Differential responses of Escherichia coli cells expressing cytoplasmic domain mutants of penicillin-binding protein 1b after impairment of penicillin-binding proteins 1a and 3

Penicillin-binding protein 1b (PBP1b) is the major high-molecular-weight PB P in Escherichia call. Although it is coded by a single gene, it is usually found as a mixture of three isoforms which vary with regard to the length of their N-terminal cytoplasmic tail. We show here that although the cytopl asmic tail seems to play no role in the dimerization of PBP1b as was origin ally suspected, only the full-length protein is able to protect the cells a gainst lysis when both PBP1a and PBP3 are inhibited by antibiotics. This su ggests a specific role for the full-length PBP1b in the multienzyme peptido glycan-synthesizing complex that cannot be fulfilled by either PBP1a or the shorter PBP1b proteins. Moreover, we have shown by alanine-stretch-scannin g mutagenesis that (i) residues R-11 to G(13) are major determinants for co rrect translocation and folding of PBP1b and that (ii) the specific interac tions involving the full-length PBP1b can be ascribed to the first six resi dues at the N-terminal end of the cytoplasmic domain. These results are dis cussed in terms of the interactions with other components of the murein-syn thesizing complex.