Cytoplasmic domain of human Fcalpha/mu receptor is required for ligand internalization

The Fcalpha/mu receptor (Fcα/μR), a type I transmembrane protein, is an immunoglobulin Fc receptor for both IgA and IgM. Its functions in immune defense are not clear at present. In this work, human Fcα/μR was expressed in CHO, 293T, and COS-7 cells to study its biochemical functions. Fcα/μR expressed by CHO and 293T was only in monomer form in cytoplasma and the monomeric receptor could not bind IgA or IgM. In comparison, Fcα/μR expressed by COS-7 cells had both monomer and dimer forms. The binding assay showed that Fcα/μR expressed by COS-7 cells could bind IgM strongly and IgA weakly, implying that dimeric receptor could be expressed on cell membrane and functioned. The bound IgM could be internalized and the internalization was abolished when the cytoplasmic domain of Fcα/μR was truncated. Therefore, the cytoplasmic portion of human Fcα/μR is required in the internalization.