THE NON-PENICILLIN-BINDING MODULE OF THE TRIPARTITE PENICILLIN-BINDING PROTEIN 3 OF ESCHERICHIA-COLI IS REQUIRED FOR FOLDING AND OR STABILITY OF THE PENICILLIN-BINDING MODULE AND THE MEMBRANE-ANCHORING MODULE CONFERS CELL SEPTATION ACTIVITY ON THE FOLDED STRUCTURE/

The ftsI-encoded multimodular class B penicillin-binding protein 3 (PB P3) is a kev dement of the cell septation machinery of Escherichia col i. Altered ftsI genes were overexpressed, and the gene products were a nalyzed with respect to the level of production, stability, penicillin affinity, and cell septation activity. In contrast to the serine beta -lactamases and low-molecular-mass PBPs which are autonomous folding e ntities, the S-259-to-V-577 penicillin-binding module of M-1-to-V-577 PBP3 lacks the amino acid sequence information for correct folding, Th e missing piece of information is provided by the associated G-57-to-E -258 non-penicillin-binding module which functions as a noncleaved, ps eudointramolecular chaperone. Key elements of the folding information reside within the motif 1-containing R-60-to-W-110 polypeptide segment and within G-188-to-D-197 motif 3 of the n-PB module. The intermodule interaction is discussed in the light of the known three-dimensional structure (at 3.5-Angstrom [0.35-nm] resolution) of the analogous clas s B PBP2x of Streptococcus pneumoniae (S. Pares, N. Mouz, Y. Petillot, R, Hakenbeck, and O. Dideberg, Nature Struct. Biol, 3:284-289, 1996), Correct folding and adoption of a stable penicillin-binding conformat ion are necessary but not sufficient to confer cell septation activity to PBP3 in exponentially growing cells, The in vivo activity of PBP3 also depends on the M-1-to-E-56 amino-terminal module which encompasse s the cytosol, the membrane, and the periplasm and which functions as a noncleaved pseudo-signal peptide.