Evidence for the Supramolecular Organization of a Bacterial Outer-Membrane Protein from In Vivo Pulse Electron Paramagnetic Resonance Spectroscopy

In
the outer membrane of Gram-negative bacteria, membrane proteins
are thought to be organized into domains or islands that play a role
in the segregation, movement, and turnover of membrane components.
However, there is presently limited information on the structure of
these domains or the molecular interactions that mediate domain formation.
In the present work, the Escherichia coli outer membrane
vitamin B12 transporter, BtuB, was spin-labeled, and double
electron–electron resonance was used to measure the distances
between proteins in intact cells. These data together with Monte Carlo
simulations provide evidence for the presence of specific intermolecular
contacts between BtuB monomers that could drive the formation of string-like
oligomers. Moreover, the EPR data provide evidence for the location
of the interacting interfaces and indicate that lipopolysaccharide
mediates the contacts between BtuB monomers.