Abstract
p27^Kip1R^ is an isoform of p27^Kip1^, having a distinct Cโterminus. 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 153โ168 and residues K168 and I169 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 as a 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 Nโterminus containing the binding regions for cyclins and cyclinโdependent 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 Nโterminal region. J. Cell. Biochem. 89: 191โ202, 2003. ยฉ 2003 WileyโLiss, Inc.