A Beckwith–Wiedemann-Associated CDKN1C Mutation Allows the Identification of a Novel Nuclear Localization Signal in Human p57(Kip2)

p57(Kip2) protein is a member of the CIP/Kip family, mainly localized in the nucleus where it exerts its Cyclin/CDKs inhibitory function. In addition, the protein plays key roles in embryogenesis, differentiation, and carcinogenesis depending on its cellular localization and interactors. Mutations of CDKN1C, the gene encoding human p57(Kip2), result in the development of different genetic diseases, including Beckwith–Wiedemann, IMAGe and Silver–Russell syndromes. We investigated a specific Beckwith–Wiedemann associated CDKN1C change (c.946 C>T) that results in the substitution of the C-terminal amino acid (arginine 316) with a tryptophan (R316W-p57(Kip2)). We found a clear redistribution of R316W-p57(Kip2), in that while the wild-type p57(Kip2) mostly occurs in the nucleus, the mutant form is also distributed in the cytoplasm. Transfection of two expression constructs encoding the p57(Kip2) N- and C-terminal domain, respectively, allows the mapping of the nuclear localization signal(s) (NLSs) between residues 220–316. Moreover, by removing the basic RKRLR sequence at the protein C-terminus (from 312 to 316 residue), p57(Kip2) was confined in the cytosol, implying that this sequence is absolutely required for nuclear entry. In conclusion, we identified an unreported p57(Kip2) NLS and suggest that its absence or mutation might be of relevance in CDKN1C-associated human diseases determining significant changes of p57(Kip2) localization/regulatory roles.