Extracellular matrix stiffness regulates degradation of the Hippo kinase MST2 via SCFβTrCP

Tumor microenvironments display disrupted mechanical properties, including altered extracellular matrix (ECM) rigidity. ECM stiffening perturbs cell tensional homeostasis resulting in activation of mechanosensing transcriptional co-activators, such as the Hippo pathway effectors YAP and TAZ. The Hippo pathway plays central roles in development and tumorigenesis, but how the proteostasis of the Hippo kinase MST2 is regulated remains unknown. Here, we show that ECM stiffness induces MST2 degradation via proteasome degradation. MST2 degradation is enhanced in human breast epithelial cells (HMEC) that are cultured in stiffer microenvironments due to integrin and integrin-linked kinase activation. MST2 knockdown resulted in increased nucleus-to-cytoplasm ratio of YAP in physiological and breast tumor rigidities and altered mechanoregulated cellular processes in HMEC. We found that MST2 is ubiquitinated by the SCF βTrCP ubiquitin ligase. Site-directed mutagenesis combined with computational molecular dynamics studies revealed that βTrCP binds MST2 via a noncanonical degradation motif. Our study uncovers the underlying biochemical mechanisms controlling MST2 degradation and demonstrates how changes in the microenvironment rigidity regulate the proteostasis of a central Hippo pathway component.