Don't let sleeping dogmas lie: new views of peptidoglycan synthesis and its regulation

Abstract

Bacterial cell wall synthesis is the target for some of our most powerful antibiotics and has thus been the subject of intense research focus for more than 50 years. Surprisingly, we still lack a fundamental understanding of how bacteria build, maintain and expand their cell wall. Due to technical limitations, directly testing hypotheses about the coordination and biochemistry of cell wall synthesis enzymes or architecture has been challenging, and interpretation of data has therefore often relied on circumstantial evidence and implicit assumptions. A number of recent papers have exploited new technologies, like single molecule tracking and real-time, high resolution temporal mapping of cell wall synthesis processes, to address fundamental questions of bacterial cell wall biogenesis. The results have challenged established dogmas and it is therefore timely to integrate new data and old observations into a new model of cell wall biogenesis in rod-shaped bacteria.

Figure 1. Unified (A) and Interdependent (B) models of peptidoglycan (PG) synthesis complexes

(A) In the unified model, RodAZ, MreB, aPBPs and bPBPs form one protein complex: guided by MreB, the aPBPs produce peptidoglycan strands via their TG domains while both aPBPs and bPBPs crosslink these strands into a tight PG mesh. (B) In the interdependent model, RodAZ, bPBP and MreB form one complex, while aPBP works in a different spatial and temporal frame. Glycan strands are produced by the transglycosylase RodA and are crosslinked by bPBP to existing PG. PG synthesis provides the force for pushing circumferential MreB movement. aPBPs exhibit a different movement pattern distinct from MreB, including two modes of movement: fast diffusion and slow movement (pause). These two systems are spatially distinct, but functionally interdependent for PG synthesis.

Figure 2. “Break before Make” model of peptidoglycan (PG) synthesis complexes

Rod/SEDS/MreB-associated endopeptidases locally cleave crosslinks in mature PG. RodA generates a PG template, which is attached to the sacculus via bPBPs (only one strand is shown here, note that in principle this could also be a raft structure of multiple parallel strands). The aPBPs then generate additional strands, which are crosslinked with nascent PG on one side and mature PG on the other, ensuring maintenance of structural integrity. Whether the Rod/SEDS/MreB complex interacts with aPBPs remains an open question. Crosslinked pentapeptide (asterisk) is formed when a nascent PG strand containing pentapeptide is crosslinked with another one. PBP-independent 3,3 crosslinks also exist albeit at low abundance under normal growth conditions.