Uncovering the Biological Function of the Peptidoglycan Hydrolase PcsB in Streptococcus pneumoniae

In the human commensal Gram-positive bacterial pathogen Streptococcus pneumoniae, the essential cell-division-associated peptidoglycan (PG) hydrolase PcsB interacts directly with the membrane-bound FtsEX complex (1-3). PcsB contains a cysteine, histidine-dependent amidohydrolase/peptidase (CHAP) domain responsible for PG hydrolysis, and a coiled-coil domain required for interaction with FtsEX (1,4). ATP hydrolysis of FtsE in the cytoplasm drives conformational changes in FtsX in the cytoplasmic membrane, which ultimately regulates the PG hydrolase on the outside of the cell (5). In this work we show that the CHAP domain of PcsB predominately functions as an D-iso-Glutaminyl-Lysyl D,L-endopeptidase, with substrate specificity for Lys-containing, amidated PG. The catalytic activity of PcsB is regulated by conformational changes of the coiled-coil region and by a short helical region immediately adjacent to the CHAP domain, to guard against PcsB hydrolytic activation outside of its cell division specific functional requirement. This work supports a model for the overall biological activity of the FtsEX-PcsB complex, allowing stem peptide cleavage that splits the septal disk and marks a region of the peptidoglycan sacculus for subsequent cell division remodelling.