The SUMO ligase PIAS1 regulates UV-induced apoptosis by recruiting Daxx to SUMOylated foci

Abstract

SummaryThe small ubiquitin-like modifier (SUMO) ligase PIAS1 (Protein Inhibitor of Activated Stat-1) has been shown to play a role in cellular stress response by SUMOylating several proteins that are involved in DNA repair, apoptosis and transcription. In this paper, we show that PIAS1 regulates ultraviolet (UV)-induced apoptosis by recruiting Death-associated protein 6 (Daxx) to PIAS1-generated SUMOfoci. Cells that ectopically express PIAS1, but not other PIASes, show increased sensitivity to UV irradiation, suggesting that PIAS1 has a distinct function in UV-dependent apoptosis. Domain analysis of PIAS1 indicates that both PIAS1 SUMO-ligase activity and the specific localization of PIAS1 through its N-terminal and C-terminal domains are essential for UV-induced cell death. Daxx colocalizes with PIAS1-generated SUMOylated foci, and the reduction of Daxx using RNAi alleviates UV-induced apoptosis in PIAS1-expressing cells. PIAS1-mediated recruitment of Daxx and apoptosis following UV irradiation are dependent upon the Daxx C-terminal SUMOinteracting motif (SIM). Overall, our data suggest that the pro-apoptotic protein Daxx specifically interacts with one or more substrates SUMOylated by PIAS1 and this interaction leads to apoptosis following UV irradiation.Key words: PIAS1, SUMO, Apoptosis, UV irradiation, Daxx IntroductionThe small ubiquitin-like modifier (SUMO) proteins regulate the activity, localization and/or stability of various cellular proteins through a reversible covalent modification or noncovalent interactions with their substrates (Geiss-Friedlander and Melchior, 2007). Three well-characterized SUMO isoforms, numbered 1 to 3, exist in vertebrates, and the gene for a fourth potential SUMO isoform (SUMO-4) has been identified; however, it is unclear whether SUMO-4 is expressed in all cells and whether it can be conjugated to other proteins. SUMO-1, SUMO-2 and SUMO-3 covalently attach to a substrate by forming an isopeptide linkage with one or more specific lysine residues on the protein. In addition, SUMO-2 and SUMO-3, which share 95% sequence identity, are known to form poly-SUMO chains (Johnson, 2004). SUMO modification involves the sequential transfer of a processed active SUMO protein from an E1 SUMO-activating enzyme to an E2 SUMO-conjugating enzyme and the subsequent covalent conjugation to the substrate in a reaction that involves a SUMO-E3 ligase. In their substrate-conjugated form, SUMO proteins can interact with other proteins containing a SUMO-interacting motif (SIM), which is characterized by YYXY or YXYY residues (where Y is a hydrophobic amino acid) and is often flanked by negatively charged residues (Song et al., 2004;Wilkinson and Henley, 2010). Therefore, protein modification by SUMO can lead to new protein-protein interaction modules.Since the first SUMO-conjugated protein was identified, the repertoire of SUMO-modified substrates has steadily increased. The diversity of SUMO substrates is presumably governed by a small number of SUMO E3 ligases that exist in cells. The PIAS (Protein...

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“…Previous studies have shown that PIAS1 promotes apoptosis in response to various stimuli [52], [65], [66]. These observations suggest that necdin targets PIAS1 to suppress its proapoptotic activities.…”

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