Three-dimensional (3D) image reconstruction from electron micrographs is providing a wealth of new structural information on complexes formerly intractable to solution. Nowhere will this technique be more important than in the analysis of multisubunit membrane-embedded complexes, where the nature of the membrane interactions makes crystallization of the intact complex exceedingly difficult. This is especially true in the case of highly dynamic membrane-embedded complexes, for which formation of perfect crystals is highly improbable.
The study by Ahting et al. 1999 represents a case in point. The TOM complex in the mitochondrial outer membrane consists of at least six distinct protein subunits, and functions as the gateway through which nuclear-encoded polypeptides can be imported into a mitochondrion. The TOM complex must recognize mitochondrial precursor proteins from amongst all other nascent polypeptides synthesized in the cytosol, bind the mitochondrial precursor proteins...
