Walleye dermal sarcoma virus Orf C is targeted to the mitochondria

Walleye dermal sarcomas are associated with the presence of a complex retrovirus, walleye dermal sarcoma virus (WDSV). These sarcomas develop and regress seasonally in naturally infected fish. In addition to gag, pol and env, WDSV contains three open reading frames (ORFs), designated orf a, orf b and orf c. orf c is located between the 5' long terminal repeat and gag. Developing tumours contain low levels of orf a and orf b transcripts, whereas regressing tumours contain high levels of genomic transcripts and virus particles. Orf C protein is encoded by the full-length, genomic transcript and can be detected in tumour extracts with anti-Orf C-specific antisera. To determine the subcellular location of WDSV Orf C, cultured cells were transfected with an expression vector encoding haemagglutinin-tagged Orf C and examined by immunofluorescence. Orf C was observed throughout the cytoplasm and accumulated in cytoplasmic organelles. Dual-antibody staining for Orf C and mitochondrial cytochrome c demonstrated colocalization of Orf C with mitochondria and loss of the normal distribution of mitochondria in the cytoplasm. Cells transiently expressing Orf C exhibited apoptotic morphology and increased levels of surface phosphatidylserine and were unable to retain MitoTracker Orange, a dye that accumulates in active mitochondria. These results imply a functional role for WDSV Orf C in an alteration of mitochondrial function that results in apoptosis contributing to tumour regression.