Mini ReviewHigh resolution structural studies of complex icosahedral viruses: a brief overview
Introduction
X-ray crystallography and electron cryomicroscopy have been the most useful structural tools for studying viruses and virus like particles (Baker and Johnson, 1997, Grimes et al., 1999, Johnson and Chiu, 2000, Thuman-Commike and Chiu, 2000). Each technique has its advantages and drawbacks. X-ray crystallography allows structures to be determined close to or at atomic resolution. However, it depends upon the availability of large amount (tens of mg) of highly purified particles, which must be able to form large crystals (in fraction of mm3 size). Such material preparations may be difficult or impossible to obtain. In contrast, at present although, electron cryomicroscopy can only provide structures at intermediate (∼7 Å) (Böttcher et al., 1997, Conway et al., 1997, Zhou et al., 2001, Zhou et al., 2000) to low resolution, it can be carried out using much smaller amounts of sample and does not require crystal formation. Consequently, it can be used for less purified samples and for samples containing heterogeneous mixtures of particle types (Thuman-Commike et al., 1998).
It is important to note that these two methodologies are not mutually exclusive. In some cases, more information may be gained from combining the two approaches than either approach can generate on its own (Grimes et al., 1999). Thus, the low resolution structures generated by electron cryomicroscopy do not provide sufficient structural information to understand biological processes in detail. For example, it is not possible to determine definitively which residues are involved in particular interactions or the nature of any chemical processes actually taking place. However, such low resolution structures can still provide valuable information on the overall organization of protein subunits in a complex viral particle (Baker et al., 1999). They can be particularly informative when the crystal structure of a viral particle in one functional state is known. Lower resolution electron cryomicroscopic structures of other functional states can then be interpreted using this high resolution template to help in understanding the changes taking place. This approach has proved very useful for studying virus morphogenesis (Lata et al., 2000) and for determining the sites of interaction with cellular receptors (Belnap et al., 2000, He et al., 2000, Rossmann et al., 1994) or with antibodies (Chiu and Smith, 1994, Smith and Mosser, 1997). The combination of crystallographic and cryomicroscopic analyses is proving to be a very powerful tool for analyzing the functional and mechanistic processes affecting virus particles.
Although both structural techniques have been widely used for examining small to medium sized viral particles, studying large viral particles (with diameters larger than 700 Å) by either X-ray crystallography or electron cryomicroscopy remains a technical challenge. In the case of crystallography, the initial challenge lies primarily in the search for successful crystallization conditions (Fry et al., 1999). For electron cryomicroscopy, the problem has been the lack of suitable software for dealing with the very noisy images and handling the large number of large size particle images (Saad et al., 2001, Zhou et al., 1998). However, technical advances have been made in both the fields that allow the study of large particles at higher resolution. To illustrate the increasing capabilities of these different approaches, we will describe two examples of icosahedral viral particles of considerable complexity and large size that have been studied using these structural techniques.
Section snippets
Bluetongue virus (BTV)
The BTV virion consists of an outer capsid shell surrounding a core particle that contains the ten double stranded RNA genome segments. The core particle, which remains intact throughout the early stages of infection, is made up of one protein, VP7 (38 kDa), surrounding an inner layer of a second protein, VP3 (100 kDa). This entire particle has been crystallized, forming crystals with a longest dimension of between 0.3 and 0.4 mm (Grimes et al., 1998). The unit cell dimensions are a=795.6, b
Herpesvirus
Herpesvirus virions are large particles with diameters of ∼2000 Å (Steven and Spear, 1997). They are composed of four compartments, envelope, tegument, nucleocapsid and double stranded DNA core. The tegument and envelope are poorly defined structurally, and can vary markedly in composition and dimensions. This makes the intact virion a poor subject for structural investigation. By contrast, the nucleocapsid is extremely uniform and has been extensively studied by electron cryomicroscopy. Upon
Future possibilities
The structural results shown in the above two examples have answered some, but not all, of the basic questions relevant to the particles functions. There will be many challenges in further extending these studies but the potential reward in terms of improved understanding of important biological processes makes the effort worthwhile. One such challenge is to apply the existing technologies to the study of viruses in different functional states so that particular biochemical or biological
Acknowledgments
We thank Matthew Baker for preparing the figures. This research was supported by NIH (grant numbers AI38469 and P41RR02250).
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