Essential PcsB putative peptidoglycan hydrolase interacts with the essential FtsXSpn cell division protein in Streptococcus pneumoniae D39

The connection between peptidoglycan remodeling and cell division is poorly understood in ellipsoid-shaped ovococcus bacteria, such as the human respiratory pathogen Streptococcus pneumoniae. In S. pneumoniae, peptidoglycan homeostasis and stress are regulated by the WalRK (VicRK) two-component regulatory system, which positively regulates expression of the essential PcsB cysteine-and histidine-dependent aminohydrolases/peptidases (CHAP)-domain protein. CHAP-domain proteins usually act as peptidoglycan hydrolases, but purified PcsB lacks detectable enzymatic activity. To explore the functions of PcsB, its subcellular localization was determined. Fractionation experiments showed that cell-bound PcsB was located through hydrophobic interactions on the external membrane surface of pneumococcal cells. Immunofluorescent microscopy localized PcsB mainly to the septa and equators of dividing cells. Chemical cross-linking combined with immunoprecipitation showed that PcsB interacts with the cell division complex formed by membrane-bound FtsX(Spn) and cytoplasmic FtsE(Spn) ATPase, which structurally resemble an ABC transporter. Far Western blotting showed that this interaction was likely through the large extracellular loop of FtsXSpn and the amino terminal coiled-coil domain of PcsB. Unlike in Bacillus subtilis and Escherichia coli, we show that FtsXSpn and FtsESpn are essential in S. pneumoniae. Consistent with an interaction between PcsB and FtsXSpn, cells depleted of PcsB or FtsXSpn had strikingly similar defects in cell division, and depletion of FtsXSpn caused mislocalization of PcsB but not the FtsZ(Spn) early-division protein. A model is presented in which the interaction of the FtsEX(Spn) complex with PcsB activates its peptidoglycan hydrolysis activity and couples peptidoglycan remodeling to pneumococcal cell division.