Differential expression and ligand binding indicate alternative functions for zebrafish polymeric immunoglobulin receptor (pIgR) and a family of pIgR-like (PIGRL) proteins
The skin mucus IgM is an important molecule in the mucosal immune system of teleost skin. However, the transport mechanism associated with this molecule has yet to be clarified. In this study, we isolated a gene encoding a polymeric Ig receptor (pIgR) from a species of teleost fish, Takifugu rubripes (fugu). This gene is known to be an Ig transporter in the intestine of mammals. Our studies further demonstrated that fugu pIgR was expressed in the skin and that a fragment of pIgR bound to tetrameric IgM in the skin mucus. These results indicate that the skin pIgR transports tetrameric IgM into the skin mucus. The fugu pIgR exhibits a unique structure containing only two Ig-like domains corresponding to domain 1 and domain 4/5 of mammalian pIgR. This structure was sufficient for successful binding to tetrameric IgM. Teleost skin thus adopts the same Ig transport system as mammalian intestine via a unique pIgR.