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 (Az 1 ) and antizyme inhibitor (AzIN). Az 1 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 Az 1 binding. The structural basis of the Az 1 -mediated regulation of polyamine homeostasis has remained elusive. Here we report crystal structures of human Az 1 complexed with either ODC or AzIN. Structural analysis revealed that Az 1 sterically blocks ODC homodimerization. Moreover, Az 1 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 Az 1 for ubiquitin-independent proteasome recognition. Dynamic and functional analyses further indicated that the Az 1 -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-Az 1 structure suggests how AzIN may effectively compete with ODC for Az 1 to restore polyamine production. Taken together, our findings offer structural insights into the Az-mediated regulation of polyamine homeostasis and proteasomal degradation.polyamine homeostasis | ornithine decarboxylase | antizyme | antizyme inhibitor | ubiquitin-independent proteolysis P olyamines are multivalent organic cations that are ubiquitous and essential in eukaryotes (1). With their polycationic characteristics, these compounds are known to modulate the structural and functional properties of nucleic acids and proteins via electrostatic interactions, in turn affecting cell growth and differentiation by influencing the underlying cellular processes (1, 2). Consistent with their crucial regulatory roles, fluctuations in intracellular polyamine levels are rigorously controlled during cell growth and differentiation via fine-tuning the balance between the biosynthesis, degradation, and uptake of polyamines. Aberrant accumulation of polyamines is associated with pathological consequences, including many types of cancer (3-5).Regulation of polyamine homeostasis is achieved mainly by adjusting the catalytic activity and protein abundance of ornithine decarboxylase (ODC), a homodimeric and pyridoxal 5ʹ-phosphatedependent enzyme that catalyzes the committed and rate-limiting step in polyamine biosynthesis, through the actions of the regulatory proteins antizyme isoform 1 (Az 1 ) and antizyme inhibitor (AzIN) (3, 6). E...

Background: LMBD1 is a transmembrane protein encoded by the lmbrd1 gene. Its biological function remains unclear. Results: lmbrd1 knockdown hinders the internalization of the insulin receptor, allowing it and its downstream signaling molecules to remain activated. Conclusion: LMBD1 functions as an insulin receptor-specific adaptor for endocytosis. Significance: LMBD1 is the first identified protein that participates in the regulation of insulin receptor endocytosis.

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The results suggested that Lmbrd1 gene not only plays a significant role in mediating the energy homeostasis in cardiac tissue, it may also be a key factor in the regulation of cardiac function in mice.