3D reconstruction from electron micrographs: its potential and practical limitations.

The 3D structure of stain-penetrated transmembrane channels of E. coli outer membrane porin has been reconstructed from electron microscopic projections. The averaged unit cells of the projected hexagonal porin lattices were calculated by correlation averaging. Amplitude and phase values along the continuous lattice lines were obtained from the unit cell transforms, and continuous curves were fitted using either a cubic spline-fit or by convolution with an appropriate sinc function. The reconstructed model shows that the three channels of a porin triplet merge to a single channel while traversing the membrane. This is in excellent agreement with results of functional studies. Some technical difficulties and limitations as well as the potential of the reconstruction process are discussed.