probed against patient sera and revealed that only full-length CadF retained reactivity. Binding assays showed that CadF₂₄ retained Fn-binding capability, while CadF₁₄ did not bind Fn. These data suggest that the immunogenic epitope of CadF is cleaved to generate smaller Fnbinding polypeptides, which are not recognized by the host humoral response. CadF cleavage therefore may be associated with virulence in C. jejuni.