X-linked Inhibitor of Apoptosis Protein promotes the degradation of its antagonist, the pro-apoptotic ARTS protein.

ARTS (Sept4_i2) is a mitochondrial pro-apoptotic tumor suppressor protein. In response to apoptotic signals, ARTS translocates to the cytosol where it promotes caspase activation through caspase de-repression and proteasome mediated degradation of X-linked Inhibitor of Apoptosis Protein (XIAP). Here we show that XIAP regulates the levels of ARTS by serving as its ubiquitin ligase, thereby providing a potential feedback mechanism to protect against unwanted apoptosis. Using both in vitro and in vivo ubiquitination assays we found that ARTS is directly ubiquitinated by XIAP. Moreover, we found that XIAP-induced ubiquitination and degradation is prevented by removal of the first four amino acids in the N-terminus of ARTS, which contains a single lysine residue at position 3. Thus, this lysine at position 3 is a likely target for ubiquitination by XIAP. Importantly, although the stabilized ARTS lacking its first 4 residues binds XIAP as well as the full length ARTS, it is more potent in promoting apoptosis than the full length ARTS. This suggests that increased stability of ARTS has a significant effect on its ability to induce apoptosis. Collectively, our data reveal a mutual regulatory mechanism by which ARTS and XIAP control each other's levels through the ubiquitin proteasome system.