Abstract
The Hippo signaling pathway regulates tissue growth and homeostasis through WW domain-containing proteins, which often target the same PPxY-containing partners. However, the mechanisms that determine how multiple WW proteins compete for a single substrate remain unclear. Here, we investigate how Kidney and Brain Expressed Protein (KIBRA) and Yes-Associated Protein (YAP), two tandem WW domain effectors of the Hippo pathway, interact with the dual-PPxY motifs of the Large Tumor Suppressor 1 (LATS1) kinase. Using complementary biophysical methods, including isothermal titration calorimetry, analytical ultracentrifugation, multi-angle light scattering, and nuclear magnetic resonance spectroscopy (NMR) spectroscopy, we show that KIBRA preferentially recognizes the second motif (P2), while YAP simultaneously engages both motifs. Although YAP binds LATS1 with approximately twice the affinity of KIBRA, competition experiments demonstrate that neither protein has a competitive advantage. Instead, they form mutually exclusive binary complexes that remain in a dynamic equilibrium, with no evidence of a stable ternary KIBRA-YAP-LATS1 assembly. Residue-level NMR titrations further reveal that KIBRA and YAP occupy overlapping but distinct interfaces at the P2 site, with binding governed by site-specific conformational dynamics rather than affinity alone. This dynamic, interface-specific mode of competition uncovers a mechanism in which WW domain selectivity is modulated through structural adaptability. By revealing how WW domain competition operates at the molecular level, these findings highlight new avenues for therapeutic intervention in growth control.