Arp2/3 complex activation, the Arp2/3 complex activator N-WASP, and the N-WASP-controlling and ProSAP-binding protein Abp1 are crucial for ProSAP-induced enlargements of spine heads. A–G, Images of spine morphology of primary hippocampal neurons at 14 DIV expressing a cell-filling molecule alone (A) and of cells cotransfected with ProSAP1 + N-WASP CA (B), ProSAP1 + Arp3 RNAi (C), ProSAP1 + N-WASP RNAi (D), and ProSAP2 + Abp1 RNAi (E), as well as with N-WASP CA (F) and Abp1 RNAi alone (G), respectively. Cell morphologies shown represent the fluorescence signals for the cell-filling molecules used. For examples of enlarged spines observed during expression of ProSAP1 and ProSAP2, respectively, see Figure 8, B and D. H, Quantitative analysis of head widths of mushroom spines (means ± SEM in percentage deviation from control) show that ProSAP1- and ProSAP2-induced head enlargements are dependent on Arp2/3 complex activity and sufficient supplies of Arp3, N-WASP, and Abp1. *p < 0.05. Scale bar, 5 μm.