The Rac1-CD2AP interaction requires the Rac1 C-terminal proline-rich and poly-basic regions and the CD2AP SH3 domains. A, the peptide encoding the C-terminal part of Rac1 (Rac1-C), or peptides containing a mutated proline-rich (PPP → AAA) or poly-basic C-terminal region (RKR → AAA) were used to test their association to CD2AP in Jurkat T-cell lysates. Long and short exposed (Exp.) blots are shown. B, full-length CD2AP and its N- and C-terminal fragments, transfected as GFP fusions into HEK293T (human embryonic kidney) cells were tested for their binding in cell lysates to the Rac1 C terminus. C and D, full-length CD2AP with separate point mutations in each of the three SH3 domains (mSH3-1, -2, -3) substituting a tryptophan for a lysine at positions 37, 144, or 306, respectively (C), or mutants with point mutations in two of the three SH3 domains (D) were transfected in HEK293T cells and their binding in lysates to the Rac1 C terminus was tested and analyzed by Western blotting (WB) for GFP.