Characterization of a new N-terminally acetylated extra-mitochondrial isoform of frataxin in human erythrocytes

Lili Guo, Qingqing Wang, Liwei Weng, Lauren A. Hauser, Cassandra J. Strawser, Clementina Mesaros, David R. Lynch & Ian A. Blair; Scientific Reports volume 8, Article number:17043 (2018) DOI:10.1038/s41598-018-35346-y

The mitochondrial form of frataxin has long been thought to be present in erythrocytes even though paradoxically, erythrocytes lack mitochondria. We have discovered that erythrocyte frataxin is in fact a novel isoform of frataxin (isoform E) with 135-amino acids and an N-terminally acetylated methionine residue. There is three times as much isoform E in erythrocytes (20.9 ± 6.4 ng/mL) from the whole blood of healthy volunteers (n = 10) when compared with the mature mitochondrial frataxin present in other blood cells (7.1 ± 1.0 ng/mL). Isoform E lacks a mitochondrial targeting sequence and so is distributed to both cytosol and the nucleus when expressed in cultured cells. When extra-mitochondrial frataxin isoform E is expressed in HEK 293 cells, it is converted to a shorter isoform identical to the mature frataxin found in mitochondria, which raises the possibility that it is involved in disease etiology. The ability to specifically quantify extra-mitochondrial and mitochondrial isoforms of frataxin in whole blood will make it possible to readily follow the natural history of diseases such as Friedreich’s ataxia and monitor the efficacy of therapeutic interventions.

 Characterization of a new N-terminally acetylated extra-mitochondrial isoform of frataxin in human erythrocytes