Two Faces of CwlM, an Essential PknB Substrate, in Mycobacterium tuberculosis

Tuberculosis claims >1 million lives annually, and its causative agent Mycobacterium tuberculosis is a highly successful pathogen. Protein kinase B (PknB) is reported to be critical for mycobacterial growth. Here, we demonstrate that PknB-depleted M. tuberculosis can replicate normally and can synthesize peptidoglycan in an osmoprotective medium. Comparative phosphoproteomics of PknB-producing and PknB-depleted mycobacteria identify CwlM, an essential regulator of peptidoglycan synthesis, as a major PknB substrate. Our complementation studies of a cwlM mutant of M. tuberculosis support CwlM phosphorylation as a likely molecular basis for PknB being essential for mycobacterial growth. We demonstrate that growing mycobacteria produce two forms of CwlM: a non-phosphorylated membrane-associated form and a PknB-phosphorylated cytoplasmic form. Furthermore, we show that the partner proteins for the phosphorylated and non-phosphorylated forms of CwlM are FhaA, a fork head-associated domain protein, and MurJ, a proposed lipid II flippase, respectively. From our results, we propose a model in which CwlM potentially regulates both the biosynthesis of peptidoglycan precursors and their transport across the cytoplasmic membrane. PknB controls growth and peptidoglycan biosynthesis in Mycobacterium tuberculosis. Turapov et al. show that CwlM, a major PknB substrate, is produced in two forms: a non-phosphorylated membrane-associated CwlM and a PknB-phosphorylated cytoplasmic CwlM. The phosphorylated CwlM binds FhaA, a fork head-associated domain protein, while non-phosphorylated CwlM interacts with MurJ (MviN), a proposed lipid II flippase.