THE STRUCTURE AND FUNCTION OF ESCHERICHIA-COLI PENICILLIN-BINDING PROTEIN-3

Escherichia coli penicillin-binding protein PBP3 is a key element in c ell septation. It is presumed to catalyse a transpeptidation reaction during biosynthesis of the septum peptidoglycan but, in vitro, its enz ymatic activity has only been demonstrated with thiolester analogues o f the natural peptide substrate. It has no detectable transglycosylase activity with lipid II as substrate. This tripartite protein is const ructed of an N-terminal membrane anchor-containing module that is esse ntial for cell septation, a non-penicillin-binding (n-PB) module of un known function and a C-terminal penicillin-binding (PB) module exhibit ing all the characteristic motifs of penicilloyl serine transferases. The n-PB module, which is required for the folding and stability of th e PB module, may provide recognition sites for other cell division pro teins. Initiation of septum formation is not PBP3-dependent but rests on the appearance of the FtsZ ring, and is thus penicillin-insensitive . The control of PBP3 activity during the cell cycle is briefly discus sed.