The poly-immunoglobulin receptor (pIgR) present in the epithelium throughout the body transports polymeric immunoglobulin (IgA and IgM) from their site of production into mucosal secretions by an active process called transcytosis. Although the receptor is constitutively transcytosed, it has been shown first in vitro using the pIgR-expressing MDCK epithelial cell line, and more recently in vivo, that upon ligand binding transcytosis of the pIgR is stimulated. In addition, we recently reported that in pIgR expressing MDCK cells dIgA binding to pIgR stimulates a PTK-dependent signaling pathway that controls dIgA stimulated pIgR transcytosis through the activation of the PLCg-1. In rodents pIgR is highly express in liver. After rat liver fractionation we isolated an endosomal fraction highly enriched in pIgR. Using this fraction we have succeeded to co immunoprecipitate and semi-purify a PTK activity together with the pIgR. This PTK activity is specifically associated to the pIgR but not to other receptors also enriched in this endosomal fraction, such as Tf-R, LDL-R nor ASGP-R. Injection of an excess of purified dIgA stimulates the PTK activity co-immunoprecipitated with pIgR. The biochemical characterization of the PTK activity associated to pIgR strongly suggested that it belonged to the src family. Immunoblot and co-immunoprecipitation experiments definitively proved that the PTK activity associated with pIgR and stimulated by dIgA was p62yes and not p60src although present in the fraction as well. Although p62yes knock-out mice have a comparable total amount of dIgA in the secretion estimated by ELISA (intestinal liquid, feces, vaginal liquid, saliva, bile liquid), the rate of IgA transcytosis, measured after injection in the blood of iodinated dIgA and collection of bile liquid, was cut down by almost 50%. Altogether these data revealed an unexpected role for p62yes in the efficient transport of IgA into the secretions and the mucosal immune response.
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