TRANSCYTOSIS OF THE POLYMERIC IMMUNOGLOBULIN RECEPTOR IS REGULATED INMULTIPLE INTRACELLULAR COMPARTMENTS

Transcytosis of the polymeric immunoglobulin receptor (pIgR) can be ex perimentally divided into three steps: 1) internalization from the bas olateral plasma membrane and delivery to basolateral early endosomes, 2) microtubule-dependent movement from basolateral early endosomes to apical recycling endosomes, and 3) delivery from apical recycling endo somes to the apical surface and cleavage of the pIgR to secretory comp onent, which is released into the apical medium. Transcytosis of the p IgR is stimulated by two signals, phosphorylation of Ser-664 in the cy toplasmic domain of the pIgR and binding of the ligand, dimeric IgA, t o the pIgR. These signals do not detectably alter step 1 of transcytos is. Here, we show that phosphorylation of Ser-664 stimulates both step s 2 and 3, whereas binding of dimeric IgA stimulates only step 3 of tr anscytosis.