The nuclear localization signal (NLS) of PDX-1 is part of the homeodomain and represents a novel type of NLS

The beta-cell homeodomain transcription factor PDX-1 has vital functions bo th in controlling the expression of pancreatic polypeptide hormones and in the development of the pancreas. The transactivating and DNA-binding proper ties of PDX-1 have been well characterized, but nuclear transport is still undefined. Here we show that PDX-1 bears a nuclear localization signal (NLS ) that is part of helix 3 of the homeodomain. PDX-1 deletion mutants were t agged with enhanced green fluorescent protein (EGFP) and expressed in COS-7 cells. Subcellular localization of the respective PDX-1-EGFP fusion protei ns was analyzed by direct fluorescence microscopy and Western immunoblottin g using an anti-(GFP). As a result we were able to demonstrate that the hom eodomain or helix 3 alone was sufficient and necessary for transport into t he nucleus. Point mutations of basic amino acid residues within helix 3 led to identification of an NLS with six amino acids being crucial for nuclear transport of PDX-1. Because this NLS does not match known examples of NLSs , the PDX-1 NLS may represent a novel class of NLS.