Ornithine decarboxylase is degraded by the 26S proteasome without ubiquitination

Abstract

Ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, is the most rapidly turned over mammalian enzyme. We have shown that its degradation is accelerated by ODC antizyme, an inhibitory protein induced by polyamines. This is a new type of enzyme regulation and may be a model for selective protein degradation. Here we report the identification of the protease responsible for ODC degradation. Using a cell-free degradation system, we demonstrate that immunodepletion of proteasomes from cell extracts causes almost complete loss of ATP- and antizyme-dependent degradation of ODC. In addition, purified 26S proteasome complex, but not the 20S proteasome, catalyses ODC degradation in the absence of ubiquitin. These results strongly suggest that the 26S proteasome, widely viewed as specific for ubiquitin-conjugated proteins, is the main enzyme responsible for ODC degradation. The 26S proteasome may therefore have a second role in ubiquitin-independent proteolysis.

Antizyme, a mediator of ubiquitin-independent proteasomal degradation and its inhibitor localize to centrosomes and modulate centriole amplification

“…We have now identified AZI overexpression as an alternative cause for daughter centriole overamplification again suggesting that the antizyme 1/AZI system regulates centriole synthesis. Our results also show that AZI overexpression leads to significantly higher levels of cyclin D1 and ODC whose stability are both regulated by antizyme 1 (Murakami et al, 1992;Newman et al, 2004). Overexpression of cyclin D1 has been reported to induce centrosome abnormalities, but it led to hyperamplification of centrosomes with equal numbers of daughter and mother centrioles (Nelsen et al, 2005).…”

“…The most abundant and best characterized family member is antizyme 1. Antizyme 1 binds ODC with high affinity, thereby blocking its enzymatic activity and promoting the ubiquitin-independent degradation of ODC via the 26S proteasome (Murakami et al, 1992). Today, its role is seen in a considerably broader context, as antizyme 1 has been shown to affect the stability of several additional proteins such as cyclin D1 and Smad1 (Lin et al, 2002;Newman et al, 2004).…”

Antizyme1 mediates AURKAIP1-dependent degradation of Aurora-A

“…The attachment of Az causes conformational changes in ODC, thereby exposing its C-terminal degradation signal for recognition by 26S proteasome (Li and Coffino, 1993). Unlike Ub, Az is usually spared from destruction and released from the ODCAz complex at the proteasome (Murakami et al, 1992). Therefore, a single Az molecule can catalyze multiple rounds of ODC degradation.…”

Overexpression of antizyme-inhibitor in NIH3T3 fibroblasts provides growth advantage through neutralization of antizyme functions

“…Ornithine decarboxylase is one of the most rapidly degraded mammalian proteins, and its degradation is regulated by polyamines via an ubiquitin independent mechanism. This is achieved by a unique mechanism in which a polyamine-induced protein termed antizyme (Az) binds and inactivates ODC, and subsequently targets it for rapid degradation by the 26S proteosome Rosenberg-Hasson et al, 1989;Murakami et al, 1992a). In addition to its role in regulating ODC activity and degradation, antizyme was also demonstrated to regulate polyamine transport across the plasma membrane via a yet unknown mechanism Mitchell et al, 1994;Suzuki et al, 1994;Sakata et al, 2000).…”