Cloning of novel binding protein, which links Keratin filaments to the nuclear envelops.

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In this study, we tried to clone and characterize proteins that interact with various keratins by yeast two-hybrid system. Employing the head and tail domains of epidermal type I and type II keratins as baits, we made several rounds of screening of the human skin cDNA. A novel protein termed okuribin was cloned as a type I keratin binding protein. Okuribin directly binds with the head domain of type I keratins K14, K16 and K18 but not with type II keratins. The okuribin mRNA in size of 4.1 kilo-nucleotides was expressed in the various epithelial tissues including skin as well as non-epithelial tissues like muscles. The okuribin protein predicted as a105 kDa protein encoded by 950 amino acids possessing eight nuclear localization signals as well as a signal peptides and one putative transmembrane segment. When the okuribin cDNA was fused with green fluorescence protein (GFP) or flag tag at its carboxyl terminus, and used for the forced expression into cultured keratinocytes, GFP and flag signals were observed predominantly along with the nuclear membrane. The endogenous okuribin in keratinocytes was observed mainly at the outer nuclear envelope, and occasionally in the nuclei depending on the maturation of the nuclear envelope verified by the staining intensity of lamin. The matured nuclei, where lamin and okuribin were observed strongly at the inner and outer envelop, tend to be tightly surrounded or enclosed by the keratin network in the immune-electron microscopy. From these data, okuribin is considered to act as a missing link connecting the keratin filaments and juxta-nuclear space.