The Cancer/Testis Antigen FATE1 Antagonizes Fission and Preserves Mitochondrial Network Integrity under Cytotoxic Stress

FATE1 (Fetal and Adult Testis Expressed 1), also known as BJ-HCC-2, is a cancer-testis antigen with highly restricted expression in normal tissues but aberrant activation in diverse tumor types, where it localizes to both the outer mitochondrial membrane and the endoplasmic reticulum (ER) and functions as a key survival factor. Although FATE1 shares sequence homology with the mitochondrial fission factor Mff, its role in regulating mitochondrial architecture has not been mechanistically defined. Here, we identify FATE1 as a novel modulator of mitochondrial morphology that acts through a mechanism distinct from Mff. Unlike Mff, FATE1 does not recruit Drp1 to mitochondria and therefore lacks canonical fission activity. Instead, FATE1 promotes mitochondrial hyperfusion and protects against mitochondrial fragmentation triggered by cytokines and mitochondrial uncoupler treatment. The pro-fusion activity of FATE1 requires its mitochondrial targeting and interaction with Mitofusin-2 (Mfn2). Our findings establish FATE1 as a cancer-selective regulator of mitochondrial dynamics that antagonizes fission and preserves network integrity under cytotoxic stress, revealing a potential mechanism by which tumor cells evade mitochondria-driven apoptotic signaling.