Phosphorylation-independent ubiquitylation and endocytosis of FcγRIIA

Endocytosis of the Fc receptor Fc gamma RIIA depends on a functional ubiquitin conjugation system, and the receptor becomes ubiquitylated upon ligand binding. Phosphorylation of tyrosines in Fc gamma RIIA by Src family kinases is thought to be the initiating event in its signaling. However, although the Src family kinase inhibitor PP1 inhibited both ligand-induced phosphorylation of Fc gamma RIIA and phagocytosis in ts20 cells expressing Fc gamma RIIA, it did not inhibit receptor ubiquitylation or endocytosis of soluble ligands. Conversely, genistein and the proteasomal inhibitor MG132 did not inhibit receptor phosphorylation but strongly inhibited both receptor ubiquitylation and endocytosis. A region of the receptor lying within the immunoreceptor tyrosine-based activation motif was found to be necessary for both ubiquitylation and endocytosis. Ubiquitylation occurs at the plasma membrane before internalization. Endocytosis of Fc gamma RIIA is dependent on clathrin but independent of the adaptor protein AP-2. These findings point to a novel mechanism for ubiquitylation and endocytosis of this immunoreceptor.