Fig. 5. The KKR or RR motif in the cytoplasmic tail of FasL contributes to the potential SH3-binding region. R45-R46 (RR-EE mutation) and K73-K74-R75 (KKR-EEE mutation) were mutated to EE or EEE, respectively, in the FasL-GFP construct, by site-directed mutagenesis, and were transiently expressed in RBL. The effects of these mutations on FasL surface expression was assayed by confocal (a) and FACS analysis (b) as previously described. The positive charge from the side chains of KKR and/or RR are thought to contribute to the binding of the proline-rich region of FasL to an SH3 domain.