Phosphorylation of Pkp1 by RIPK4 regulates epidermal differentiation and skin tumorigenesis

Tissue homeostasis of skin is sustained by epidermal progenitor cells localized within the basal layer of the skin epithelium. Post‐translational modification of the proteome, such as protein phosphorylation, plays a fundamental role in the regulation of stemness and differentiation of somatic stem cells. However, it remains unclear how phosphoproteomic changes occur and contribute to epidermal differentiation. In this study, we survey the epidermal cell differentiation in a systematic manner by combining quantitative phosphoproteomics with mammalian kinome cDNA library screen. This approach identified a key signaling event, phosphorylation of a desmosome component, PKP1 (plakophilin‐1) by RIPK4 (receptor‐interacting serine–threonine kinase 4) during epidermal differentiation. With genome‐editing and mouse genetics approach, we show that loss of function of either Pkp1 or Ripk4 impairs skin differentiation and enhances epidermal carcinogenesis in vivo . Phosphorylation of PKP1's N‐terminal domain by RIPK4 is essential for their role in epidermal differentiation. Taken together, our study presents a global view of phosphoproteomic changes that occur during epidermal differentiation, and identifies RIPK‐PKP1 signaling as novel axis involved in skin stratification and tumorigenesis. Synopsis Phospho‐proteome analysis of epidermal basal layer progenitors coupled to kinome‐wide screening and genome editing reveals that phosphorylation of the desmosome component plakophilin 1 (PKP1) by RIPK4 kinase is critical for normal and malignant epithelial cells. Quantitative proteomics identifies changes in cell junction protein phosphorylation during skin development. Kinase‐refractory PKP1 mutants impair epidermal differentiation in skin grafts. RIPK4 phosphorylates PKP1 in the N‐terminal head domain and regulates epidermal stratification. Skin‐specific knockout of RIPK4 perturbs embryonic skin development and adult epidermal homeostasis, and increases susceptibility to skin carcinogenesis in vivo . RIPK4 phosphorylation of PKP1 promotes binding to scaffold protein SHOC2 and blocking of RAS/MAPK signalling. Graphical Abstract Desmosomal junction protein plakophilin 1 is a RIPK4 kinase substrate in skin cell progenitors, whose phosphorylation is critical for epidermal development and carcinogenesis.