Regulation of Peptidoglycan Synthesis by Outer-Membrane Proteins

Abstract

SummaryGrowth of the meshlike peptidoglycan (PG) sacculus located between the bacterial inner and outer membranes (OM) is tightly regulated to ensure cellular integrity, maintain cell shape and orchestrate division. Cytoskeletal elements direct placement and activity of PG synthases from inside the cell but precise spatiotemporal control over this process is poorly understood. We demonstrate that PG synthases are also controlled from outside the sacculus. Two OM lipoproteins, LpoA and LpoB, are essential for the function respectively of PBP1A and PBP1B, the major E. coli bifunctional PG synthases. Each Lpo protein binds specifically to its cognate PBP and stimulates its transpeptidase activity, thereby facilitating attachment of new PG to the sacculus. LpoB shows partial septal localization and our data suggest that the LpoB-PBP1B complex contributes to OM constriction during cell division. LpoA/ LpoB and their PBP docking regions are restricted to γ-proteobacteria, providing models for niche-specific regulation of sacculus growth.

Cofactor bypass variants reveal a conformational control mechanism governing cell wall polymerase activity

“…LpoA is specifically required for PBP1a function, whereas LpoB is specifically required for PBP1b function (9,10). Although they are unrelated, LpoA and LpoB both span the periplasm to interact directly with their cognate PBP (11)(12)(13), and when added to in vitro reactions, they modulate the PGT and TP activities of their target synthases via different mechanisms (9,10,14). LpoA activates the TP activity of PBP1a, which indirectly stimulates its PGT activity, indicating a coupling between the two active sites (10,14).…”

“…We used this synthetic lethal phenotype to identify outer membrane lipoproteins that are required for the in vivo function of the aPBPs (9). LpoA is specifically required for PBP1a function, whereas LpoB is specifically required for PBP1b function (9,10). Although they are unrelated, LpoA and LpoB both span the periplasm to interact directly with their cognate PBP (11)(12)(13), and when added to in vitro reactions, they modulate the PGT and TP activities of their target synthases via different mechanisms (9,10,14).…”

“…Although they are unrelated, LpoA and LpoB both span the periplasm to interact directly with their cognate PBP (11)(12)(13), and when added to in vitro reactions, they modulate the PGT and TP activities of their target synthases via different mechanisms (9,10,14). LpoA activates the TP activity of PBP1a, which indirectly stimulates its PGT activity, indicating a coupling between the two active sites (10,14). Similarly, LpoB stimulates the PGT activity of PBP1b, which in turn provides more material for crosslinking by the TP domain (9,12).…”

Dynamic protein complexes for cell growth

“…A similar redundancy exists for the hydrolases involved in cell elongation (8). Furthermore, in E. coli, the bifunctional PG synthases, PBP1A and PBP1B, have dedicated regulators, LpoA and LpoB, which localize independently to elongation and divisions sites, respectively (9,10). In contrast, some PG hydrolases require their regulators for localization (11,12), whereas others do not (13).…”

“…It is expected that the process will be significantly different among microbes: many subunits are not ubiquitously conserved, cytoskeletal elements can vary in structure and function (14), recruitment hierarchy is different even in organisms with conserved subunits (15), and niche-specific regulators of PG growth and interchangeability of enzymatic components seem to be prevalent (4,10). Mapping the differences and also linking them to the physiology of diverse organisms will be in the core of future studies in the field and will help us dissect the general underlying principles of these machineries from the nichespecific ones.…”