Section 8.1.8.4.  Multi-macromolecular complexes

Multi-macromolecular complexes, such as viruses (Rossmann et al., 1985; Acharya et al., 1989; Liddington et al., 1991) (Fig. 8.1.8.2), the nucleosome (Luger et al., 1997), light-harvesting complex (McDermott et al., 1995) and the 13-subunit membrane-bound protein cytochrome c oxidase (Tsukihara et al., 1996), and large-scale molecular assemblies like muscle (Holmes, 1998) are very firmly recognizable as biological entities whose crystal structure determinations rely on SR. These single-crystal structure determinations involve extremely large unit cells and are now tractable despite very weak scattering strength. The crystals often show extreme sensitivity to radiation (hundreds, even a thousand, crystals have been used to constitute a single data set). Cryocrystallography radiation protection is now used extensively in crystallographic data collection on whole ribosome crystals (Hope et al., 1989); SR is essential for this structure determination (Yonath, 1992; Yonath et al., 1998; Ban et al., 1998). These large-scale molecular assemblies often combine electron-microscope and diffraction techniques with SR X-ray crystallography and diffraction for low-to-high resolution detail, respectively. A major surge in results has come from the ESRF, where the X-ray undulator radiation, of incredible intensity and collimation in a number of beamlines (Helliwell, 1987; Miller, 1994; Branden, 1994; Lindley, 1999), has been harnessed to yield atomic level crystal structures of the 780 Å diameter blue tongue virus (Grimes et al., 1997, 1998) and the nucleosome core particle (Luger et al., 1997). A very large multi-protein complex solved using data from the Daresbury SRS wiggler is the F₁ ATPase structure (Fig. 8.1.8.3), for which a share in the Nobel Prize for Chemistry in 1997 was awarded to John Walker in Cambridge. The structure (Abrahams et al., 1994; Abrahams & Leslie, 1996) and the amino-acid sequence data, along with fluorescence microscopy, show how biochemical energy is harnessed to drive the proton pump across biological membranes, thus corroborating hypotheses about this process made over many years. This study, made tractable by the SRS wiggler high-intensity protein crystallography station (Fig. 8.1.4.1), illustrates the considerable further scope possible with yet stronger, more brilliant SR undulator and multipole wiggler sources.

Figure 8.1.8.2| top | pdf | A view of SV40 virus (based on Liddington et al., 1991) determined using data recorded at the SRS wiggler station 9.6 (Fig. 8.1.4.1a).

Figure 8.1.8.3| top | pdf | The protein crystal structure of F1 ATPase, one of the largest non-symmetrical protein structure complexes, solved using SR data recorded at the SRS wiggler 9.6, Daresbury. The scale bar is 20 Å long. Reprinted with permission from Nature (Abrahams et al., 1994). Copyright (1994) MacMillan Magazines Limited.