Class A PBPs have a distinct and unique role in the construction of the pneumococcal cell wall

Abstract

In oval-shaped Streptococcus pneumoniae, septal and longitudinal peptidoglycan syntheses are performed by independent functional complexes: the divisome and the elongasome. Penicillin-binding proteins (PBPs) were long considered the key peptidoglycan-synthesizing enzymes in these complexes. Among these were the bifunctional class A PBPs, which are both glycosyltransferases and transpeptidases, and monofunctional class B PBPs with only transpeptidase activity. Recently, however, it was established that the monofunctional class B PBPs work together with transmembrane glycosyltransferases (FtsW and RodA) from the shape, elongation, division, and sporulation (SEDS) family to make up the core peptidoglycan-synthesizing machineries within the pneumococcal divisome (FtsW/PBP2x) and elongasome (RodA/PBP2b). The function of class A PBPs is therefore now an open question. Here we utilize the peptidoglycan hydrolase CbpD that targets the septum of S. pneumoniae cells to show that class A PBPs have an autonomous role during pneumococcal cell wall synthesis. Using assays to specifically inhibit the function of PBP2x and FtsW, we demonstrate that CbpD attacks nascent peptidoglycan synthesized by the divisome. Notably, class A PBPs could process this nascent peptidoglycan from a CbpD-sensitive to a CbpD-resistant form. The class A PBP-mediated processing was independent of divisome and elongasome activities. Class A PBPs thus constitute an autonomous functional entity which processes recently formed peptidoglycan synthesized by FtsW/PBP2×. Our results support a model in which mature pneumococcal peptidoglycan is synthesized by three functional entities, the divisome, the elongasome, and bifunctional PBPs. The latter modify existing peptidoglycan but are probably not involved in primary peptidoglycan synthesis. class A PBPs | CbpD | peptidoglycan | Streptococcus pneumoniae

Structural Characterization of the Essential Cell Division Protein FtsE and Its Interaction with FtsX in Streptococcus pneumoniae

“…We previously reported that pneumococci form very long chains when PBP2b, a key component of the elongasome, is depleted. Furthermore, transmission electron microscopy analysis of these cells revealed that their septal cross walls have not been cleaved (40). Thus,…”

“…We previously reported that pneumococci form very long chains when PBP2b, a key component of the elongasome, is depleted. Furthermore, transmission electron microscopy analysis of these cells revealed that their septal cross walls have not been cleaved ( 40 ). Thus, when the elongasome is no longer functional, the separation of daughter cells mediated by the FtsEX-PcsB complex does not take place.…”

Organization of Peptidoglycan Synthesis in Nodes and Separate Rings at Different Stages of Cell Division ofStreptococcus pneumoniae

“…PG synthesis is carried out by distinct septal PG (sPG) and peripheral PG (pPG) modes (9)(10)(11). sPG synthesis produces the cross wall that separates daughter cells (12)(13)(14), whereas concurrent pPG synthesis elongates daughter cells from midcell to form ovoid-shaped cells (Fig. S1) (11)(12)(13)(14).…”

A CozE Homolog Contributes to Cell Size Homeostasis of Streptococcus pneumoniae

“…Bifunctional class A PBPs in S. pneumoniae were recently shown to have unique roles, independent of the monofunctional class B PBPs, in S. pneumoniae ( 21 ). The exact biological function of class A PBPs during cell wall synthesis remains to be determined, but recent results indicate that their main role is repair, remodeling, or strengthening of PG rather than primary PG synthesis ( 21 , 22 , 39 ). Consequently, the activity of these PBPs needs strict control in time and space to maintain the cell wall homeostasis ensuring the cell shape and integrity during the division process.…”

“…In contrast, the role of class A PBPs is less clear, and notably, their numbers vary from one species to the other. It was recently shown, however, that class A PBPs can work independently of the SEDS–class B PBP complexes and contribute to the remodeling of the PG mesh required during expansion and/or after cell wall damages ( 21 , 22 ). The class A PBPs PBP1a, PBP1b, and PBP2a encoded by the pneumococcus are not essential, and individual deletion leads to weak morphological and growth defects.…”