The heavy chain of neonatal Fc receptor for IgG is sequestered in endoplasmic reticulum by forming oligomers in the absence of β2-microglobulin association

Authors:
Publisher website
Google Scholar
Abstract
The heavy chain (HC) of the neonatal Fc receptor (FcRn) for IgG is non-convalently associated with β2-microglobulin (β2m). In β2m-/- mice, FcRn functions are greatly impaired. We sought to determine how FcRn HC, particularly its structure and biogenesis, is affected by the absence of β2m. Human FcRn HC, expressed from the β2m-null cell line FO-1FcRn, was present as a monomeric 45-kDa protein under reducing conditions but primarily as a 92-kDa oligomeric protein under non-reducing conditions. Two-dimensional electrophoresis and MS analysis showed that the 92-kDa protein was a dimer of the 45-kDa HC. Immunostaining showed that FcRn HC in FO-1FcRn was co-localized with the endoplasmic reticulum (ER) protein Bip/GRP78 but not with an endosome protein, EEA1. In contrast, FcRn HC in FO-1FcRn+β2m was detected in both the ER and endosome. The dimeric HC in FcRn oligomers was free of β2m association in FO-1FcRn+β2m. Mutation of non-paired cysteine residues at positions 48 and 251 within the human FcRn cDNA failed to eliminate the oligomers. The FcRn HC oligomers could be reduced by reconstitution of FO-1FcRn with β2m or by balanced expression of FcRn HC with β2m, or β2m fused with a KDEL retention sequence. Similarly, the majority of FcRn HC isolated from neonatal β2m-/- mice was in a dimeric form under non-reducing conditions. The amount of FcRn HC was significantly decreased in β2m-/- mice and FO-1FcRn. Furthermore, β2m-free FcRn HC was sensitive to endoglycosidase digestion. These results indicate that FcRn HC alone can form disulphide-bonded oligomers in the ER, which may represent a misfolded protein. The β2m association with FcRn HC is critical for correct folding of FcRn and exiting the ER for routing to endosomes and the cell surface.

This publication has 47 references indexed in Scilit: