Structure and Dynamics of Porin Loops in Asymmetric Outer Membranes of E. coli.
Biological membranes are usually asymmetric lipid bilayers that possess an oligosaccharide coat on the outside into which loops of transmembrane proteins extend. Structurally, the protein-bilayer interface is not well explored. As a paradigmatic example, we examine the structure of the bacterial outer membrane protein G (OmpG) expressed into E. coli outer membranes (OM). We investigate two strands of the β-sheet and loops 6 and 7 that extend into the lipopolysaccharide (LPS) layer and which are involved in pore opening (pH > 6.0) and closing (pH < 6.0). Previous x-ray crystallography studies described these loops as well-ordered, whereas solid-state NMR described them as fully disordered, largely due to a lack of signal. Here, solid-state NMR measurements of OmpG in OM vesicles at 1.2 GHz 1H frequency now yield extended sets of cross peaks for His and Trp residues. Two alternative conformations are observed for two His residues, which are likely H231 and H261. Trp sidechain cross peak intensities in cross polarization spectra are in agreement with an increase of conformational freedom toward the surface. The oligosaccharide layer appears to protect the loops while promoting flexibility and multiple conformations. Our investigation highlights the importance of structural investigations in a quasi-native environment.