Structure of the oligogalacturonate-specific KdgM porin.

The phytopathogenic Gram-negative bacterium Dickeya dadantii (Erwinia chrysanthemi) feeds on plant cell walls by secreting pectinases and utilizing the oligogalacturanate products. An outer membrane porin, KdgM, is indispensable for the uptake of these acidic oligosaccharides. Here, the crystal structure of KdgM determined to 1.9 A resolution is presented. KdgM is folded into a regular 12-stranded antiparallel β-barrel with a circular cross-section defining a transmembrane pore with a minimal radius of 3.1 A. Most of the loops that would face the cell exterior in vivo are disordered, but nevertheless mediate contact between densely packed membrane-like layers in the crystal. The channel is lined by two tracks of arginine residues facing each other across the pore, a feature that is conserved within the KdgM family and is likely to facilitate the diffusion of acidic oligosaccharides.