Molecular mechanism for degradation of ornithine decarboxylase by 26 S proteasome

鸟氨酸脱羧酶（ODC）是最不稳定的蛋白质之一，它是由26S蛋白酶体在不泛素化的情况下进行atp依赖性降解的。降解依赖于抗酶，一种多胺诱导的蛋白质，与ODC结合，抑制其活性。本研究旨在了解26s蛋白酶体识别和降解odc -抗酶复合物的分子机制。首先，我们通过(1)分光光度法分析ODC与抗酶结合引起的结构变化，(2)分析抗酶的功能区域和分析突变ODC的降解来检验抗酶的作用。其次，我们通过(1)分析ODC的降解产物，(2)分析各种模拟ODC序列不同部分的合成肽对ODC降解的抑制作用，以及(3)鉴定催化组成型ODC降解的酶，来检测26s蛋白酶体识别的ODC位点。结果表明，抗酶与ODC单体结合，通过c端半段（122-218）失活ODC单体，引起ODC与相邻区域（113-118）的构象变化，使ODC的隐性降解信号暴露于26s蛋白酶体。多催化蛋白酶似乎通过其蛋白内水解功能在许多位点（主要是中性/疏水氨基酸残基的羧基侧）切割ODC，产生由5-11个氨基酸残基组成的寡肽。在ODC降解过程中，抗酶未被降解。结果表明，抗酶在其分子内不携带降解信号，但通过增强ODC与蛋白酶的结合，加速了26 S蛋白酶体催化的ODC的组成性降解。ODC的降解信号有待于进一步的研究。

Ornithine decarboxylase (ODC), one of the most unstable protein, is ATP-dependently degraded by the 26S proteasome without ubiquitination. The degradation is dependent on antizyme, a polyamine-induced protein that binds to ODC,inhibiting its activity. The present study aimed to understand molecular mechanism whereby 26 S proteasome recognizes and degrades ODC-antizyme complex. First, we examined the role of antizyme by (1) spectrophotometric analysis of the structural changes of ODC caused by binding with antizyme, (2) analysis of functional regions of antizyme and analysis of degradation of mutant ODC.Second, we examined the site of ODC recognized by 26 S proteasome by (1) analysis of degradation products of ODC,(2) analysis of inhibitory effects of various synthetic peptides, each simulating different part of ODC sequence, on ODC degradation, and (3) identification of the enzyme which catalyses constitutive ODC degradation. The results suggested that antizyme binds to ODC monomer and inactivates it with C-terminal half (122-218) and elicits a conformational change of ODC with an adjacent region (113-118), resulting in exposure of a hidden degradation signal of ODC to 26 S proteasome. The multicatalytic proteinase was seemed to cleave ODC at many sites (mainly carboxyl sides of neutral/hydrophobic amino acid residues) by its endoproteolytic function, generating oligopeptides consisting of 5-11 amino acid residues. Antizyme was not degraded during ODC degradation. It was suggested that antizyme does not carry degradation signal within its molecule but accelerates constitutive ODC degradation, also catalyzed by the 26 S proteasome, by enhancing the association of ODC to proteinase. The degradation signal of ODC remained to be clarified by future work.