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Description

IgG receptor FcRn large subunit p51 (UniProt P55899, also known as FcRn, IgG Fc fragment receptor transporter alpha chain, Neonatal Fc receptor) is encoded by the FCGRT gene (also known as FCRN) (Gene ID 2217) in human. Transfer of passive immunity from mother to the fetus or newborn involves the transport of IgG across several epithelia. Depending on the species, IgG is transported prenatally across the placenta and yolk sac or is absorbed from colostrum and milk by the small intestine of the suckling newborn. In both cases, apical to basolateral transepithelial transport of IgG is mediated by FcRn. FcRn is a heterodimer of an alpha-chain and beta(2)-microglobulin (beta(2)m) and differs from other IgG Fc receptors in that it is structurally related to MHC class I molecules. The assembly of the FcRn alpha-chain with beta(2)m is important for both cell surface expression and pH-dependent high affinity IgG binding. Proteins endocytosed from serum are degraded in the lysosomes. However, serum albumin (SA) and IgG bind to FcRn at low pH in the endosome after endocytosis, and are transported back to the cellular surface, where they are released into the bloodstream, resulting in an extended serum half-life.