The C-terminal third of Arp3B promotes the formation of short actin tails. (A) Alignment of human Arp3 and Arp3B showing conservative residues (black) and nonconservative amino acid differences (red). The C-terminal third of Arp3/3B responsible for long or short actin tail phenotypes is boxed in yellow. (B) Schematic representation of RNAi-resistant human Arp3 (blue) and Arp3B (red) together with the three different chimeras (Ch1, Ch2, and Ch3; blue + red = purple). Residue positions at the splice sites are indicated. (C) The immunoblot shows the expression level of RNAi-resistant GFP-tagged Arp3 and Arp3B together with the three different chimeras in HeLa cells treated with Arp3 and Arp3B siRNA. (D) Images of actin tails labeled with Alexa Fluor 568 phalloidin (magenta) together with quantification of their length in HeLa cells stably expressing RNAi-resistant GFP-tagged Ch1, Ch2, or Ch3 (green) and treated with Arp3 and Arp3B siRNA. All error bars represent SEM from n = 3 independent experiments in which the length of 100 tails was analyzed per condition. Tukey’s multiple comparisons test was used to determine statistical significance; **** P < 0.0001; *** P < 0.001; ** P < 0.01; * P < 0.05. Scale bar = 5 µm.