Dual Localization of Human DNA Topoisomerase IIIα to Mitochondria and Nucleus

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 99; no. 19; pp. 12114 - 12119
• Main Authors: Wang, Yong, Lyu, Yi Lisa, Wang, James C.
• Format: Journal Article
• Language: English
• Published: National Academy of Sciences 17.09.2002; National Acad Sciences
• Subjects: Amino acids; Antibodies; Biological Sciences; Codons; DNA; Enzymes; Fluorescence; HeLa cells; Mitochondria; Mitochondrial DNA; Proteins
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.192449499

The human TOP3α gene encoding DNA topoisomerase IIIα (hTop3α) 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 hTop3α 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 hTop3α 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 hTop3α 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 IIIα 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.