The open reading frame 1-encoded (‘36K’) protein of Carnation Italian ringspot virus localizes to mitochondria

The localization of the 36 kDa (‘36K’) protein encoded by open reading frame 1 of Carnation Italian ringspot virus was studied in infected cells and in cells transiently expressing the 36K protein fused to green fluorescent protein (GFP). Subcellular fractionation demonstrated that the 36K protein accumulated in fractions containing mostly mitochondria. Fluorescence microscopy of transiently transformed cells showed that the 36K–GFP fusion protein accumulated in structures which could be stained with the mitochondrial-specific dye MitoTracker. However, these structures were larger than normal mitochondria and were irregular in shape and distribution in the cytoplasm. Electron microscopy showed severe alterations of mitochondria, which were often clumped. The stroma was more electron-opaque, the cristae were irregularly shaped, the intermembrane space was enlarged and the outer membrane was covered with an electron-dense amorphous material whose nature could not be determined. The organelle-targeted 36K protein seems to promote the overgrowth of the mitochondrial outer membrane.