Peptidoglycan hydrolase of an unusual cross-link cleavage specificity contributes to bacterial cell wall synthesis

Bacteria are surrounded by a protective exoskeleton, peptidoglycan (PG), a cross-linked mesh-like macromolecule consisting of glycan strands interlinked by short peptides. Because PG completely encases the cytoplasmic membrane, cleavage of peptide crosslinks is a prerequisite to make space for incorporation of nascent glycan strands for its successful expansion during cell growth. In most bacteria, the peptides consist of L-alanine, D-glutamate, meso-diaminopimelic acid (mDAP) and D-alanine (D-Ala) with cross-links occurring either between D-Ala and mDAP or two mDAP residues. In Escherichia coli, the D-Ala-mDAP cross-links whose cleavage by specialized endopeptidases is crucial for expansion of PG predominate. However, a small proportion of mDAP-mDAP cross-links also exist, yet their role in the context of PG expansion or the hydrolase(s) capable of catalyzing their cleavage is not known. Here, we identified an ORF of unknown function, YcbK (renamed MepK), as an mDAP-mDAP cross-link cleaving endopeptidase working in conjunction with other elongation-specific endopeptidases to make space for efficient incorporation of nascent PG strands into the sacculus. E. coli mutants lacking mepK and another D-Ala-mDAP-specific endopeptidase (mepS) were synthetic sick, and the defects were abrogated by lack of L,D-transpeptidases, enzymes catalyzing the formation of mDAP cross-links. Purified MepK was able to cleave the mDAP cross-links of soluble muropeptides and of intact PG sacculi. Overall, this study describes a PG hydrolytic enzyme with a hitherto unknown substrate specificity that contributes to expansion of the PG sacculus, emphasizing the fundamental importance of cross-link cleavage in bacterial peptidoglycan synthesis.