Fas Ligand (FasL, CD178) is a cytokine that may be secreted or expressed as a transmembrane ligand at the cell Surface, and induces apoptosis by binding to the "death receptor" Fas (CD95). Here, we show that Grb2, an SH3 domain-containing adaptor Protein, binds to the proline-rich domain of FasL and regulates its cell surface expression. We found that knocking down Grb2 expression decreased the amount of FasL at the cell surface and increased the abundance of intracellular vesicles containing FasL. Furthermore, we showed that Grb2 acts as an adaptor for FasL to interact with adaptin P, a molecule known to regulate trafficking, Our data reveal that Grb2 facilitates the association of FasL with adaptin beta, and promotes Sorting of FasL to the cell Surface. As FasL is a potent regulator of cell death, dynamic regulation of its cell Surface localization is critical for controlling local tissue remodeling and inflammation. (C) 2008 Elsevier Inc. All rights reserved.