Figure 3.
Atomic models of 2D actin arrays cross-linked by fimbrin. The spatial relationships between the filaments for the two types of arrays were deduced from the observed diffraction patterns (Fig. 1B and Fig. C). The filaments in the micrographs with straight crossbands are in register. The filaments in the micrographs with slanted crossbands are rotated by 26.66° in respect to their neighbors. This operation exposes the same actin interface only one notch down (also Fig. 2 C and its corresponding legend). (A–C) Model of the straight crossband with filaments in register. Actin is shown in gray, ABD1 in pink, ABD2 in blue, and the NH2-terminal calcium-binding domain (EF-hands) in cyan. The spatial relationship between the filaments and the ABDs was taken from the atomic model of actin–ABD1 (Hanein et al. 1998). The position of the calcium-binding domain was deduced from the difference peak between the docked model and the observed 3D reconstruction (solid red peak in Figure 3 of Hanein et al. 1998). ABD2 and ABD1 could be exchanged in principle; however, steric clashes between ABD2 and the calcium-binding domain would result if we exchange the ABD1 with ABD2. (A) Shows a side view of the arrays (as seen by the microscope). The actin monomers are enumerated, crossbridges occur at positions 0, 13, and 26 (as in Fig. 2 D). (B) Shows a view looking down the filament axes. The lipid layer is located on top of the figure. (C) A magnified view of a cross-link. (D) Fourier transform of the 2D array in A. Note how well the pattern matches Fig. 1 B. (E) Overlay of an enhanced version of an observed micrograph and a scaled version (light gray) of the atomic model in A. Note how well the crossband and filament distances correspond. (F–G and J) Model of the slanted crossband with adjacent filaments rotated by 26.66°, equivalent to a 55-Å downward translation. This geometry can be achieved by rotating fimbrin by 180° around its center of mass parallel to the filament axes and accommodating for the slight offset of fimbrin's geometry from exact rotational geometry. This symmetry is immediately evident by comparing B and F. Note the seemingly different position of the calcium-binding domain (C and G) is due to slightly different views. (F) A view looking down the filaments. The lipid layer is located on top of the figure. The criterion for handedness in this arrangement would be a preference for maximum distance from the lipid layer. (J) A side view, and (G) a magnified view of a cross-link. (H) Overlay of an enhanced version of an observed micrograph and a scaled version (light gray) of the atomic model in J. Note how well the crossband and filament distances correspond. (I) Fourier transform of the 2D array in J. Note how well the pattern matches Fig. 1 C.