Murine polyomavirus has repeatedly provided insights into tumorigenesis, revealing key control mechanisms such as tyrosinephosphorylation and phosphoinositide 3-kinase (PI3K) signaling. We recently demonstrated that polyomavirus small T antigen(ST) binds YAP, a major effector of Hippo signaling, to regulate differentiation. Here we characterize YAP as a target of middle Tantigen (MT) important for transformation. Through a surface including residues R103 and D182, wild-type MT binds to theYAPWWdomains. Mutation of either R103 or D182 of MT abrogates YAP binding without affecting binding to other signalingmolecules or the strength of PI3K or Ras signaling. Either genetic abrogation of YAP binding to MT or silencing of YAP via shorthairpin RNA (shRNA) reduced MT transformation, suggesting that YAP makes a positive contribution to the transformed phenotype.MT targets YAP both by activating signaling pathways that affect it and by binding to it. MT signaling, whether fromwild-type MT or the YAP-binding MT mutant, promoted YAP phosphorylation at S127 and S381/397 (YAP2/YAP1). Consistentwith the known functions of these phosphorylated serines, MT signaling leads to the loss of YAP from the nucleus and degradation.Binding of YAP to MT brings it together with protein phosphatase 2A (PP2A), leading to the dephosphorylation of YAP inthe MT complex. It also leads to the enrichment of YAP in membranes. Taken together, these results indicate that YAP promotesMT transformation via mechanisms that may depart from YAP's canonical oncogenic transcriptional activation functions.
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