Abstract
The neonatal Fc receptor for IgG (FcRn) functions to transport maternal IgG to the fetal/neonatal animals and protects IgG from catabolism. The present study identified two pFcRn cDNAs (1.071 kb and 0.795 kb) from intestinal epithelial cells. The corresponding mRNA transcripts were detected in porcine kidney cell line LLC-PK1, peripheral blood mononuclear cells and porcine tissues by RT-PCR and Northern blot. Sequence analysis showed that the 1.071 kb cDNA encodes the full-length pFcRn (pFcRn-L); whereas the 0.795 kb cDNA codes for a truncated pFcRn (pFcRn-S) with deletion of 92 amino acids matching to the alpha2 domain of pFcRn-L. The pFcRn-L was constitutively expressed by epithelial cells; however, pFcRn-S was not detectable in porcine tissues and cell lines although its transcript was abundant. Despite of the lack of native pFcRn-S, pFcRn-S was readily detected in transfected cells. Recombinant pFcRn-L was confirmed to bind IgG at pH 6.0, but not pH 7.5; however, pFcRn-S failed to bind IgG at both pH 5.0–6.0 and 7.5. The pFcRn-L was expressed on the cell surface and mainly localized in early endosomes. In contrast, pFcRn-S was absent from cell surface and primarily localized in the lysosome and pFcRn-S trafficking to lysosomes was independent of β2m. The accumulation of pFcRn-S in the lysosome may explain the absence detection of native pFcRn-S protein expression. In addition, the trafficking of pFcRn-S to the lysosomal compartment suggests that in addition to sorting signals in its cytoplasmic tail, the FcRn structural integrity may be important for proper intracellular trafficking and function.