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 PBP in Escherichia coli. 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 cytoplasmic tail seems to play no role in the dimerization of PBP1b, as was originally suspected, only the full-length protein is able to protect the cells against lysis when both PBP1a and PBP3 are inhibited by antibiotics. This suggests a specific role for the full-length PBP1b in the multienzyme peptidoglycan-synthesizing complex that cannot be fulfilled by either PBP1a or the shorter PBP1b proteins. Moreover, we have shown by alanine-stretch-scanning mutagenesis that (i) residues R(11) to G(13) are major determinants for correct translocation and folding of PBP1b and that (ii) the specific interactions involving the full-length PBP1b can be ascribed to the first six residues at the N-terminal end of the cytoplasmic domain. These results are discussed in terms of the interactions with other components of the murein-synthesizing complex.

青霉素结合蛋白1b（Penicillin-binding protein 1b，PBP1b）是大肠杆菌（Escherichia coli）中主要的高分子量青霉素结合蛋白。尽管其由单个基因编码，但通常以三种亚型的混合物形式存在，各亚型的N端细胞质尾长度存在差异。本研究显示，尽管细胞质尾并未如最初推测的那样参与PBP1b的二聚化过程，但仅全长型PBP1b能够在PBP1a与PBP3均被抗生素抑制时，保护细胞免于裂解。这提示全长型PBP1b在多酶肽聚糖合成复合体（peptidoglycan-synthesizing complex）中承担着特异性功能，且该功能无法由PBP1a或截短型PBP1b替代。此外，本研究通过丙氨酸串列扫描诱变（alanine-stretch-scanning mutagenesis）证实：其一，第11位精氨酸至第13位甘氨酸残基是PBP1b正确转位与折叠的关键决定因素；其二，与全长型PBP1b相关的特异性相互作用可归因于细胞质结构域N端的前6个残基。本文最后就该蛋白与肽聚糖合成复合体（murein-synthesizing complex）其他组分的相互作用展开了讨论。