Projected structure of the pore-forming OmpC protein from Escherichia coli outer membrane.

A single-projection structure analysis of a bacterial outer membrane protein, OmpC, has been carried out by electron microscopy of frozen hydrated specimens. Two distinct crystal polymorphs have been observed in the frozen-hydrated samples, and projection structures of both forms have been obtained to a resolution of 13.5 A. Preliminary examination of negatively stained samples revealed the expected, trimeric appearance of pores in the OmpC specimens. Electron microscopy of unstained, frozen-hydrated OmpC reveals the trimeric pore structure with equal clarity. In addition, the overall molecular envelope of the protein is readily discerned, and a major lipid-containing domain can also be seen. Because of the small coherent patch size, mosaic disorder, and unpredictable polymorphism of the presently available specimens, three-dimensional reconstruction of frozen-hydrated OmpC has not been carried out.