Transpeptidase activity of penicillin-binding protein SpoVD in peptidoglycan synthesis conditionally depends on the disulfide reductase StoA

Endospore cortex peptidoglycan synthesis is not
required for bacterial growth but essential for endo-
spore heat resistance. It therefore constitutes an
amenable system for research on peptidoglycan bio-
genesis. The Bacillus subtilis sporulation-specific
class B penicillin-binding protein (PBP) SpoVD and
many homologous PBPs contain two conserved cys-
teine residues of unknown function in the transpepti-
dase domain – one as residue x in the SxN catalytic
site motif and the other in a flexible loop near the cat-
alytic site. A disulfide bond between these residues
blocks the function of SpoVD in cortex synthesis.
With a combination of experiments with purified pro-
teins and B. subtilis mutant cells, it was shown that
in active SpoVD the two cysteine residues most prob-
ably interact by hydrogen bonding and that this is
important for peptidoglycan synthesis in vivo. It was
furthermore demonstrated that the sporulation-
specific thiol-disulfide oxidoreductase StoA reduces
SpoVD and that requirement of StoA for cortex syn-
thesis can be suppressed by two completely different
types of structural alterations in SpoVD. It is con-
cluded that StoA plays a critical role mainly during
maturation of SpoVD in the forespore outer mem-
brane. The findings advance our understanding of
essential PBPs and redox control of extra-
cytoplasmic protein disulfides in bacterial cells.