Hsiang-Yi Wua, Shin-Fu Chena,1, Ju-Yi Hsiehb,1, Fang Choua, Yu-Hsuan Wangb, Wan-Ting Lina,c, Pei-Ying Leea,c, Yu-Jen Yua, Li-Ying Lina,c, Te-Sheng Lina,c, Chieh-Liang Lina, Guang-Yaw Liud, Shiou-Ru Tzenga,2, Hui-Chih Hungb,e,2 & Nei-Li Chana,c,2
aInstitute of Biochemistry and Molecular Biology, College of Medicine, National Taiwan University, Taipei, Taiwan. bDepartment of Life Sciences and Institute of Genomics and Bioinformatics, National Chung Hsing University, Taichung, Taiwan. cInstitute of Biochemistry, National Chung Hsing University, Taichung, Taiwan. dInstitute of Microbiology and Immunology, Chung Shan Medical University, and Division of Allergy, Immunology, and Rheumatology, Chung Shan Medical University Hospital, Taichung, Taiwan. eAgricultural Biotechnology Center, National Chung Hsing University, Taichung, Taiwan.
Proc. Natl. Acad. Sci. (USA) 2015; 112:11229-34
Polyamines are organic polycations essential for cell growth and differentiation; their aberrant accumulation is often associated with diseases, including many types of cancer. To maintain polyamine homeostasis, the catalytic activity and protein abundance of ornithine decarboxylase (ODC), the committed enzyme for polyamine biosynthesis, are reciprocally controlled by the regulatory proteins antizyme-1 (Az1) and antizyme inhibitor (AzIN). Az1 suppresses polyamine production by inhibiting the assembly of the functional ODC homodimer and, most uniquely, by targeting ODC for ubiquitin-independent proteolytic destruction by the 26S proteasome. In contrast, AzIN positively regulates polyamine levels by competing with ODC for Az1 binding. The structural basis of the Az1-mediated regulation of polyamine homeostasis has remained elusive. Here we report crystal structures of human Az1 complexed with either ODC or AzIN. Structural analysis revealed that Az1 sterically blocks ODC homodimerization. Moreover, Az1-binding triggers ODC degradation by inducing the exposure of a cryptic proteasome-interacting surface of ODC, which illustrates how a substrate protein may be primed upon association with Az1 for ubiquitin-independent proteasome recognition. Dynamic and functional analyses further indicated that the Az1-induced binding and degradation of ODC by proteasome can be decoupled, with the intrinsically disordered C-terminal tail fragment of ODC being required only for degradation but not binding. Finally, the AzIN-Az1 structure suggests how AzIN may effectively compete with ODC for Az1 to restore polyamine production. Taken together, our findings offer structural insights into the Az-mediated regulation of polyamine homeostasis and proteasomal degradation.