Electron micrographs of F-actin:villin crosslinks. (A) Electron micrograph of a well ordered segment from Tomogram 1 (Table 1). Protein is white. (B) Fourier transform of the raft shown in (A). The meridional and equatorial axes are shown in black dashed lines, layer lines and row lines are in gray. (C) Projection image of the 3D global average of ~6,500 aligned villin crosslink repeats. (D) Averaged 2D projection image of ice embedded specimens, for comparison.

Model of villin crosslinking F-actin. (A) Placement of the villin homology model atomic coordinates within the averaged villin density. (B) Schematic of villin domain topology with the approximate location of linkers shown

Villin and gelsolin binding sites do not overlap. Villin density map and atomic model are shown next to gelsolin crystal structures (colored copper) bound to actin. (A) N-terminus of gelsolin bound to actin as in the crystal structure. G2 binds via its long α-helix, the “actin binding helix”, between actin subdomains 1 and 3; G3 does not make contact with actin. G1 bound in this manner effectively caps the barbed end of the filament (PDB 1RGI). (B) C-terminus of gelsolin bound to actin. G4 makes contact with actin subdomain 3. G5 and G6 are not bound to actin; however, the long α-helix of G6 is in close proximity to helices of actin subdomains 3 and 4 (PDB 1H1V).

Class averages based on the variance within the villin crosslink. Labels across the top are in order of relatedness from the classification. Numbers at the bottom are the number of class members. (A) Projection images. Class average 1 shows incomplete “half-villin” density. Average 0 has no villin crosslink. Class average 2 illustrates a fraction of crosslinks that are highly variable at one end. (B) Same averages as above, but shown as volumes. All averages in (B) are contoured to the same threshold.

Close-up of villin-actin interactions. (A) A top-down view of the model in Figure 3. The N-terminus of the long V2 helix, residues 151-158, points between two actin subunits. On the left hand filament, which is interacting with villin domain V2 (red), the actin subdomain 1 (S.D. 1) helices, residues 112-126 and 358-365 of one subunit and on the next subunit up, actin subdomain 4 (S.D. 4) helix residues 222-236, all of which are involved in villin interaction, are shown in red. Likewise, V1 (blue) is positioned between the same regions of actin on the right hand filament. (B) Position of the villin head piece. The head piece helix residues 62-76, which include the “crown of positive charge”, are positioned towards actin and in the vicinity of helix residues 308-320 on actin subdomain 3 (S.D. 3). The disordered linker can be readily accommodated within the linker density. (C) Replacement of V2 by the head piece. The head piece is too small for the V2 density. (D) Replacement of head piece by V1. The gelsolin-like domain is too large for this density. Thus, the head piece placement determines the overall orientation of the villin cross-link model.

Fimbrin and the F-actin:villin crosslink model. To demonstrate compatibility between these two co-crosslinkers we placed the atomic coordinates of fimbrin ABD2 (orange) on the left-hand actin filament and ABD1 and calmodulin-like domain (violet) on the closest right-hand actin monomer. It is not known exactly how these domains interact with one another in a crosslink but all are accommodated within the context of our model without a steric clash. (A) Model built with the two actin filaments offset by 17Å. (B) Model built with the two actin filaments in axial register. In this model, the interaction between V2 and actin must be broken and the head piece linker rebuilt.