Residues 293/295 are surface exposed in activated Arp2/3. (A) Top: 3.9-Å cryo-EM structure of S. pombe Arp2/3 in its short-pitch conformation after activation by Dip1 (Shaaban et al., 2020; Protein Data Bank accession no. 6W17). The structure is visualized as molecular surface, and the Arp2/3 subunits, Dip1, and the daughter actin filament subunits (indicated as n, n+1…etc) are color coded as in Protein Data Bank accession no. 6W17. Middle: The structure is rotated ∼180° in the direction indicated by the black curved arrow to allow visualization of residues of Leu304 and Pro306 of Schizosaccharomyces pombe Arp3 (highlighted in dark blue) that correspond to Met293 and Ser295 in human Arp3B. The dashed rectangle is enlarged at the bottom of the figure. (B) Top: 9.0-Å cryo-EM structure of human Arp2/3 at the branch point between a mother and daughter filament (Fäßler et al., 2020; Protein Data Bank accession no. 7AQK). The structure is visualized as molecular surface, and the Arp2/3 subunits, mother filament subunits (indicated as M1–M8), and daughter filament subunits (indicated as n, n + 1…etc.) are color coded as in Protein Data Bank accession no. 7AQK. The red arrow below the branch point indicates the point of view represented in the middle panel after a ∼90° rotation in the direction shown by the black curved arrow. This structure contains Arp3; hence, the position of Thr293 and Pro295 is highlighted in dark blue and indicated by a dashed black line. The dashed rectangle is enlarged at the bottom of the figure.