Sequence requirement for nuclear localization and growth inhibition of p27Kip1R, a degradation-resistant isoform of p27Kip1

p27(Kip1R) is an isoform of p27(Kip1), having a distinct C-terminus. The sequences of p27(Kip1R) required for nuclear localization and growth inhibition were determined in HeLa cells using a green fluorescence protein (GFP) as a reporter molecule. Region 153-168 and residues K168 and 1169 were determined to play a critical role in the nuclear localization of p27(Kip1R). Aliphatic amino acid was found to be a substitute for the basic residue in the typical nuclear localization signal, while its functional substitution was incomplete, thereby causing a significant cytoplasmic retention of p27(Kip1R). p27(Kip1R) is thus the first example of an atypical bipartite nuclear localization signal with aliphatic amino acid asa functional residue. Despite cytoplasmic retention, p27(Kip1R) inhibited the cell growth as well as p27(Kip1), while GFP alone had no effect. The mutants lacking an N-terminus containing the binding regions for cyclins and cyclin-dependent kinases also showed a significant degree of nuclear localization, but failed to inhibit cell growth. The growth inhibition by p27(Kip1R) as well as p27(Kip1) was thus suggested to originate in the common N-terminal region. (C) 2003 Wiley-Liss, Inc.