Elucidation of the structure of the membrane anchor of penicillin-binding protein 5 of Escherichia coli.

Penicillin-binding protein 5 (PBP 5) of Escherichia coli is a membrane-bound cell wall dd-carboxypeptidase, localized in the outer leaflet of the cytosolic membrane of this Gram-negative bacterium. Not only is it the most abundant PBP of E. coli, but it is as well a target for penicillins and is the most studied of the PBP enzymes. PBP 5, as a representative peripheral membrane protein, is anchored to the cytoplasmic membrane by the 21 amino acids of its C-terminus. Although the importance of this terminus as a membrane anchor is well recognized, the structure of this anchor was previously unknown. Using natural isotope abundance NMR, the structure of the PBP 5 anchor peptide within a micelle was determined. The structure conforms to a helix-bend-helix-turn-helix motif and reveals that the anchor enters the membrane so as to form an amphiphilic structure within the interface of the hydrophilic/hydrophobic boundary regions near the lipid head groups. The bend and the turn within the motif allow the C-terminus to exit from the same side of the membrane that is penetrated. The PBP anchor sequences represent extraordinary diversity, encompassing both N-terminal and C-terminal anchoring domains. This study establishes a surface adherence mechanism for the PBP 5 C-terminus anchor peptide, as the structural basis for further study toward understanding the role of these domains in selecting membrane environments and in the assembly of the multienzyme hyperstructures of bacterial cell wall biosynthesis.