Cingulin unfolds ZO-1 and organizes myosin-2B and γ-actin to mechanoregulate apical and tight junction membranes

How junctional proteins regulate the mechanics of the plasma membrane and how actin and myosin isoforms are selectively localized at epithelial cell-cell junctions is poorly understood. Here we show by atomic force indentation microscopy, immunofluorescence analysis and FLIM membrane tension imaging that the tight junction (TJ) protein cingulin maintains apical surface stiffness and TJ membrane tortuosity and down-regulates apico-lateral membrane tension in MDCK cells. KO of cingulin in MDCK, mCCD and Eph4 cells results in a decrease in the juxta-membrane accumulation of labeling for cytoplasmic myosin-2B (NM2B), γ-actin, phalloidin and ARHGEF18, but no detectable effect on myosin-2A (NM2A) and β-actin. Loss of paracingulin leads to weaker mechanical phenotypes in MDCK cells, correlating with no detectable effect on the junctional accumulation of myosins and actins. Cingulin and paracingulin form biomolecular condensates, bind to the ZU5 domain of ZO-1, and are recruited as clients into ZO-1 condensates in a ZU5-dependent manner. Cingulin binding to ZO-1 promotes the unfolding of ZO-1, as determined by interaction with DbpA in cells lacking ZO-2 and in vitro. Cingulin promotes the accumulation of a pool of ZO-1 at the TJ and is required in a ZU5-dependent manner for the recruitment of phalloidin-labelled actin filaments into ZO-1 condensates, suggesting that ZU5-cingulin interaction promotes ZO-1 interaction with actin filaments. Our results indicate that cingulin tethers the juxta-membrane and apical branched γ-actin-NM2B network to TJ to modulate ZO-1 conformation and the TJ assembly of a pool of ZO-1 and fine-tune the distribution of forces to apical and TJ membranes.