Two faces of an essential PknB substrate in mycobacteria

Tuberculosis claims nearly 2 million lives annually and its causative agent Mycobacterium tuberculosis is a highly successful pathogen which explores multiple signalling mechanisms to grow and survive in humans for decades. Well-studied protein kinase B (PknB), one of the 11 serine/threonine kinases present in M. tuberculosis, is critical for growth in vitro and in vivo. Here we demonstrate that PknB-depleted mycobacteria can normally replicate and synthesise peptidoglycan in osmoprotective medium. Comparative phosphoproteomics of PknB-producing and PknB-depleted mycobacteria identifies CwlM as a major PknB substrate. Complementation studies of cwlM mutant further support CwlM phosphorylation as the molecular basis for the PknB essentiality. We demonstrate that in growing mycobacteria CwlM is produced in two forms with distinct functions which are essential for growth: a non-phosphorylated membrane-associated CwlM and a PknB-phosphorylated cytoplasmic CwlM. Immunoprecipitation and mycobacterial two hybrid assays reveal partner proteins for phosphorylated and non-phosphorylated CwlM forms, as FhaA, a forkhead-associated domain protein, and MurJ (MviN), a predicted Lipid II flippase, respectively. We propose that PknB is not required for mycobacterial division but coordinates peptidoglycan polymerization and cross-linking by a novel CwlM-mediated mechanism