Importin α as a molecular marker for investigating the microenvironment of micronuclei

Micronuclei (MN) are nuclear membrane-enclosed cellular structures that contain DNA/chromatin fibers that can lead to genomic instability. Here, we report that importin α, known as the nuclear transport factor, is highly concentrated in some of the MN in cultured human cancer cells. We observed that importin α1 accumulated in the MN together with other subtypes, indicating that this represented a conserved feature of the importin α family. Indirect immunofluorescence analysis revealed that the MN localization of importin α1 was regulated independently of canonical nuclear transport-related factors such as importin β1, CAS/CSE1L, nucleoporins, and lamins. In contrast, importin α1 signals merged with histone H3 modified by trimethylation at lysine 4 in MN, indicating that importin α1 was associated with euchromatin in MN. Furthermore, we found a mutually exclusive relationship between importin α1 and RAD51, a homologous recombination repair protein, in relation to MN accumulation in cells exposed to etoposide, a DNA damaging reagent. Our findings suggest that importin α is a previously overlooked molecular marker for assessing the microenvironment and quality control of MN in human cancer cells. Summary statement Accumulation of importin α in micronuclei, followed by alteration of their microenvironment, suggests the non-canonical function of importin α in genome instability and cancer development. ### Competing Interest Statement The authors have declared no competing interest.