Pneumococcal CbpD is a murein hydrolase that requires a dual cell envelope binding specificity to kill target cells during fratricide

Abstract

SummaryPneumococci that are competent for natural genetic transformation express a number of proteins involved in binding, uptake, translocation and recombination of DNA. In addition, they attack and lyse noncompetent sister cells present in the same environment. This phenomenon has been termed fratricide. The key effector of pneumococcal fratricide is CbpD, a secreted protein encompassing an N-terminal CHAP domain, two SH3b domains and a C-terminal choline-binding domain (CBD). CbpD is believed to degrade the cell wall of target cells, but experimental evidence supporting this hypothesis has been lacking. Here, we show that CbpD indeed has muralytic activity, and that this activity requires functional CBD and SH3b domains. To better understand the critical role played by the non-catalytic C-terminal region of CbpD, various translational fusions were constructed between the CBD and SH3b domains and green fluorescent protein (GFP). The results showed that the SH3b domains specifically recognize and bind peptidoglycan, while the CBD domain functions as a localization signal that directs CbpD to the septal region of the pneumococcal cell. Intriguingly, transmission electron microscopy analysis revealed that target cells attacked by CbpD ruptures at the septal region, in accordance with the binding specificity displayed by the CBD domain.

Cell Death of Streptococcus mutans Induced by a Quorum-Sensing Peptide Occurs via a Conserved Streptococcal Autolysin

“…The amidase CbpD activates two specific autolysins, LytA and LytC, which in the presence of CbpD and the twopeptide bacteriocin CibAB efficiently degrade the cell walls of noncompetent siblings (40). The lytic enzymes CbpD and LytA and the CibAB bacteriocin are encoded by late competence genes and are consequently an integral part of the CSP-ComDE quorum-sensing system of the bacteria (41,42).…”

Minimal Peptidoglycan (PG) Turnover in Wild-Type and PG Hydrolase and Cell Division Mutants of Streptococcus pneumoniae D39 Growing Planktonically and in Host-Relevant Biofilms

“…S3 in the supplemental material). Once in a biofilm, PG lysis of noncompetent cells by the fratricidal PG hydrolases, CbpD endopeptidase, LytC lysozyme, and LytA amidase (67,(70)(71)(72), releases PG fragments and substantial bacterial DNA. However, these potent innate immune signals must be largely confined to the biofilm matrix, thus preventing interaction with host cells and strong induction of innate and secondary adaptive immune responses.…”

“…PG hydrolases are required for PG turnover in E. coli and B. subtilis (10,12,13). In S. pneumoniae, PG hydrolases carry out three broad functions: remodeling during PG synthesis (e.g., PcsB:FtsEX, LytB, Pmp23, DacA, and DacB) (see references 37, 43, and 50), autolysis in late stationary phase and during antibiotic stress responses (LytA) (71,73), and fratricide during competence (LytA, CbpD, and LytC) (67,(70)(71)(72). PG hydrolases involved in PG remodeling are mostly attached to the outer surface of the cell membrane, and deficiency of these PG hydrolases causes cell morphology defects, including cell rounding, nonparallel division, and chaining ( Fig.…”

Characterization of Mutants Deficient in the l,d -Carboxypeptidase (DacB) and WalRK (VicRK) Regulon, Involved in Peptidoglycan Maturation of Streptococcus pneumoniae Serotype 2 Strain D39

“…Finally, S. pneumoniae produces three PG hydrolases, LytA, CbpD, and LytC (Table 1; Fig. 1), that mediate the autolysis of stationary-phase cultures and the fratricide of noncompetent cells by competent cells (12,32). These three hydrolases do not seem to play obligatory roles in pneumococcal cell division.…”