C/EBP proteins contain nuclear localization signals imbedded in their basic regions.

The C/EBP-related proteins (C/EBPalpha, CRP1, C/EBPbeta, and C/EBPdelta) form a subfamily of bZIP (basic region/leucine zipper) transcription factors that display sequence homology within the bZIP domain. The conserved basic region contains two motifs that exhibit significant homology to the bipartite nuclear localization signal (NLS) first described in nucleoplasmin. CRP1 and C/EBPbeta proteins bearing deletions of the basic region accumulate in the cytoplasm, in contrast to their normal nuclear location. Analysis of chimeric proteins consisting of CRP1 basic region sequences fused to beta-galactosidase revealed that the CRP1 basic region contains a single NLS that differs from conventional bipartite signals in two ways. First, mutation of a pair of arginine residues at the N-terminus of the proposed NLS does not disrupt its function. Second, the CRP1 NLS requires additional nonbasic residues at its C-terminus. A basic residue within the CRP1 NLS that is not conserved within the C/EBP family is occupied instead by an uncharged residue in C/EBPalpha and C/EBPbeta. When this nonconserved arginine residue was changed to alanine the CRP1 NLS behaved as a classical bipartite signal, suggesting that bipartite NLSs are present in all family members but that NLSs of the individual members differ slightly. Additionally, mutation of critical NLS residues in the intact CRP1 and C/EBPbeta proteins showed that these elements exhibit more bipartite-like characteristics when present in their normal sequence context. Finally, we observed that a C/EBPbeta protein lacking its NLS can be localized to the nucleus when coexpressed with C/EBPalpha, indicating that a single NLS is sufficient to promote nuclear transport of a bZIP dimer.