International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 19.2, p. 427   | 1 | 2 |

Section 19.2.5. Future development

W. Chiua*

aVerna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA
Correspondence e-mail: wah@bcm.tmc.edu

19.2.5. Future development

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Electron crystallography has proven to be a high-resolution structural tool for two-dimensional protein crystals, to the point where the polypeptide backbone can be traced and atomic coordinates derived. Needless to say, there is still much to be learned about how to make highly ordered two-dimensional crystals from either membrane or soluble proteins. Research in this direction is critical for the growth of electron crystallography. Recent results have promoted optimism; there has been an increase in the number of membrane proteins crystallized into two-dimensional arrays from which at least 6 to 8 Å structures can be obtained (Walz et al., 1997[link]; Auer et al., 1998[link]; Zhang et al., 1998[link]; Unger et al., 1999[link]).

In the most recent high-resolution structural study of tubulin, a 3.7 Å map was obtained from 100 electron diffraction patterns and 150 electron images. Effectively, this structure was the result of a computational average of about one million tubulin dimers. It took six years to determine the structure from the time when the first high-resolution crystal structure was reported (Downing & Jontes, 1992[link]; Nogales et al., 1998[link]). All the experimental and computational procedures were basically the same as those developed for bacteriorhodopsin (Henderson et al., 1990[link]). An obvious future development in protein electron crystallography would be aimed at improving the throughput of the structural determination. This entails a search for better solutions to some of the technical problems mentioned above as well as the introduction of automation in both data collection and processing.

Finally, another potentially exciting aspect of electron crystallography is the ability to detect charged residues from the high scattering differences between neutral and charged atoms. This physical property may make electron crystallography a unique method for detecting the ionization state of the amino-acid residues in proteins (Mitsuoka et al., 1999[link]). Furthermore, there is also a good prospect of extending the structure close to 2 Å resolution, as the next generation of electron cryomicroscope will be equipped with a field emission gun operated at 300 keV, a liquid helium cryo-specimen stage and an energy filter. This combination of instrumental features is likely to bring electron crystallography a step closer to its ultimate potential for structural biology research at the atomic level.

References

Auer, M., Scarborough, G. A. & Kühlbrandt, W. (1998). Three-dimensional map of the plasma membrane H+-ATPase in the open conformation. Nature (London), 392, 840–843.Google Scholar
Downing, K. H. & Jontes, J. (1992). Projection map of tubulin in zinc-induced sheets at 4 Å resolution. J. Struct. Biol. 109, 152–159.Google Scholar
Henderson, R., Baldwin, J. M., Ceska, T. A., Zemlin, F., Beckmann, E. & Downing, K. H. (1990). Model for the structure of bacteriorhodopsin based on high-resolution electron cryo-microscopy. J. Mol. Biol. 213, 899–929.Google Scholar
Mitsuoka, K., Hirai, T., Murata, K., Miyazawa, A., Kidera, A., Kimura, Y. & Fujiyoshi, Y. (1999). The structure of bacteriorhodopsin at 3.0 Å resolution based on electron crystallography: implication of the charge distribution. J. Mol. Biol. 286, 861–882.Google Scholar
Nogales, E., Wolf, S. G. & Downing, K. H. (1998). Structure of the alpha beta tubulin dimer by electron crystallography. Nature (London), 391, 199–203.Google Scholar
Unger, V. M., Kumar, N. M., Gilula, N. B. & Yeager, M. (1999). Three-dimensional structure of a recombinant gap junction membrane channel. Science, 283, 1176–1180.Google Scholar
Walz, T., Hirai, T., Murata, K., Heymann, J. B., Mitsuoka, K., Fujiyoshi, Y., Smith, B. L., Agre, P. & Engel, A. (1997). The three-dimensional structure of aquaporin-1. Nature (London), 387, 624–627.Google Scholar
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