Fig. 4. The polyproline region of FasL can be modelled to bind the SH3 domain of Fyn. The Fyn SH3 domain is shown as a blue dot surface and the FasL peptide as gold van der Waals spheres. An area of negatively charged surface of the Fyn SH3 is illustrated in red (a). A magnified view of that area (b) shows that the Arg backbone (green) of the FasL peptide lies in proximity to the negatively charged area, enabling electrostatic interactions between the peptide Arg and the SH3 Asp. Also, the close fit of the peptide and SH3 are shown by the van der Waals and the SH3 dot surface.