Structural basis of antizyme-mediated regulation of polyamine homeostasis

Proc Natl Acad Sci U S A. 2015 Sep 8;112(36):11229-34. doi: 10.1073/pnas.1508187112. Epub 2015 Aug 24.


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 isoform 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.

Keywords: antizyme; antizyme inhibitor; ornithine decarboxylase; polyamine homeostasis; ubiquitin-independent proteolysis.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Biocatalysis
  • Carrier Proteins / chemistry*
  • Carrier Proteins / metabolism
  • Crystallography, X-Ray
  • Enzyme Inhibitors / chemistry
  • Enzyme Inhibitors / metabolism
  • Homeostasis*
  • Humans
  • Kinetics
  • Models, Molecular
  • Molecular Sequence Data
  • Ornithine Decarboxylase / chemistry*
  • Ornithine Decarboxylase / metabolism
  • Polyamines / chemistry*
  • Polyamines / metabolism
  • Proteasome Endopeptidase Complex / metabolism
  • Protein Binding
  • Protein Conformation
  • Protein Multimerization
  • Protein Structure, Secondary
  • Protein Structure, Tertiary
  • Proteins / chemistry*
  • Proteins / metabolism
  • Proteolysis
  • Sequence Homology, Amino Acid


  • AZIN1 protein, human
  • Carrier Proteins
  • Enzyme Inhibitors
  • Polyamines
  • Proteins
  • ornithine decarboxylase antizyme
  • Proteasome Endopeptidase Complex
  • ATP dependent 26S protease
  • Ornithine Decarboxylase

Associated data

  • PDB/4ZGY
  • PDB/4ZGZ