RAC-LATS1/2 signaling regulates YAP activity by switching between the YAP-binding partners TEAD4 and RUNX3

The tumor-suppressor RUNX3 has a critical role in a lineage determination, cell cycle arrest and apoptosis. Lozenge ( Lz ), a Drosophila homolog of mammalian RUNX family members, has integral roles in these processes and specifically in eye cell fate determination. To elucidate the genetic modifiers of Lz/RUNX3 , we performed a large-scale functional screen in a fly mutant library. The screen revealed genetic interactions between the Lz , Rac and Hippo pathways. Analysis of interactions among these genes revealed that the defective phenotype resulting from activation of Yki , an end point effector of the Hippo pathway, was suppressed by Lz and enhanced by Rac-Trio . Molecular biological analysis using mammalian homologs reveled that LATS1/2-mediated YAP phosphorylation-facilitated dissociation of the YAP-TEAD4 complex and association of the YAP-RUNX3 complex. When cells were stimulated to proliferate, activated RAC-TRIO signaling inhibited LATS1/2-mediated YAP phosphorylation; consequently, YAP dissociated from RUNX3 and associated with TEAD, thereby replacing the YAP-RUNX3 complex with YAP-TEAD. RUNX3 contributed to both association and dissociation of YAP-TEAD complex, most likely through the formation of the YAP-TEAD-RUNX3 ternary complex. Ectopic expression of RUNX3 in MKN28 gastric cancer cells reduced tumorigenicity, and the tumor-suppressive activity of RUNX3 was associated with its ability to interact with YAP. These results identify a novel regulatory mechanism, mediated by the Hippo and RAC-TRIO pathways, that changes the binding partner of YAP.