PBP1 of Staphylococcus aureus has multiple essential functions in cell division

Bacterial cell division is a complex process requiring the coordination of multiple components, to allow the appropriate spatial and temporal control of septum formation and cell scission. Peptidoglycan (PG) is the major structural component of the septum, and our recent studies in the human pathogen Staphylococcus aureus have revealed a complex, multi-stage PG architecture that develops during septation. Penicillin binding proteins (PBPs) are essential for the final steps of PG biosynthesis -- their transpeptidase activity links together the peptide sidechain of nascent glycan strands together. PBP1 is required for cell division in S. aureus and here we demonstrate that it has multiple essential functions associated with its enzymatic activity and as a regulator of division. Loss of PBP1, or just its C-terminal PASTA domains, results in cessation of division at the point of septal plate formation. The PASTA domains can bind PG and thus coordinate the cell division process. The transpeptidase activity of PBP1 is also essential but its loss leads to a strikingly different phenotype of thickened and aberrant septa, which is phenocopied by the morphological effects of adding the PBP1-specific {beta}-lactam, meropenem. Together these results lead to a model for septal PG synthesis where PBP1 enzyme activity is responsible for the characteristic architecture of the septum and PBP1 protein molecules coordinate cell division allowing septal plate formation.