Electron tomography of actin filament branches mediated by the Arp2/3 complex. (A) Central slice showing several branches (boxes), individual Arp2/3 complexes (circles), and gold markers (arrowheads). (B and C) Enlarged view of individual complexes (B) and branches (C1–3). (D) Automatically segmented branch junctions show individual actin subunits and substructure in the junction. (E–G) Central slices through the averages of branch junctions from A. castellanii (E), cow (F), and budding yeast (G). (H) Composite of three x-y slices through a dual-axis cryotomogram. Because the branches are not contained within a single slice, we concatenated three slices for visualization purposes. (I) Central slices through the three branches marked in H after reorientation to coincide with the x-y plane. (J) Detail of a filament from H showing individual actin subunits after automatic segmentation. Bars: (A, C, D, H, and I) 20 nm; (G and J) 10 nm.