It was shown previously that endotubin, an integral membrane protein of endosomes, regulates the trafficking of tight junction proteins between endosomes and the tight junctions. Here it is shown that endotubin regulates YAP localization on endosomes through its interaction with AMOT and thus may play a role in contact inhibition.

Epithelial permeability is regulated by targeted insertion and recycling of tight junction proteins. Rab14 regulates the lysosomal targeting of the leaky claudin, claudin-2, and depletion of Rab14 results in increased transepithelial resistance and aberrant morphogenesis in three-dimensional culture.

GOPC (FIG/PIST/CAL) is a PDZ-domain scaffolding protein that regulates the trafficking of a wide array of proteins, including small GTPases, receptors, and cell surface molecules such as cadherin 23 and CFTR. In Madin-Darby Canine Kidney (MDCK) cells, we find that GOPC localizes to the Trans-Golgi Network (TGN), but not the cis or trans Golgi cisternae. There is also colocalization with the early endosome Rab GTPase Rab5 and a TGN/endosome marker Rab14, but not with Rab11, a marker of recycling endosomes. There is no localization of GOPC to the lateral membranes or tight junctions. We find that knockdown of GOPC in MDCK cells results in decreased transepithelial resistance and increased paracellular flux. This may be due to compromised trafficking of tight junction components from the TGN, as GOPC knockdown cells have decreased lateral labeling of the tight junction protein claudin-1 and decreased protein levels of claudin-2. GOPC may mediate trafficking of newly synthesized tight junction proteins from the TGN to the cell surface or recycling of these proteins from specialized endosomal compartments.