Abstract: The major penicillin-binding proteins (PBPs) of Escherichia coli play vital roles in cell wall biosynthesis and are located in the inner membrane. The high Mr PBPs 1A, 1B, 2 and 3 are essential bifunctional transglycosylases/transpeptidases which are thought to be type II integral inner membrane proteins with their C-terminal enzymatic domains projecting into the periplasm. The low Mr PBP4 is a DD-carboxypeptidase/endopeptidase, whereas PBPs 5 and are DD-carboxypeptidases. All three low Mr, PBPs act in the modification of peptidoglycan to allow expansion of the sacculus and are thought to be periplasmic proteins attached with varying affinities to the inner membrane via C-terminal amphiphilic α-helices. It is possible that the PBPs and other inner membrane proteins form a peptidoglycan synthesizing complex to coordinate their activities.