FAT10, a Ubiquitin-Independent Signal for Proteasomal Degradation

Abstract

FAT10 is a small ubiquitin-like modifier that is encoded in the major histocompatibility complex and is synergistically inducible by tumor necrosis factor alpha and gamma interferon. It is composed of two ubiquitin-like domains and possesses a free C-terminal diglycine motif that is required for the formation of FAT10 conjugates. Here we show that unconjugated FAT10 and a FAT10 conjugate were rapidly degraded by the proteasome at a similar rate. Fusion of FAT10 to the N terminus of very long-lived proteins enhanced their degradation rate as potently as fusion with ubiquitin did. FAT10-green fluorescent protein fusion proteins were not cleaved but entirely degraded, suggesting that FAT10-specific deconjugating enzymes were not present in the analyzed cell lines. Interestingly, the prevention of ubiquitylation of FAT10 by mutation of all lysines or by expression in ubiquitylation-deficient cells did not affect FAT10 degradation. Thus, conjugation with FAT10 is an alternative and ubiquitin-independent targeting mechanism for degradation by the proteasome, which, in contrast to polyubiquitylation, is cytokine inducible and irreversible.

ACKNOWLEDGMENTS

We thank E. Naidoo for excellent technical support and C. M. Pickart for valuable advice. We acknowledge M. Piechaczyk for contributing ts20 cells, F. Levy for DHFR constructs, and M. Basler for HA-ubiquitin plasmids.

We declare that we have no financial conflict of interest. This work was funded by the Swiss National Science Foundation (grant 31-63387.00) in its initial phase and by the German Research Foundation (grants GR 1517/-2-1 and GR 1517/3-1) in its later phase.