Class-A penicillin binding proteins do not contribute to cell shape but repair cell-wall defects

Cell shape and cell-envelope integrity of bacteria are determined by the
peptidoglycan cell wall. In rod-shaped Escherichia coli, two conserved
sets of machinery are essential for cell-wall insertion in the cylindrical
part of the cell: the Rod complex and the class-A penicillin-binding
proteins (aPBPs). While the Rod complex governs rod-like cell shape, aPBP
function is less well understood. aPBPs were previously hypothesized to
either work in concert with the Rod complex or to independently repair
cell-wall defects. First, we demonstrate through modulation of enzyme
levels that aPBPs do not contribute to rod-like cell shape but are
required for mechanical stability, supporting their independent activity.
By combining measurements of cell-wall stiffness, cell-wall insertion, and
PBP1b motion at the single-molecule level, we then present evidence that
PBP1b, the major aPBP, contributes to cell-wall integrity by repairing
cell wall defects.