Crk mediates Csk-Hippo signaling independently of Yap tyrosine phosphorylation to induce cell competition

Src family kinases (SFKs), including Src, Fyn and Yes, play important roles in development and cancer. Despite being first discovered as the Yes-associated protein, the regulation of Yap by SFKs remains poorly understood. Here, through single-cell analysis and genetic lineage tracing, we show that the pan-epithelial ablation of C-terminal Src kinase (Csk) in the lacrimal gland unleashes broad Src signaling but causes cell competition-mediated extrusion and apoptosis of only acinar progenitors. Csk mutants can be phenocopied by constitutively active Yap and rescued by the genetic ablation of Yap or Taz, indicating remarkable functional equivalence between Src and Yap signaling. Although Csk inactivation induces tyrosine phosphorylation of Yap, mutating these tyrosine residues in both Yap and Taz fails to alleviate the Csk lacrimal gland phenotype. In contrast, Yap loses Hippo signaling-dependent serine phosphorylation and translocates into the nucleus in Csk mutants. Chemical genetics studies further demonstrate that acute inhibition of Csk enhances Crk/CrkL phosphorylation and Rac1 activity, whereas the deletion of either Crk/CrkL or Rac1/Rap1 rescues the Csk mutant phenotype. These results show that Csk controls Hippo-Yap signaling through the Crk/CrkL-Rac/Rap axis to regulate cell competition. Overall design: Whole lacrimal glands were isolated from P2 control and Csk conditional knockout (Csk(CKO)) mice and analyzed using scRNAseq.