Possible role of both the alpha and beta gamma subunits of the heterotrimeric G protein, Gs, in transcytosis of the polymeric immunoglobulin receptor.

In endosomes the polymeric immunoglobulin receptor (pIgR) is sorted into transcytotic vesicles. Transcytosis of the pIgR in polarized Madin-Darby canine kidney (MDCK) cells is regulated both by phosphorylation of Ser664 and by binding of ligand, dimeric IgA (dIgA). Mutation of Ser664 to Ala (pIgR-Ala664) greatly reduces transcytosis of the pIgR without dIgA bound. Here we report that cholera toxin, which specifically activates the heterotrimeric G protein, Gs, stimulates transcytosis of pIgR-Ala664 when dIgA is not bound. However, transcytosis of the wild-type pIgR without dIgA bound was not stimulated by cholera toxin. To further analyze the involvement of Gs in transcytosis, we reconstituted part of the transcytotic pathway. Vesicles containing endocytosed ligand bound to the pIgR could be released from MDCK cells whose apical surface had been mechanically perforated. Release of these vesicles was controlled by phosphorylation of the pIgR and by binding of dIgA. The use of perforated cells allowed us to directly address the involvement of Gs. Our data suggest that both subunits of Gs, Gs alpha and beta gamma, function together to control vesicular release and may therefore be involved in transcytosis in vivo.