Formation of finger-like actin-based protrusions at the site of fusion. (A) Survey view of the cytoplasmic surface of the plasma membrane from control muscle cells differentiated for 24 h. Cell 1: Pseudocolored in purple forms finger-like protrusions that are traceable below the acceptor cell 2. (A′) Higher-magnification view from A of the finger-like protrusions denoted with yellow arrows. (B) Higher-magnification view of the finger-like protrusions from a double unroofed cell–cell contact. White arrows, actin cables from attacking cell 1; yellow arrows, finger-like protrusions traceable below cell 2; *, branched actin filaments. (C) Survey view of the cytoplasmic surface of the plasma membrane from two unroofed control muscle cells differentiated for 24 h that have successfully fused. Cell 1 presenting the protrusions is pseudocolored in purple. White arrowheads denote the fusion site. (C′) Higher-magnification view of the fusion site from boxed region in C. Yellow arrows denote finger-like protrusions below the fusion site. (D) Survey view of the cytoplasmic surface from mutant muscle cells differentiated for 24 h. Note clusters of MBs (each MB is a different pseudocolor) unable to fuse. (D′ and D″) Higher-magnification views of cell contact sites from the boxed regions in C. (E) Survey view of the cytoplasmic surface from Mut/Act⁺ cells differentiated for 24 h. (E′) Higher-magnification view from E.