G1P3/IFI6, an interferon stimulated protein, promotes the association of RAB5+ endosomes with mitochondria in breast cancer cells

G1P3/IFI6 is an interferon stimulated gene with antiapoptotic, prometastatic, and antiviral functions. Despite its pleiotropic functions, subcellular localization of G1P3 remains unclear. Using biochemical- and confocal microscopic approaches, this study identified the localization of G1P3 in organelles of the endomembrane system and in the mitochondria of breast cancer cells. In cell fractionation studies, both interferon-induced endogenous- and stably expressed G1P3 cofractionated with affinity-isolated mitochondria. Results of the protease protection assay have suggested that ~24% of mitochondrial G1P3 resides within the mitochondria. Conforming to this, confocal microscopy studies of cells stably expressing epitope-tagged G1P3 (MCF-7/G1P3-FLAG), identified its localization in mitochondria (~38%) as well as in ER, trans-Golgi network (TGN), lysosomes, and in RAB5 positive (RAB5+ ) endosomes. These results suggested the trafficking of G1P3 from TGN into endolysosomes. Both G1P3 and RAB5 were known to confer apoptosis resistance through mitochondrial stabilization. Therefore, the effects of G1P3 on the localization of RAB5 in mitochondria were tested. Compared to vector control, the co-occurrence of RAB5 with the mitochondria was increased by 1.5-fold in MCF-7/G1P3-FLAG expressing cells (p ≤ .005). Taken together, our results demonstrate a role for G1P3 to promote the association of RAB5+ endosomes with mitochondria and provide insight into yet another mechanism of G1P3-induced cancer cell survival.