title: Pneumococcal CbpD is a murein hydrolase that requires a dual cell envelope binding specificity to kill target cells during fratricide.
abstract: Pneumococci 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 non-competent 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.
authors.affiliation: Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, N-1432, As, Norway.
authors.forename: Vegard
authors.initials: V
authors.lastname: Eldholm
authors.forename: Ola
authors.initials: O
authors.lastname: Johnsborg
authors.forename: Daniel
authors.initials: D
authors.lastname: Straume
authors.forename: Hilde Solheim
authors.initials: HS
authors.lastname: Ohnstad
authors.forename: Kari Helene
authors.initials: KH
authors.lastname: Berg
authors.forename: Juan A
authors.initials: JA
authors.lastname: Hermoso
authors.forename: Leiv Sigve
authors.initials: LS
authors.lastname: Håvarstein
mesh_terms: D000595:Amino Acid Sequence; D002473:Cell Wall / Q000378:metabolism*; D002794:Choline / Q000378:metabolism*; D046529:Microscopy, Electron, Transmission; D008969:Molecular Sequence Data; D009238:N-Acetylmuramoyl-L-alanine Amidase / Q000235:genetics / Q000378:metabolism*; D010457:Peptidoglycan / Q000378:metabolism; D013296:Streptococcus pneumoniae / Q000201:enzymology* / Q000235:genetics / Q000648:ultrastructure; D018909:src Homology Domains
chemical_list: D010457:Peptidoglycan; D009238:N-Acetylmuramoyl-L-alanine Amidase; D002794:Choline