PBP1 ofStaphylococcus aureushas multiple essential functions in cell division

Abstract

Bacterial cell division is a complex process requiring the coordination of multiple components, to allow the appropriate spatial and temporal control of septum formation and cell scission. Peptidoglycan (PG) is the major structural component of the septum, and our recent studies in the human pathogen Staphylococcus aureus have revealed a complex, multi–stage PG architecture that develops during septation. Penicillin binding proteins (PBPs) are essential for the final steps of PG biosynthesis — their transpeptidase activity links together the peptide sidechain of nascent glycan strands together. PBP1 is required for cell division in S. aureus and here we demonstrate that it has multiple essential functions associated with its enzymatic activity and as a regulator of division. Loss of PBP1, or just its C–terminal PASTA domains, results in cessation of division at the point of septal plate formation. The PASTA domains can bind PG and thus coordinate the cell division process. The transpeptidase activity of PBP1 is also essential but its loss leads to a strikingly different phenotype of thickened and aberrant septa, which is phenocopied by the morphological effects of adding the PBP1–specific β–lactam, meropenem. Together these results lead to a model for septal PG synthesis where PBP1 enzyme activity is responsible for the characteristic architecture of the septum and PBP1 protein molecules coordinate cell division allowing septal plate formation.

The cell cycle of Staphylococcus aureus: An updated review

“…As mentioned above, the septum consists of separate layers (an inner core of ordered PG with ring-like architecture between two layers of meshlike PG, Figure 1f) (Pasquina-Lemonche et al, 2020) which are likely to be synthesized by independent PG synthesis machinery (Straume et al, 2021). Interestingly, it has also been observed that the very first PG in the septum (often termed "piecrust") is formed independently of PBP1 (Turner et al, 2010;Wacnik et al, 2022). A hypothesis for the division of labor between PG-synthesizing enzymes is that PBP2 is recruited to the septum immediately after the MurJ-mediated flipping of lipid II starts and forms the initial PG (Monteiro et al, 2018;Wacnik et al, 2022).…”

“…Interestingly, it has also been observed that the very first PG in the septum (often termed "piecrust") is formed independently of PBP1 (Turner et al, 2010;Wacnik et al, 2022). A hypothesis for the division of labor between PG-synthesizing enzymes is that PBP2 is recruited to the septum immediately after the MurJ-mediated flipping of lipid II starts and forms the initial PG (Monteiro et al, 2018;Wacnik et al, 2022). Subsequently, the FtsW-PBP1-complex synthesizes the ordered PG in the inner septum core, and this layer then works as a framework for the PBP2-and PBP4-mediated synthesis of the mesh-like outer layers (Wacnik et al, 2022).…”

“…Furthermore, although PBP1 is essential, mutants lacking PBP1 TPase activity are still viable, implying that this protein has other functions in addition to being the primary TPase. These functions are still elusive, however, PBP1 indeed plays a role in septum formation and progression independent of its TPase activity (Wacnik et al, 2022). It could therefore possibly act as a stabilizer of the FtsW-PBP1 complex in the divisome at midcell (Reichmann et al, 2019) and/or stimulate the essential TGase activity of FtsW (Taguchi et al, 2019).…”

“…Despite being essential, its exact function is not known. It has been suggested that while the SEDS-bPBP pairs synthesize the primary PG in the septum, the bifunctional PBP2, may be involved in the synthesis of the mesh-like PG layers (Section 4.2) and/or repair and maintenance of the PG mesh (Straume et al, 2021;Wacnik et al, 2022).…”

“…PBP2a (see Fishovitz et al, 2014 for review) has, in contrast to PBP1-4, low affinity for most β-lactams, and MRSA strains are thus able to perform crosslinking even in the presence of such antibiotics. PBP2a can replace the TPase function of the otherwise essential PBP1 and PBP2 in staphylococcal cells, but the TGase activity of PBP2 remains essential (Pereira et al, 2007;Pinho et al, 2001;Wacnik et al, 2022).…”