Identification of nuclear localization signals in the human homeoprotein MSX1

MSX1 is one of the homeoproteins with the homeodomain (HD) sequence, which regulates proliferation and differentiation of mesenchymal cells. In this study, we investigated the nuclear localization signal (NLS) in the MSX1 HD by deletion and amino acid substitution analyses. The web-based tool NLStradamus predicted 2 putative basic motifs in the N- and C-termini of the MSX1 HD. Green fluorescent protein (GFP) chimera studies revealed that NLS1 (₁₆₁RKHKTNRKPR₁₇₀) and NLS2 (₂₁₆NRRAKAKR₂₂₃) were independently insufficient for robust nuclear localization. However, they can work cooperatively to promote nuclear localization of MSX1, as was shown by the 2 tandem NLS motifs partially restoring functional NLS, leading to a significant nuclear accumulation of the GFP chimera. These results demonstrate a unique NLS motif in MSX1, which consists of an essential single core motif in helix-I, with weak potency, and an auxiliary subdomain in helix-III, which alone does not have nuclear localization potency. Additionally, other peptide sequences, other than predicted 2 motifs in the spacer, may be necessary for complete nuclear localization in MSX1 HD.