FasL associated factors and their potential role in the regulation of FasL expression

Fas Ligand (FasL) is a death factor with multiple functions. The expression and release of FasL is tightly regulated at a post-transcriptional level by sorting of FasL in so-called secretory lysosomes or directly on the cell membrane. The underlying mechanism and protein-protein interactions involved are largely unknown. Three putatively ‘FasL associated factors’ (FLAF1-3) had been identified earlier in a yeast two-hybrid screen by using the cytoplasmic portion of FasL as bait. Until today, nothing is known about their role in the context of FasL expression and function. Here we show that FLAFs interact with FasL via SH3 or WW domain binding to the proline-rich domain in the cytosolic tail of FasL. FLAF1 forms part of the formin binding protein 11 (FBP11, also called huntingtin-interacting protein A, HYPA), FLAF2 is part of the c-Cbl-associated protein SH3P12 = “sorbin and SH3 domain containing 1” and FLAF3 represents a portion of the BAI1-associated protein 2 (BAP2β). SH3P12 and BAP2β could precipitate FasL in double transfectants. The interaction of SH3P12/FasL and BAP2β/FasL results in the enhanced surface expression of FasL. These findings suggest that in contrast to other FasL interacting proteins, SH3P12 and BAP2β do not target FasL to the lysosomal compartment but rather might be involved in its surface expression.