β2-microglobulin is important for cell surface expression and pH-dependent IgG binding of human FcRn

FcRn is a heterodimer of an α-chain andβ 2-microglobulin (β2m) and differs from other IgG Fc receptors in that it is structurally related to MHC class I molecules. Several functions attributed to FcRn are affected inβ 2-microglobulin (β2m)-deficient mice,suggesting that the α-chain needs to assemble with β2m to form a functional receptor. However, the precise role ofβ 2m in FcRn function is not known. Here we expressed the human FcRn α-chain alone or in combination with β2m in human melanoma FO-1 cells. We show that β2m is important for cell surface expression of FcRn and that, in the absence of β2m,the receptor is retained in the endoplasmic reticulum. Furthermore, in the absence of β2m, IgG binding is decreased compared with that of native FcRn. Thus, assembly of the FcRn α-chain with β2m is important for both transport of FcRn from the ER to the cell surface and efficient pH-dependent IgG binding.