Cryomicroscopy helical structures

Yang, Z., Kollman, J. M., Pandi, L., and Doolittle, R. F. (2001). Crystal structure of native chicken fibrinogen at 2.7 A resolution. Biochemistry 40, 12515-12523. Yonekura, K., Maki-Yonekura, S., and Namba, K. (2003). Complete atomic model of the bacterial flagellar filament by electron cryomicroscopy. Nature 424, 643-650. Zhang, L., and Hermans, J. (1993). Calculation of the pitch of the a-helical coiled coil An addendum. Proteins 17, 217-218. 

Akiba, T., Toyoshima, C., Matsunaga, T., Kawamoto, M., Kubota, T., Fukuyama, K., Namba, K., and Matsubara, H., 1996, Three-dimensional structure of bovine cytochrome bcl complex by electron cryomicroscopy and helical image reconstruction. Nature Struct. Biol. 3 553n561. 

For more complex structures, it is possible to combine electron cryomicroscopy structures with sequence-based secondary structure predictions to interpret the observed secondary structure elements. In the outer shell protein P8 of rice dwarf virus (RDV), where nine helices were predicted in the domain formed by the N and C termini, it was possible to match the lengths of the helices identified in the 3-D density map to those predicted from a consensus secondary structure analyses (Fig. 13a see Color Insert). The connections between the helical densities can be seen in the lower domain of P8, allowing us to establish a rough backbone model for the lower domain of P8 (Zhou et at, 2001). 

Many medium resolution structures of macromolecular assemblies (e.g., ribosomes), spherical and helical viruses, and larger protein molecules have now been determined by electron cryomicroscopy in ice. Four atomic resolution structures have been obtained by electron cryomicroscopy of thin 2D crystals embedded in glucose, trehalose, or tannic acid (11-14), where specimen cooling reduced the effect of radiation damage. One of these, the structure of bacteriorhodopsin (1 l)provided the first structure of a seven-helix membrane protein. The medium resolution density distributions can often be interpreted in terms of the chemistry of the structure if a high resolution model of one or more of the component pieces has already been obtained by X-ray, electron microscopy, or NMR methods. As a result, the use of electron microscopy is becoming a powerful technique for which, in some cases, no alternative approach is possible. Useful reviews [e.g., Dubochet et al. (9), Amos et al. (15), Walz and Grigorieff (16), and Baker et al. (17)] and a book [Frank (18)] have been written. 

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