Figure 1.

Disruption of SD2–SD1 contact is a function of occupancy. Three-dimensional reconstructions of pure F-actin (a), pADF-F-actin complex, pH 6.5 (b and c), and pADF-F-actin complex, pH 7.7 (d). Surfaces are shown at the top, while cross sections of the respective reconstructions are shown at the bottom. The four actin subdomains are labeled 1–4, and four successive protomers are indicated as a–d. The additional mass due to the AC protein bound at the primary site is indicated by the AC labels in b–d. The averaged pH 6.5 complex (b) has been found to contain a variable amount of bound pADF. A subset corresponding to nearly saturated binding (c) looks very similar to the pH 7.7 complex (d) that was much more homogeneous with respect to pADF binding, showing that the differences between b and d can mainly be explained by different amounts of pADF bound. It can be seen in the cross sections that more pADF is bound in the averaged pH 7.7 reconstruction (d) than in the high occupancy pH 6.5 reconstruction (c). The red arrows (a and b) mark the density of SD2, which forms a link with SD1 of the protomer above on the same long-pitch helical strand. This link is still present in b when there is a partial occupancy by pADF, but the density is attenuated in comparison with that present in pure F-actin (a). With greater occupancy at pH 6.5 (c) or at pH 7.7 (d), this link is absent (black arrow, c and d).