Dual localization of human DNA topoisomerase IIIalpha to mitochondria and nucleus

Proc Natl Acad Sci U S A. 2002 Sep 17;99(19):12114-9. doi: 10.1073/pnas.192449499. Epub 2002 Sep 3.


The human TOP3alpha gene encoding DNA topoisomerase IIIalpha (hTop3alpha) has two potential start codons for the synthesis of proteins 1,001 and 976 aa residues in length. The sequence of the N-terminal region of the 1,001-residue form resembles signal peptide sequences for mitochondrial import, and fluorescence microscopy shows that the addition of as few as the first 34 aa of the 1,001-residue form of hTop3alpha to a green fluorescent protein can direct the chimeric protein to mitochondria. Biochemical analyses of subcellular fractions of HeLa cells further demonstrate that a distinctive fraction of hTop3alpha is present inside mitochondria, as evidenced by its resistance to proteinase K. This fraction constitutes several percent of the enzyme in the nuclear fraction, suggesting that the distribution of the mitochondrial and nuclear forms of hTop3alpha is roughly in proportion to the DNA contents of these cellular compartments. The presence of a type IA DNA topoisomerase in the mitochondria of other eukaryotes is supported by an examination of the amino acid sequences of mouse and Drosophila DNA topoisomerase IIIalpha and Schizosaccharomyces pombe DNA topoisomerase III. Given the presence of at least one type IA DNA topoisomerase in all forms of life examined to date, the finding of a type IA enzyme in mitochondria further supports the notion of a key role of such enzymes in DNA transactions.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Cell Nucleus / enzymology*
  • DNA Topoisomerases, Type I / genetics
  • DNA Topoisomerases, Type I / metabolism*
  • Endopeptidase K / pharmacology
  • Green Fluorescent Proteins
  • HeLa Cells
  • Humans
  • Luminescent Proteins / genetics
  • Luminescent Proteins / metabolism
  • Mice
  • Mitochondria / enzymology*
  • Molecular Sequence Data
  • Peptide Fragments / genetics
  • Peptide Fragments / metabolism
  • Protein Sorting Signals / genetics
  • Recombinant Fusion Proteins / genetics
  • Recombinant Fusion Proteins / metabolism


  • Luminescent Proteins
  • Peptide Fragments
  • Protein Sorting Signals
  • Recombinant Fusion Proteins
  • Green Fluorescent Proteins
  • Endopeptidase K
  • DNA Topoisomerases, Type I