Filamin A stabilizes FcγRI surface expression and prevents its lysosomal routing

Filamin A, or actin-binding protein 280, is a ubiquitously expressed cytosolic protein that interacts with intracellular domains of multiple receptors to control their subcellular distribution, and signaling capacity. In this study, we document interaction between FcγRI, a high-affinity IgG receptor, and filamin A by yeast two-hybrid techniques and coimmunoprecipitation. Both proteins colocalized at the plasma membrane in monocytes, but dissociated upon FcγRI triggering. The filamin-deficient cell line M2 and a filamin-reconstituted M2 subclone (A7), were used to further study FcγRI-filamin interactions. FcγRI transfection in A7 cells with filamin resulted in high plasma membrane expression levels. In filamin-deficient M2 cells and in filamin RNA-interference studies, FcγRI surface expression was consistently reduced. FcγRI localized to LAMP-1-positive vesicles in the absence of filamin as shown by confocal microscopy indicative for lysosomal localization. Mouse IgG2a capture experiments suggested a transient membrane expression of FcγRI before being transported to the lysosomes. These data support a pivotal role for filamin in FcγRI surface expression via retention of FcγRI from a default lysosomal pathway.