11 – 19 Additionally, it has been shown that many of the PBPs can form homo-dimers. 1, 8 – 10 Although such complexes are yet to be identified, a number of interactions have been detected, which support their existence. To coordinate these various biochemical activities, it has been hypothesized that the PBPs assemble into hetero-oligomeric protein complexes. The high molecular weight PBPs (PBP1A, 1B, 1C, 2, and 3) are transglycosylases and/or transpeptidases, whereas the low molecular weight PBPs (PBP4, 5, 6, 6B, and 7/8) are dd-carboxypeptidases and/or endopeptidases (reviewed in Refs. The PBPs have varied functions related to peptidoglycan biosynthesis. 4 This process is coordinated by a group of proteins collectively known as the penicillin-binding proteins (PBPs), as they are the targets of penicillin and penicillin-based antibiotics. 1 – 3 During bacterial growth and cell division, the peptidoglycan layer is continuously remodeled and turned over. The peptidoglycan layer is constructed from stiff glycan chains that are cross-linked with short flexible oligopeptides and is situated between the inner and outer membranes. Gram-negative bacteria are constrained by a macromolecular “net” called the peptidoglycan layer, which is important for protection against osmotic stress. This would allow the catalytic domain to have access to pentapeptides at different distances from the membrane. It suggests that the stem domains interact and the catalytic domains have freedom to move from the position observed in the crystal structure. This model extends our understanding of PBP5 function as it suggests how PBP5 can interact with the peptidoglycan layer. As the crystal structure of the soluble domain of PBP5 (i.e., lacking the membrane anchor) shows a monomer, we used our experimental data to generate a model of the homo-dimer. Both approaches indicate that PBP5 exists as a homo-oligomeric complex, most likely as a homo-dimer. In this study, we have analyzed the oligomeric state of PBP5 in detergent and in its native environment, the inner membrane. In doing so, it varies the substrates for transpeptidation and plays a key role in maintaining cell shape.
Penicillin-binding protein 5 (PBP5) is a dd-carboxypeptidase, which cleaves the terminal d-alanine from the muramyl pentapeptide in the peptidoglycan layer of Escherichia coli and other bacteria.
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