Cell wall damage reveals spatial flexibility in peptidoglycan synthesis and a non-redundant role for RodA in mycobacteria

Abstract

Cell wall peptidoglycan is a heteropolymeric mesh that protects the bacteria from internal turgor and external insults. In many rod-shaped bacteria, peptidoglycan synthesis for normal growth is achieved by two distinct pathways: the Rod complex, comprised of MreB, RodA and a cognate class B PBP, and the class A PBPs. In contrast to laterally-growing bacteria, pole-growing mycobacteria do not encode an MreB homolog and do not require SEDS protein RodA for in vitro growth. However, RodA contributes to survival of Mycobacterium tuberculosis in some infection models, suggesting that the protein could have a stress-dependent role in maintaining cell wall integrity. Under basal conditions, we find here that the subcellular distribution of RodA largely overlaps with that of the aPBP PonA1, and that both RodA and the aPBPs promote polar peptidoglycan assembly. Upon cell wall damage, RodA fortifies M. smegmatis against lysis and, unlike aPBPs, contributes to a shift in peptidoglycan assembly from the poles to the sidewall. Neither RodA nor PonA1 relocalize; instead, the redistribution of nascent cell wall parallels that of peptidoglycan precursor synthase MurG. Our results support a model in which mycobacteria balance polar growth and cell-wide repair via spatial flexibility in precursor synthesis and extracellular insertion.ImportancePeptidoglycan synthesis is a highly successful target for antibiotics. The pathway has been extensively studied in model organisms under laboratory-optimized conditions. In natural environments, bacteria are frequently under attack. Moreover the vast majority of bacterial species are unlikely to fit a single paradigm because of differences in growth mode and/or envelope structure. Studying cell wall synthesis under non-optimal conditions and in non-standard species may improve our understanding of pathway function and suggest new inhibition strategies. Mycobacterium smegmatis, a relative of several notorious human and animal pathogens, has an unusual polar growth mode and multi-layered envelope. In this work we challenged M. smegmatis with cell wall-damaging enzymes to characterize the roles of cell wall-building enzymes when the bacterium is under attack.

Polar growth protein Wag31 undergoes changes in homo-oligomeric network topology, and has distinct functions at both cell poles and the septum

“…Recent work suggests that localization of the enzymes that synthesize cell wall precursors to the subpolar Intracellular Membrane Domain (IMD) may be critical in restricting elongation to the pole (10,27). Depletion of Wag31 has been shown to delocalize the IMD (11), though it is not known if Wag31 regulates the IMD, or if the IMD localization is merely dependent on an intact cell pole.…”

“…Wag31 helps with chromosome segregation by recruiting the chromosome segregation factor ParA to the poles (26). Wag31 is not required for cell wall metabolism, but it is needed to direct this metabolism to the pole (27). Both RodA and PBP1 in Msmeg are delocalized around the entire cell instead of restricted to the poles (27).…”

“…Wag31 is not required for cell wall metabolism, but it is needed to direct this metabolism to the pole (27). Both RodA and PBP1 in Msmeg are delocalized around the entire cell instead of restricted to the poles (27).…”

“…Wag31 is not required for cell wall metabolism, but it is needed to direct this metabolism to the pole (27). Both RodA and PBP1 in Msmeg are delocalized around the entire cell instead of restricted to the poles (27). Thus, Msmeg does not regulate polar cell wall synthesis by restricting its essential peptidoglycan transglycosylases to the poles.…”

“…Thus, Msmeg does not regulate polar cell wall synthesis by restricting its essential peptidoglycan transglycosylases to the poles. However, Msmeg does restrict synthesis of many cell wall precursor enzymes near the poles, through association with the IMD(10, 27). Depletion of Wag31 destabilizes the IMD (11).…”