Novel M23 peptidases Pgp4, Pgp5, and Pgp6 contribute to helical cell shape in Campylobacter jejuni

The helical morphology of Campylobacter jejuni is maintained by its peptidoglycan (PG) layer and influences its success as a pathogen. Periplasmic PG hydrolases that cleave the PG glycan backbone and peptide sidechains (such as carboxypeptidases and endopeptidases) are critical for proper cell function and/or growth and are important in the PG remodeling required for cell shape generation and any morphological alterations. The C. jejuni shape is determined by PG hydrolases Pgp1 (DL-carboxypeptidase), Pgp2 (LD-carboxypeptidase) and Pgp3 (DD-carboxypeptidase/DD-endopeptidase), as well as a group of M23 peptidase domain containing proteins with previously uncharacterized activity: CJJ81176_1105, CJJ81176_1228, and CJJ81176_0166. Using a PG cleavage assay, we showed that 1105 and 1228 have DD-carboxypeptidase/DD-endopeptidase activity, and 0166 is a DD-carboxypeptidase. We renamed 1105, 1228, and 0166 to Pgp4 (peptidoglycan peptidase 4), Pgp5, and Pgp6, respectively. Pgp6 is the first described C. jejuni M23 peptidase with substrate selectivity on monomeric pentapeptides. Sequence comparisons between the DD-carboxypeptidase Pgp6 and the DD-carboxypeptidase/DD-endopeptidase Pgp3 (with an available crystal structure) and their corresponding orthologs revealed that Pgp6 contains insertion sequences in the M23 peptidase domain not present in Pgp3. Modeling of Pgp6 predicted that the insertion sequences would restrict the active site groove, only allowing entrance of a smaller substrate. This provides a possible explanation for the lack of Pgp6 DD-endopeptidase activity. To our knowledge, Pgp6 is the first reported DD-carboxypeptidase in the M23 peptidase superfamily. Deletions in pgp4, pgp5, and pgp6 resulted in mutants with varying curved rod morphologies and changes in PG muropeptide profiles in comparison to wild type and each other. Using these mutants, we examined the effect of deleting these genes on C. jejuni properties affecting pathogenesis and survival: motility, biofilm formation, autoagglutination, the ability to transition to a coccoid form, growth under varying pH, susceptibility to antimicrobial compounds, and adherence, invasion and intracellular survival in human epithelial cells. Each mutant showed distinct phenotypic changes to each other, indicating they are not functionally redundant. This also further supports the correlation between C. jejuni morphology and morphology-related genes with pathogenic potential.