Keratin 14 is a novel interaction partner of keratinocyte differentiation regulator: receptor-interacting protein kinase 4

Abstract

The epidermis, the outer layer of the skin, is formed by stratified keratinocyte layers. The self-renewal of the epidermis is provided by sustained proliferation and differentiation of the keratinocyte stem cells localized to the basal layer of the epidermis. Receptorinteracting protein kinase 4 (RIPK4) is an important regulator of keratinocyte differentiation, mutations of which are associated withcongenital ectodermal malformations. In an attempt to identify the molecular basis of RIPK4’s function, we applied yeast two-hybridscreen (Y2H) and found basal layer-specific keratin filament component keratin 14 (KRT14) as a novel RIPK4-interacting partner. During keratinocyte differentiation, layer-specific keratin composition is tightly regulated. Likewise, the basal layer specific KRT14/keratin5 (KRT5) heterodimers are replaced by keratin 1 (KRT1)/keratin 10 (KRT10) in suprabasal layers. The regulation of keratin turnoveris under the control of signaling associated with posttranslational modifications in which phosphorylation plays a major role. In thisstudy, we verified the KRT14–RIPK4 interaction, which was identified with Y2H, in mammalian cells and showed that the interactionwas direct by using proteins expressed in bacteria. According to our results, the N-terminal kinase domain of RIPK4 is responsible forKRT14–RIPK4 interaction; however, the RIPK4 kinase activity is dispensable for the interaction. In accordance with their interaction,RIPK4 and KRT14 colocalize within the cells, particularly at keratin filaments associated with perinuclear ring-like structures. Moreover, RIPK4 did not show any effect on KRT14/KRT5 heterodimer formation. Our results suggest that RIPK4 may regulate the keratinturnover required for keratinocyte differentiation through interacting with KRT14.