Requirement of the cytoplasmic portion for dimer formation of Fcalpha/micro receptor expressed on cell surface.

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