The ubiquitin-like protein FAT10 forms covalent conjugates and induces apoptosis

FAT10 is a ubiquitin-like protein that is encoded in the major histocompati bility complex class I locus and is synergistically inducible with interfer on-gamma and tumor necrosis factor alpha. The molecule consists of two ubiq uitin-like domains in tandem arrangement and bears a conserved diglycine mo tif at its carboxyl terminus commonly used in ubiquitin-like proteins for i sopeptide linkage to conjugated proteins. We investigated the function of F AT10 by expressing murine FAT10 in a hemagglutinin-tagged wild type form as well as a diglycine-deficient mutant form in mouse fibroblasts in a tetrac ycline-repressible manner. FAT10 expression did not affect major histocompa tibility complex class I cell surface expression or antigen presentation. H owever, we found that wild type but not mutant FAT10 caused apoptosis withi n 24 h of induction in a caspase-dependent manner as indicated by annexin V cell surface staining and DNA fragmentation. Wild type FAT10, but not its diglycine mutant, was covalently conjugated to thus far unidentified protei ns, indicating that specific FAT10 activating and conjugating enzymes must be operative in unstimulated fibroblasts. Because FAT10 expression causes a poptosis and is inducible with tumor necrosis factor alpha, it may be funct ionally involved in the programmed cell death mediated by this cytokine.