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N-terminal tails of histones H3 and H4 are known to bind several different Importins to import the histones into the cell nucleus. However, it is not known what binding elements in the histone tails are recognized by the individual Importins. Biochemical studies of H3 and H4 tails binding to seven Importins, Impβ, Kapβ2, Imp4, Imp5, Imp7, Imp9, and Impα, show the H3 tail binding more tightly than the H4 tail. The H3 tail binds Kapβ2 and Imp5 with KD values of 77 and 57 nm, respectively, and binds the other five Importins more weakly. Mutagenic analysis shows H3 tail residues 11–27 to be the sole binding segment for Impβ, Kapβ2, and Imp4. However, Imp5, Imp7, Imp9, and Impα bind two separate elements in the H3 tail: the segment at residues 11–27 and an isoleucine-lysine nuclear localization signal (IK-NLS) motif at residues 35–40. The H4 tail also uses either one or two basic segments to bind the same set of Importins with a similar trend of relative affinities as the H3 tail, albeit at least 10-fold weaker. Of the many lysine residues in the H3 and H4 tails, only acetylation of the H3 Lys14 substantially decreased binding to several Importins. Lastly, we show that, in addition to the N-terminal tails, the histone fold domains of H3 and H4 and/or the histone chaperone Asf1b are important for Importin-histone recognition.

Robust animal development, tissue homeostasis, and stem cell renewal requires precise control of the Wnt/β-catenin signaling axis. In the embryo of the fruit fly Drosophila melanogaster, the naked cuticle (nkd) gene attenuates signaling by the Wnt ligand Wingless (Wg) during segmentation. nkd mutants have been reported to exhibit abnormalities in wg transcription, Wg protein distribution and/or transport, and the intracellular response to Wg, but the relationship between each alteration and the molecular mechanism of Nkd action remains unclear. In addition, whether Nkd acts in a cell-autonomous or nonautonomous fashion in the embryo is not known. Mammalian Nkd homologs have N-terminal consensus sequences that direct the post-translational addition of a lipophilic myristoyl moiety, but fly and mosquito Nkd, while sharing N-terminal sequence homology, lack a myristoylation consensus sequence. Here we provide evidence that fly Nkd acts cell-autonomously in the embryo, with its N-terminus able to confer unique functional properties and membrane association that cannot be mimicked in vivo by heterologous myristoylation consensus sequences. In conjunction with our recent observation that Nkd requires nuclear localization for function, our data suggest that Nkd acts at more than one subcellular location within signal-receiving cells to attenuate Wg signaling.