Fas ligand (FasL), a potent mediator of apoptosis expressed by CTL and NK cells, is sorted into the inner vesicles of secretory lysosomes for release via exosome-like vesicles. Previous studies identified a proline-rich domain in the cytoplasmic tail required for sorting FasL to secretory lysosomes, but the mechanisms by which this occurs have not been identified. Here we demonstrate that the PRD of FasL binds Fgr, Fyn and Lyn tyrosine kinases, leading to phosphorylation of FasL. Loss of phosphorylation reduces internalisation of FasL into multivesicular bodies. FasL is also directly mono-ubiquitylated at lysines flanking the PRD and mutation of these lysines reduces MVB localisation of FasL. Phosphorylation is not required for ubiquitylation because FasL lacking all tyrosines undergoes mono-ubiquitylation. These studies show that phosphorylation and ubiquitin signals regulate the sorting of FasL to secretory lysosomes by controlling entry into multivesicular bodies.
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