Structural studies of the Cpx pathway activator NIpE on the outer membrane of Escherichia coli

NIpE, an outer membrane lipoprotein, functions during envelope stress responses in Gram-negative bacteria. In Escherichia coli, adhesion to abiotic surfaces has been reported to activate the Cpx pathway in an NIpE-dependent manner. External copper ions are also thought to activate the Cpx pathway mediated by NIpE. We determined the crystal structure of NIpE from E. coli at 2.6 angstrom resolution. The structure showed that NIpE consists of two P barrel domains. The N-terminal domain resembles the bacterial lipocalin Blc, and the C-terminal domain has an oligonucleotide/oligosaccharide-binding (013) fold. From both biochemical analyses and the crystal structure, it can be deduced that the cysteine residues in the CXXC motif may be chemically active. Furthermore, two monomers in the asymmetric unit form an unusual 3D domain-swapped dimer. These findings indicate that tertiary and/or quaternary structural instability may be responsible for Cpx pathway activation.