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Figure 4
Impact of specimen tilt on definition of α-helices. Gap-junction channels directly connect two adjacent cells. Each of the channel's 12 subunits adopts an α-helical fold within its membrane-spanning part (Unger et al., 1999BB28). The α-helices can be resolved as soon as the data sample views of the molecule at tilt angles equal to the inclination angle of the most highly tilted α-helix in the structure. To illustrate this fact, three 3D density maps were calculated from data corresponding to maximum specimen tilts of (a) 5°, (b) 15° and (c) 30°, respectively. An inverse B factor of −350 Å2 was applied to correct the resolution-dependent fall-off of the image-derived amplitudes. Shown are identical cross-sections from each of these maps. The sections are contoured in steps of σ with solid lines representing contour levels above the mean. Even a specimen tilt of only 5° suffices to resolve two of the α-helices. However, the more highly tilted α-helices remain ill-defined. Inclusion of data from crystal tilts up to 15° completely resolves the two α-helices that are oriented almost perpendicular to the membrane plane. Furthermore, the two more highly tilted α-helices start being recognisable. At 30° of specimen tilt, the vertical resolution is sufficient to resolve all of the α-helices. Nevertheless, the cross-section through the most highly tilted α-helix is still elliptical in shape, indicating that its tilt is very close to the maximum specimen tilt covered by the data. Addition of data from even more highly tilted crystals would improve the definition of each of the α-helices. However, the main conclusion drawn from this section of the 3D density-map will remain unchanged, i.e. each of the six connexin subunits has four α-helical transmembrane domains.

Journal logoBIOLOGICAL
ISSN: 1399-0047
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