Fig. 10. Model of FasL sorting pathways in cells with secretory lysosomes versus conventional lysosomes. FasL exits the biosynthetic pathway at the trans-Golgi network (TGN). In secretory cells, an SH3-domain-containing protein sorts FasL, via its proline-rich domain, to the secretory lysosome. The absence of this interaction in non-secretory cells results in sorting of FasL to the plasma membrane via the default exocytosis pathway. Delivery of FasL to the plasma membrane in haematopoietic secretory cells only occurs during activation (e.g. stimulation via the TcR).