1. ¹Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, Hefei, Anhui 230027, People's Republic of China
2. ²School of Biological Sciences, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Republic of Singapore
3. ³The National Laboratory of Medical Molecular Biology, Department of Biochemistry and Molecular Biology, The Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences and Peking Union Medical College, 5 DongdanSantiao, Beijing 100005, People's Republic of China
4. ⁴These authors contributed equally to this work.

Correspondence: M Wu, Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, Hefei, Anhui 230027, People's Republic of China. Tel: +86 551 3606264; Fax: +86 551 3606264; E-mail: wumian@ustc.edu.cn

Received 29 December 2005; Revised 20 March 2006; Accepted 3 April 2006; Published online 26 May 2006.

Abstract

Livin, a member of the inhibitor of apoptosis protein (IAP) family, encodes a protein containing a single baculoviral IAP repeat (BIR) domain and a COOH-terminal RING finger domain. It has been reported that Livin directly interacts with caspase-3 and -7 in vitro and caspase-9 in vivo via its BIR domain and is negatively regulated by Smac/DIABLO. Nonetheless, the detailed mechanism underlying its antiapoptotic function has not yet been fully characterized. In this report, we provide, for the first time, the evidence that Livin can act as an E3 ubiquitin ligase for targeting the degradation of Smac/DIABLO. Both BIR domain and RING finger domain of Livin are required for this degradation in vitro and in vivo. We also demonstrate that Livin is an unstable protein with a half-life of less than 4 h in living cells. The RING domain of Livin promotes its auto-ubiquitination, whereas the BIR domain is likely to display degradation-inhibitory activity. Mutation in the Livin BIR domain greatly enhances its instability and nullifies its binding to Smac/DIABLO, resulting in a reduced antiapoptosis inhibition. Our findings provide a novel function of Livin: it exhibits E3 ubiquitin ligase activity to degrade the pivotal apoptotic regulator Smac/DIABLO through the ubiquitin–proteasome pathway.