EWS (Ewing sarcoma) encodes an RNA/ssDNA binding protein that is frequently rearranged in a number of different cancers by chromosomal translocations. Physiologically, EWS has diverse and essential roles in various organ development and cellular processes. In this study, we uncovered a new role of EWS in mitochondrial homeostasis and energy metabolism. Loss of EWS leads to a significant decrease in mitochondria abundance and activity, which is caused by a rapid degradation of Peroxisome proliferator-activated receptor γ Coactivator (PGC-1α), a central regulator of mitochondria biogenesis, function, and cellular energy metabolism. EWS inactivation leads to increased ubiquitination and proteolysis of PGC-1α via proteasome pathway. Complementation of EWS in Ews-deficient cells restores PGC-1α and mitochondrial abundance. We found that expression of E3 ubiquitin ligase, FBXW7 (F-box/WD40 domain protein 7), is increased in the absence of Ews and depletion of Fbxw7 in Ews-null cells restores PGC-1α expression and mitochondrial density. Consistent with these findings, mitochondrial abundance and activity are significantly reduced in brown fat and skeletal muscles of Ews-deficient mice. Furthermore, expression of mitochondrial biogenesis, respiration and fatty acid β-oxidation genes is significantly reduced in the liver of Ews-null mice. These results demonstrate a novel role of EWS in mitochondrial and cellular energy homeostasis by controlling PGC-1α protein stability, and further implicate altered mitochondrial and energy metabolism in cancers harboring the EWS translocation.EWS | PGC-1alpha | protein stability | mitochondria homeostasis | energy metabolism E wing sarcoma breakpoint region 1 (EWSR1, herein termed EWS) was first identified from the Ewing sarcoma chromosomal breakpoint t(11, 22)(q24;q12) region as a translocationgenerated fusion gene between EWS and FLI1 (Friend leukemia integration 1) (1), an ETS-family of transcription factor. EWS is a member of the FET (or TET) family of RNA and ssDNAbinding proteins, which includes two other members, FUS/TLS (Fused in Sarcoma/Translocated in Liposarcoma) and TAF15/ hTAFII68 (TATA-binding protein-associated factor 15/human TATA-binding protein-associated factor II 68) as well as a Drosophila protein, Cabeza/SARFH (1, 2). A transcriptional role of EWS has been inferred by its association with basic transcription factors (3, 4) and by its modulation of several transcription factor activities (5-8). EWS is also involved in alternative splicing of specific genes in response to DNA damage (9, 10). Animal studies with genetic ablation of Ews have revealed a surprisingly diverse role of EWS in various cellular processes such as pre-B lymphocyte development, meiosis, mitosis and prevention of premature cellular senescence in fibroblasts and hematopoietic stem cells (11-13). Recently, it was shown that EWS is required for determining embryonic brown fat cell fate during development (8), for in vitro white fat differentiation (14), and in the regulation of microRNAs (15).The ...