Molecular cloning and expression analysis of polymeric immunoglobulin receptor in flounder (Paralichthys olivaceus)

The polymeric immunoglobulin receptor (pIgR) is one of the most important mucosal effectors mediating the transcytosis of polymeric immunoglobulins (pIgs) to protect the organisms. In this paper, a full-length cDNA of pIgR was firstly cloned from flounder (Paralichthys olivaceus) using rapid amplification of cDNA ends approaches, and it was of 1384 bp, containing an open reading frame (ORF) of 1005 bp encoding a polypeptide of 335 amino acids with the predicted molecular mass of 37.6 kDa. The flounder pIgR exhibited a unique structure containing only two immunoglobulin-like domains (ILD) corresponding to mammalian pIgR ILD1 and ILD5. The mRNA transcripts of pIgR were detected in all the tested tissues of flounder by semi-quantitative RT-PCR, and the pIgR was expressed at the highest level in liver and higher levels in intestine, gill, skin, spleen and head kidney than in stomach and muscle. The ORF was successfully expressed in Escherichia coli BL21 (DE3) and the recombinant protein displayed binding capability to the purified mucus IgM and serum IgM of flounder by ELISA. The polyclonal antibody against flounder recombinant pIgR was developed by immunization of Balb/C mice, which specifically reacted to the recombinant pIgR in Western blot. Moreover, a secretory component-like molecule was detected in the skin mucus but not in the serum of flounder, which molecular mass (about 37 kDa) was near the theoretical mass obtained from the sequence of flounder pIgR. All these results indicated that flounder pIgR probably involved in the pIgs transport and provided insights into the roles of fish pIgR in the mucosal immunity.