Requirement of the cytoplasmic portion for dimer formation of Fcα/μ receptor expressed on cell surface

Fcα/μ receptor (Fcα/μR), an Fc receptor for IgA and IgM, is the only Fc receptor for IgM identified on hematopoietic cells in human and rodents and for IgA in rodents. Fcα/μR is a type 1 transmembrane protein containing one immunoglobulin-like domain in the extracellular portion. Both human and mouse Fcα/μR mediate endocytosis of the ligands IgA and IgM, for which the cytoplasmic portion of Fcα/μR is responsible. However, molecular characteristics of Fcα/μR involved in the function have been incompletely understood. Here, we show that both monomeric and dimeric Fcα/μR are expressed in a mouse B cell line BCL1-B20 and BW5147 or Ba/F3 transfectants stably expressing Fcα/μR. We also show that the dimeric, but not monomeric, Fcα/μR is preferentially localized to the cell surface of the transfectants. BW5147 transfectant expressing mutant Fcα/μR lacking the cytoplasmic portion expressed only the monomeric Fcα/μR. These results suggest that the cytoplasmic portion is required for the dimer formation and thus for efficient cell surface expression of Fcα/μR.