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. In doing so, it varies the substrates for transpeptidation and plays a key role in maintaining cell shape. In this study, we have analyzed the oligomeric state of PBP5 in detergent and in its native environment, the inner membrane. Both approaches indicate that PBP5 exists as a homo-oligomeric complex, most likely as a homo-dimer. 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. This model extends our understanding of PBP5 function as it suggests how PBP5 can interact with the peptidoglycan layer. It suggests that the stem domains interact and the catalytic domains have freedom to move from the position observed in the crystal structure. This would allow the catalytic domain to have access to pentapeptides at different distances from the membrane.
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