Basic Residues within the Kruppel Zinc Finger DNA Binding Domains Are the Critical Nuclear Localization Determinants of EKLF/KLF-1

Abstract

EKLF/KLF-1 is an erythroid-restricted transcription factor essential for expression of the adult beta-globin gene. EKLF/KLF-1 is a 358-amino acid nuclear protein with an amino-terminal proline-rich domain and a carboxyl-terminal DNA binding domain. The nuclear localization signal (NLS) of EKLF/KLF-1 has not been empirically determined. We generated a series of epitope-tagged deletion and point mutants and assessed their subcellular localization. Our results delimit the NLS to the 83-amino acid (amino acids 276-358) DNA binding domain that consists of three Kruppel zinc fingers. All three zinc fingers are necessary for efficient nuclear localization; deletion of any individual finger results in cytoplasmic accumulation. Fusion of the three zinc fingers to green fluorescent protein (GFP) targeted GFP to the nucleus, demonstrating that the zinc finger domain is sufficient for nuclear localization. EKLF/KLF-1 containing histidine to alanine mutations that disrupt the structure of all three fingers retains appropriate nuclear localization, indicating that neither the tertiary structure of the zinc fingers nor specific DNA binding are necessary for nuclear localization. We demonstrate that basic residues within the fingers are the critical determinants for nuclear localization; mutations of these basic residues to alanine resulted in cytoplasmic mislocalization. The basic residues of all mammalian Kruppel zinc fingers are highly conserved; therefore we propose that these basic residues are a common NLS shared by all Kruppel family members.