Atomic resolution 3D structures of biological molecules by electron microscopy

The electron microscope is easily capable of atomic resolution structural analysis. Organic and biological molecules can be examined entirely in their natural state using images and electron diffraction patterns to determine the intrinsic atomic structure of the molecules. The fundamental limitation is electron radiation damage which destroys the initial structure at doses of a few electrons per square Angstrom. This limits the electron dose and ensures that all images are statistically noisy. The only way to overcome the low signal-to-noise ratio is to average images of many individual molecules. The number of images of molecules required to produce a 3D structure is much less using an electron microscope than it would be for an X-ray or neutron microscope because of lower radiation damage. Atomic structures of three different protein molecules have now been determined entirely by low-dose electron microscopy, all of them having resisted extensive efforts to analyse their structure by other methods such as X-ray diffraction. Two of these structures are membrane proteins: bacteriorhodopsin, a light-driven proton pump from Halobacteria, and the light-harvesting complex from green plants which contains most of the world's chlorophyll. The third structure is the microtubule protein tubulin which forms the 'bones' which give cells their internal structure. All three molecules are interesting and important and function at the heart of biology. The structures of all of them have been analysed by electron diffraction and microscopy of two-dimensional crystals, a method which allows the easy averaging of very many individual molecular images. Information from several million molecules has thus been averaged to produce the resulting structure. This method of structure determination by electron microscopy will become even more powerful once it is possible to determine the atomic structure of biological molecules or assemblies from single particles without the need for crystallisation. Work to this end has so far reached a resolution of 7 Angstrom with the work of Bottcher et al.(1) on the structure of hepatitis B virus shells.