Mechanical tension is an important regulator of cell proliferation, differentiation, migration and cell death. It is involved in the control of tissue architecture and wound repair and its improper sensing can contribute to cancer. The Hippo tumor suppressor pathway was recently shown to be involved in regulating cell proliferation in response to mechanical tension. The core of the pathway consists of the kinases MST1/2 and LATS1/2, which regulate the target of the pathway, the transcription co-activator YAP/ TAZ (hereafter referred to as YAP). When the Hippo pathway is inactive, YAP remains in the nucleus and promotes cell proliferation and stem cell maintenance. When the Hippo signaling pathway is turned on, MST1/2 phosphorylate and activates LATS1/2. LATS1/2 phosphorylates and inactivates YAP in the cytoplasm which is sequestered and degraded, stopping cell proliferation and promoting differentiation of stem cells. Mechanical forces are transmitted across cells and tissues through the cell-cell junctions and the actin cytoskeleton. However, the factors that connect cell-cell junctions to the Hippo signaling pathway were not clearly known. We identified a LIM domain protein called TRIP6 that functions at the adherens junctions to regulate the Hippo signaling pathway in a tension-dependent manner. TRIP6 responds to mechanical tension at adherens junctions and regulates LATS1/2 activity. Under high mechanical tension, TRIP6 sequesters and inhibits LATS1/2 at adherens junctions to promote YAP activity. Conditions that reduce tension at adherens junctions by inhibition of actin stress fibers or disruption of cell-cell junctions reduce TRIP6-LATS1/2 binding, which activates LATS1/2 to inhibit YAP. Vinculin has been shown to act as part of a mechanosensory complex at adherens junctions. We show that vinculin promotes TRIP6 inhibition of LATS1/2 in response to mechanical tension. Furthermore, we show that TRIP6 competitively inhibits MOB1 (a known LATS1/2 activator) from binding and activating LATS1/2. Together these findings reveal TRIP6 responds to mechanical signals at adherens junctions to regulate the Hippo signaling pathway in mammalian cells.

The Hippo pathway regulates the transcriptional coactivator YAP to control cell proliferation, organ size, and stem cell maintenance. Multiple factors, such as substrate stiffness, cell density, and G protein-coupled receptor signaling, regulate YAP through their effects on the F-actin cytoskeleton, although the mechanism is not known. Here we show that angiomotin proteins (AMOT130, AMOTL1, and AMOTL2) connect F-actin architecture to YAP regulation. First, we show that angiomotins are required to relocalize YAP to the cytoplasm in response to various manipulations that perturb the actin cytoskeleton. Second, angiomotins associate with F-actin through a conserved F-actin-binding domain, and mutants defective for F-actin binding show enhanced ability to retain YAP in the cytoplasm. Third, F-actin and YAP compete for binding to AMOT130, explaining how F-actin inhibits AMOT130-mediated cytoplasmic retention of YAP. Furthermore, we find that LATS can synergize with F-actin perturbations by phosphorylating free AMOT130 to keep it from associating with F-actin. Together these results uncover a mechanism for how F-actin levels modulate YAP localization, allowing cells to make developmental and proliferative decisions based on diverse inputs that regulate actin architecture.