The Imp2 SH3 domain can fulfill most functions of the Cdc15 SH3 domain. (A) The SH3 domain of Cdc15 was replaced with that of Imp2 at the endogenous cdc15 locus. Cells were grown and stained as in Fig. 1 A. (B) Rlc1-GFP and Sid4-GFP were imaged live in wild-type (wt) and cdc15-imp2SH3 backgrounds. BF, bright field. (C) Cdc15 and Imp2 are depicted as dimers at the membrane; F-BAR domains are shown as crescents, and SH3 domains are shown as dark ovals. In wild-type cells, both Cdc15 and Imp2 bind Fic1 (the C2 domain–containing protein) and possibly other proteins (Y). Cdc15 binds to Pxl1 as well. In the absence of the SH3 domain of Cdc15, some proteins (Fic1, Pxl1, and Y) continue to localize to the contractile ring through Imp2, whereas other Cdc15-specific proteins (X) fail to localize. In the absence of the SH3 domains of either Imp2 or Cdc15, none of these proteins localize properly, resulting in cytokinetic failure. Bars, 5 μm.