Projection maps of three members of the KdgM outer membrane protein family

We present the projection structures of the three outer membrane porins KdgM and KdgN from Erwinia chrysanthemi and NanC from Escherichia coli, based on 2D electron crystallography. A wide screening of 2D crystallization conditions yielded tubular crystals of a suitable size and quality to perform high-resolution electron microscopy. Data processing of untilted samples allowed us to separate the information of the two crystalline layers and resulted in projection maps to a resolution of up to 7 Å. All three proteins exhibit a similar putative β-barrel structure and the three crystal forms have the same symmetry. However, there are differences in the packing arrangements of the monomers as well as the densities of the projections. To interpret these projections, secondary structure prediction was performed using β-barrel specific prediction algorithms. The predicted transmembrane β-barrels have a high similarity in the arrangement of the putative β-strands and the loops, but do not match those of OmpG, a related protein porin whose structure was solved.