Electron Microscopy and Image Processing: An Essential Tool for Structural Analysis of Macromolecules

Macromolecular electron microscopy (EM) deals with macromolecular complexes and their placement within the cell—linking the molecular and cellular worlds as a bridge between atomic‐resolution X‐ray crystallographic or NMR studies and lower resolution light microscopy. The amount of specimen required is typically 102 to 103 times less than for X‐ray crystallography or NMR. Electron micrographs of frozen‐hydrated specimens portray native structures. Computer averaging yields enhanced images with reduced noise. Three‐dimensional reconstructions may be computed from multiple views. Under favorable circumstances, resolutions of 7 to 10 Å are achieved. Fitting atomic‐resolution coordinates of components into three‐dimensional density maps gives pseudo‐atomic models of a complex's structure and interactions. Time‐resolved experiments describe conformational changes. Electron tomography allows reconstruction of pleiomorphic complexes and sub‐cellular structures. Electron crystallography has produced near‐atomic resolution models of two‐dimensional arrays, notably of membrane proteins.