Figure 7.

A working model on Rab11-FIP1- and Rab11-FIP5-mediated pIgA transcytosis in incompletely polarized cells. In our incompletely polarized cells model, pIgA is recognized and bound by pIgR upon pIgA treatment. Subsequently, the endocytic pIgR–pIgA complex is delivered by Rab11a-positive endosomes. The complex is first transported from the basolateral plasma membrane to the vicinity of the centrosome where Rab11-FIP1 and Rab11-FIP5 bind to it. During the trafficking process, TRIM21 mediates the K11-linked polyubiquitination of Rab11-FIP1 and the K6-linked polyubiquitination of Rab11-FIP5 to promote their activation. With the assistance of Rab11-FIP1 and Rab11-FIP5, pIgR–pIgA complex is further transferred to the apical plasma membrane via Golgi apparatus and then secreted.