Fig. 2. Identification of a nucleolar localization signal. (A) Mutation of the basic amino acids in the region 143-146 (RKKR) to alanine residues prevents localization of YFP-NIK1-794 in nucleoli while still retaining its nuclear accumulation. Moderately expressing cells were analyzed by confocal microscopy. (B) Fusion of the seven basic amino acids 143-149 of NIK to either the C terminus of GFP (GFP-RKKRKKK) or the N terminus of GFP (RKKRKKK-GFP) induces accumulation of the modified GFP in nucleoli. An additional accumulation in the nucleoplasm is observed for the C-terminally tagged GFP-RKKRKKK.